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Full text of "Zoology : a textbook for universities, colleges and normal schools / Thomas Walton Galloway"

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A Text-book for Secondary 
Educational Institutions 

i2mo, xx+4i8 pages. Cloth, $1.25 














The last twenty years have been very productive of text- 
books of Zoology, both for secondary schools and for colleges. 
Many of these are admirable from the point of view of the'science 
itself. It is increasingly apparent, however, that the right 
text-book of Zoology, as of every other subject, is primarily a 
matter of psychology. The prime object of all teaching is so 
to use the subject as to produce the best possible result in the 
personality of the pupil. This means that the mental structure 
and functioning of the pupils are even more important than the 
subject matter in conditioning the presentation of any subject. 
Teachableness is the first prerequisite in any text-book in any 
subject whatsoever. Some of the best zoologies we have from 
the point of view of the science of Zoology are pedagogically 

It is the purpose of the present book, however much it may 
fall short of its aim, to subordinate certain zoological prepos- 
sessions to the mental requirements of freshmen. The method 
here adopted has been used successfully by the writer in a first 
course, for more than fifteen years. It has secured good interest 
and fine spirit. The following are some of the principles that 
have guided him in the selection and arrangement of the mate- 
rial in the present volume: 

i. The work done in a first course is primarily for pupils 
who do not take a second course. This first course should be 
handled, therefore, as a life-training rather than as satisfying 
a college requirement. It should seek to accomplish these, 
among other, things: 

The production and conservation of a vital interest in 
animals; an appreciation of the human values of animals; 
the encouragement of the attitude of raising and solving 
problems concerning animals; some ability to use the 
library, the field, and the laboratory in individual pursuit 
of these interests; the ability to sustain interest in these 


problems through considerable periods ; a sense of the way 
in which organisms respond to the environing conditions ; 
an elementary conception of development and of the evolu- 
tionary series of animals ; some experience in classification 
of organisms theoretical and practical; a conception of 
the place of man in the biological series, along with the 
conviction that this does not invalidate, but rather 
heightens, the meaning of all the higher human qualities. 

2. The first thing to be sought therefore is a thorough-going 
appreciation on the part of the student of the attractiveness, 
the scope, and importance of animals and their activities. 

3. A first course should really be a foundation course, and 
as such should give the student a broad and catholic view of 
the whole subject. It should utilize all the main departments 
of Zoology, because each department contains matter which 
should be familiar to all persons of ordinary education. Further- 
more, the departments of morphology, physiology, ecology, dis- 
tribution, and classification furnish exercises which have dis- 
tinct, and yet complementary, pedagogical value. Any single 
phase of the subject, however important or interesting, gives 
a false and therefore an unscientific view of the wonderful science 
of Zoology, unless it is supplemented by the others. Therefore 
a book, if it is to serve the pedagogical needs of beginners, should 
contain fairly representative matter from all the main depart- 
ments of the science; and it should at the same time provide 
both for the descriptive work and for the practical work in the 
field and laboratory. 

4. Laboratory work and field work are essential, both to 
proper interest and to proper results, and should not be merely 
illustrative of text or lecture work, but as far as possible should 
be the foundation and point of departure of the lectures and 
the text. No instrumentality open to the teacher is better than 
the laboratory as a means of securing real interest and mental 
growth for the pupils. However, in order to attain this end 
it is essential that this work shall really be vitally done. It is 
not enough that a pupil be induced to observe and to record 
his observations. The pupil's mind should always be en- 
couraged to "follow through" to whatever response in the way 
of conclusion or explanation seems sound in the light of his 
knowledge at the time. It is much more important that these 


inner reactions be allowed, be complete, and be the student's 
own, than that they be rigorously right. It is easy enough to 
add new facts in order both to teach the pupil that conclusions 
are liable to be wrong, and to force his conclusion closer to that 
moving equilibrium which we call truth. This process is much 
more easy than to re-galvanize the soul whose interest and 
ability to reach conclusions have been aborted by a continual 
denial of the right. Much of the failure in our laboratory work 
is due to this "taxation without representation" in respect to 

5. On the other hand it is equally important that the work 
shall not be confined to the field and the laboratory. "There 
are many things in the infinite concourse of particulars which 
we cannot afford to verify by experiment." The chief end of 
laboratory work is gained for the elementary student when he 
comes to appreciate the method and spirit by which sound in- 
vestigation proceeds, has acquired enough technical skill to fol- 
low elementary investigation on his own behalf, and has learned 
how to appreciate, and if necessary to verify, the statements of 
others. It is as easy to waste time in the laboratory as in reading 

6. The time in an elementary course should be about equally 
apportioned (i) to laboratory work (chiefly in physiology and 
in the larger problems of morphology rather than in minute 
dissection); (2) to field observation on physiology, life histories, 
and the simpler problems of distribution, classification, and 
life relations; (3) to the body of the descriptive text; and (4) to 
classes of questions demanding reference to classical zoological 

It is a great mistake not to impress upon the student the 
immense amount of work already done and to heighten his re- 
spect for the library as one of his sources of information and 

7. The matter of greater native interest should underlie and 
sustain that of less. It should not, however, exclude or efface 
the latter. The most interesting is often the least important. 

8. Certain of the general facts and principles which the 
beginner cannot be expected to discover for himself should be 


presented early, in order to give the student a skeleton or 
dimensions, so to speak in which he shall later insert the par- 
ticulars which he discovers. He must have this in order to 
unify his own results in the brief time at his disposal. The 
lack of this unifying result is the ground of the just complaint 
concerning much of the unorganized and unrelated laboratory 
instruction in the secondary schools and early college classes. 

9. While it is necessary to bring our materials from various 
departments of Zoology and is desirable that the student should 
be able to recognize whether a given problem is primarily one 
of structure or function or relation, the total result of an ele- 
mentary course of Zoology should be a sense of unity, of con- 
tinuity, and of interdependence. The final view of the student 
should be of life and organic progress, and not of a disjointed 
science, dissected in the house of its friends. 

10. The teacher should have some latitude in the choice of 
matter and emphasis, in order that both may be properly suited 
to his equipment and locality. It should be impossible for the 
teacher or the class to use a text-book in a slavish, or parasitic 
fashion. Therefore a text-book should contain and suggest 
much more than one teacher or one class can use in the time 
allowed. This not only gives the teacher a chance (and makes 
it necessary for him) to mould his own course, but causes the 
student -to realize that he is a mere beginner when he has com- 
pleted his first course. 

In attempting to apply these principles to the present book 
the author has made use of the following devices : 

i. The book is divided into three portions: (i) a general 
part dealing largely with broad biological problems and princi- 
ples, which constitute the foundations of the science and are 
felt to be for the most part, beyond even the verification of the 
elementary student (Chapters I-VIII) ; (2) a special part (Chap- 
ters IX-XXV), in which the various principal phyla of animals 
are taken up in succession, beginning with the lowest. The 
purpose has been to make this part particularly illustrative of 
the principles laid down in the general portion; and (3) a group 
of synthetic chapters (XXVI-XXIX) to induce the student to 
gather up the details of his course by a new reorganization of 
the materials. 


2. Each chapter of the general part contains the following 
elements: (i) the general statement of principles or facts; 
(2) interspersed with this are such practical exercises for labo- 
ratory, field, or library, as have been found practicable for 
elementary classes. These are intended to compensate for the 
enforced brevity and abstractness of definitions and description, 
by causing the student to find concrete illustration of the princi- 
ples; (3) an analytic summary of the most important general 
truths of the chapter in outline, at the close of the chapter; 
and finally (4), a list of supplementary topics for individual 
laboratory or library investigation and report. These supple- 
ment and illustrate the text, and enrich the review by introduc- 
ing a new viewpoint and new matter. It is not intended that 
all of these topics shall be demanded of the whole class. The 
writer has got best results in interest and knowledge by allow- 
ing each member of the class to select some topic in which he is 
interested and to make a brief report of his investigation before 
the whole group. 

3. In the chapters of the special part each phylum is intro- 
duced by field and laboratory work on some representatives 
taken as types. This is purposely made brief and suggestive 
in order to stimulate the teacher and class to build up their 
own detailed program. This is followed, corrected, and en- 
larged by a brief discussion of the typical condition of the organs 
and functions in the group as a whole. This serves to unify the 
isolated and local observations of the student. Next follows 
a brief statement of the most important facts of classification, 
together with ecological and economic suggestions. Finally, 
each chapter concludes with a list of supplementary questions 
calling for field, laboratory, and library work in review, and as 
a brief view of new material. 

4. The figures are carefully selected, the majority of them 
being specially made for this book. With each figure of special 
moment is a brief list of queries designed to assist the student 
in the study of the figure. It is a common complaint among 
teachers that it is difficult to get students to appreciate and to 
use illustrations intelligently, and to relate them to the text. 
A sane emphasis on these questions will solve this problem. 


5. The concluding chapter consists of practical questions 
and special exercises which necessitate a review by the studen 
of all that is essential in the book, from a new point of view. 
It is intended to do for the whole book what the suggestive 
topics at the end of each chapter may do for the chapters. 

6. The headings of paragraphs are printed in black-faced 
type, in order to emphasize the analysis of subject matter. 
Technical terms are in italics the first time they appear. The 
author does not agree that all technical language should be 
omitted from even an elementary course. 

The author extends most cordial thanks to the many pub- 
lishers and authors whose courtesy enables him to reproduce 
classic illustrations from their copyrighted works. Recognition 
is given to the sources in immediate connection with the figures. 
The thanks of the author are also due to many fellow teachers 
for suggestions and criticisms during the progress of the work, 
and since the appearance of the second edition. 













PHYLUM I. PROTOZOA (Primitive Animals) 149 


PHYLUM II. PORIFERA (Pore Bearing) 164 





fishes, Etc.) 173 


UNSEGMENTED WORMS (at least four Phyla, including Flat-worms, Thread- 
worms, Rotifers, Polyzoa, Etc.) 190 


PHYLUM VIII. ECHINODERMATA (Starfish, Sea-urchins, Sand-dollars, Sea- 
lilies) 207 

PHYLUM IX. ANNUL ATA (Segmented Worms) 222 





CHORDATA (CONT.) : SUB-PHYLUM VERTEBRATA (Fishes, Amphibians, Reptiles, 

Birds, and Mammals) . 3 J 5 


CLASS II. AMPHIBIA (Frogs, Toads, Salamanders) . 375 


CLASS III. REPTILIA (Lizards, Crocodiles, Tortoises, Snakes) 384 

CLASS IV. AVES (Birds) 398 

CLASS V. MAMMALIA (Mammals) 437 











INDEX 533 




1. Nature presents to man, as he looks upon it, a great and 
interesting variety of material objects. Each member of the 
race gathers in his lifetime, by means of experience and infer- 
ence, a certain limited knowledge of these objects and of the 
changes which they undergo. The knowledge, thus collected 
and systematized in the course of the history of the human 
race, constitutes the so-called Natural Sciences. Every one 
of us, whether he deliberately chooses or not, must be in some 
degree a natural scientist. The beauty and interest of the 
work has attracted and charmed thousands of people of all 
conditions, in all parts of the world. 

We commonly speak of material objects as either living or 
non-living as organic and inorganic. The study of living 
things in all their relations we call Biology. Physics and Chem- 
istry are often considered as dealing exclusively with inor- 
ganic matter, and are therefore placed in contrast with Biology. 
Their principles apply, however, in the realm of living things 
just as truly as in the non-living, and one must not imagine 
because of this antithesis that the phenomena of life can be 
explained apart from chemical and physical laws. The term 
Biology was first introduced about the beginning of the nine- 
teenth century, and is intended to express the fact that plants 
and animals are similar in their most essential structure and 
activities. The term Natural History is sometimes used 
synonymous with Biology. 

2. Zoology. Owing to the fundamental likeness of all 
living matter, there is great theoretical difficulty in distinguish- 
ing between the plant and animal kingdoms. The practical 


difficulty however is confined to the very lowest and simplest 
forms of life. The plants and animals which come under the 
common observation of the student are readily distinguished^ 
It is only the deeper study which reveals the underlying simi- 
larity of all living objects. The branch of Biology which 
treats of plants is called Botany; that which deals with animals, 

3. The Value of the Study of Zoology. The student, on 
taking up a new subject has a perfect right to ask whether that 
subject is worth while. A subject may have a great deal of 
value in practical ways and not mean much in education; or 
it may have high value in educating human beings and have 
very little of practical worth. Zoology is strong in both par- 
ticulars. Animals constitute one of the most interesting and 
important features 'in the surroundings of man. They arouse 
our interest, they appeal to our sense of beauty, they furnish 
us with food and clothing, they attack our crops, they produce 
diseases in us and in those animals we most use. In the second 
place, Zoology adds to our knowledge of the structure and 
activities of man himself since we are enabled through it to 
study ourselves in proper relation to the other animals. We 
may gain much light on the means of preserving human health 
and making right adjustments by the study of animals. Finally, 
the very method by which we study zoology is of the greatest 
value in our mental growth. The scientific method demands 
that we observe at first hand as many facts as possible; that we 
compare and contrast these facts with one another and with 
those which other people have observed; that we discriminate 
between important and unimportant facts; that we learn to 
draw right conclusions from our facts ; and that we always hold 
our minds open for new facts even after we have reached our 
conclusions. It is worth while to get these ideals and form 
these habits. 

To the investigator, the ultimate object of zoological study 
is to find the real nature of animal life as it exists, the mode of 
its development, and the causes which have brought it to its 
present exquisite variety and adjustment. These larger and 


more general questions constitute what may be called theo- 
retical Zoology, or the principles of Zoology. 

4. Practical Exercises. Cause the student to select ten or more kinds of 
animals with which he is partially acquainted, and, from his observation and ex- 
perience, to enumerate the points at which they touch human welfare. Are they, 
in each instance, to be classed as helpful? as harmful? or merely as indifferent? 
Is their influence upon man's interest direct or indirect? 

What animals have, in the past, most appealed to your interest? Select that 
particular quality in which you have been most interested (structure, beauty, 
powers, instincts, habits) and show how the attempt to study or explain any one 
takes you at once into all the others. 

5. Divisions of the Science. The facts and principles 
which have been, and are yet to be, discovered concerning 
animals are so numerous and various in their bearings, and in- 
vestigators approach the subject from such different points of 
view that it is necessary, in order to express these results, to 
divide zoology into several branches or departments. It must 
be held in mind, however, that these divisions are more or less 
artificial, and that the facts of each department are to be con- 
sidered in connection with those of all the others, if they are 
really to be understood. With all its departments, animal life 
is to be thought of as a whole. Structures exist for the per- 
formance of function, and the activities are intended to adjust 
the animal to its whole life-relation. 

6. Morphology is the branch of the science which deals 
with form or structure in its broadest sense, whether internal 
or external, partial or total. In its most general sense it em- 
braces the study of animals from the standpoint of symmetry, 
that is, the form of the organism with reference to certain 
planes passing through the body. For example, the human 
body may be so divided by a single plane that two essentially 
similar parts result, the right and the left. Again similar 
parts may succeed each other in a linear series, as in the seg- 
ments of the earth-worm; or they may radiate from a central 
point, as in the arms of the star-fish. This is the most funda- 
mental kind of morphology. It -relates the organism to space. 
It is called Promorphology, and is related to Zoology some- 
what as the study of crystals is to Mineralogy. 

Anatomy is that department of morphology which treats 


of the structure of parts of the individual, as the organs and 
systems of organs, the tissues, the cells, and so forth. This is 
known as gross anatomy if the study pertains to the larger 
units, as organs; it is called Histology, if the constituent ele- 
ments of these organs (as tissues and cells) are to be considered. 
Thus far we have thought of structure as stationary or per- 
manent. As a matter of fact we know that each organism 
begins life in a very modest way, as a single "cell," and grows 
more complex by fairly well-defined stages until the adult con- 
dition is reached. This is development. The science of 
Embryology is the record of this history of the successive stages 
which the individual animal assumes in becoming adult, or 
at least until its organs are essentially formed. 

7. In Physiology are considered the facts and laws relating 
to the activities or functions of the^ organism and of its separate 
parts. It includes the tracing back of the adult activities to 
their lowest form, as found in the simplest animals or the 
youngest stages of the higher animals. It includes the powers 
of the single cell; the chemical and physical processes which 
seem to underlie all the functional activities; the division and 
more perfect performance of the primitive functions as the 
various organs arise and come to do their special work. 
Finally it includes the relation of the animal as a whole to other 
animals of the same or of different species, to plants, and to 
the inanimate surroundings. The term Ecology is applied to 
the branch of physiology which treats of the relation of the 
organism to the complex and wonderful conditions in which 
it finds itself. Of recent years much emphasis is being given 
to this branch of zoology. 

8. Animals may be studied as to their distribution or occur- 
rence in the world. For example, we find lions in Africa and 
Asia only, and the African and Asiatic lions are of different 
varieties; the giraffe is found only in Africa; man is found 
over the most of the habitable globe, but before the era of easy 
communication between distant countries the men of different 
regions were conspicuously different. Again we can easily see 
that the animals that live in the various bodies of water are 


very different from those living on the land; those in the frigid 
zones are different from those in the temperate and torrid. 
All such topics are treated under the head of distribution, or 
geographical distribution. This is distribution in space. 

The hard parts of animals are found as fossils in many of 
the surface rocks of the earth. The various systems of rock- 
strata are characterized by more or less different fossil remains, 
indicating a variation in the animal life during the successive 
periods of the earth's history. This distribution of animals 
in time is the subject-matter of Palao zoology. The facts 
of palaeozoology and the conclusions resting thereon are 
among the most important in the whole realm of Zoology, 
inasmuch as they supplement the facts gained from the study 
of embryology and morphology of living species, thus enabling 
the investigator to trace the history of the various races of 
animals into the remote past. In this way we also learn much 
of the history of the earth itself. 

9. Practical Exercises. Let the student submit a written report on the dis- 
tribution of the animals in his immediate neighborhood, based on his own observa- 
tions. The report need not be exhaustive in order to convince the student of the 
effect of the environment, which includes everything in the surroundings, on the 
distribution of animals. Some classification should be made of the varieties of 
territory included; as river, pond, lowland, woodland, prairie, mountain, and the 
like. Determine, by reference to the authorities available, the geographical 
distribution of the following: the elephant, the camel, the kangaroo, the horse, 
the white bear, the seal, the salmon, the crocodile, the reef-forming coral, the 
sponge of commerce. Select five other species having a personal interest. 

10. Classification. In studying animals and plants one is 
soon impressed with the fact that among the thousands of 
individuals, even of the same general kind, there are no two 
exactly alike; and yet among them all, with their manifest dif- 
ferences, there are numerous points of similarity. These two 
facts make it possible to group those most alike into more or 
less coherent classes, separating them at the same time from 
other classes. The forming, naming, and defining of these 
groups and subgroups we call Taxonomy or Classification. 
Manifestly, true classification must depend upon the facts de- 
rived from the completest possible study of the structure and 
relations of organisms, and can only be perfect when we know 


all that is to be known about them. In addition to displaying 
our present knowledge of the relationship of animals, classifi- 
cation serves a most important end in giving us more rapid 
power of using that knowledge in getting further knowledge 
that is needed. 

11. Historical. Zoology as a science can scarcely be said 
to be more than three hundred years old, although Aristotle, 
more than three hundred years before Christ, wrote much of 
value concerning animals. Later many facts of general anat- 
omy were discovered in connection with the study of medicine, 
and about 1600 the invention of the microscope opened up the 
field of histology. Toward the end of the seventeenth century 
an effort was made to establish a scientific classification of 
animals. Since that time very much of the attention of students 
of zoology has been turned in this direction. During the 
last century however there has been a constantly increasing 
interest in the study of embryology, of histology, and in the 
general theoretical questions, the answers to which depend 
on the bringing together of the results of studies in all depart- 
ments. Such are the problems of race development or evolu- 
tion, of heredity, of man's place in nature, and the like. The 
most notable development of the subject in recent years has 
been in connection with the study of the finer structure of the 
cell, in more exact methods of studying physiology, and in 
extending its scope to take in the lower organisms as well as 
the higher and the single cell as well as the organs. It is 
important to add that all this work is now being done in a 
comparative way. The necessity of comparing the histology, 
the embryology, and the physiology of one animal with that 
of another arises from the belief in the unity of animal life, 
and that all animals are really akin. If animals of different 
kinds are really related, their likenesses and differences take on 
a new meaning to the student, and classification comes to ex- 
press the degree of kinship, as well as to serve the convenience 
of the investigator. 

12. Summary. 

I. Natural Science embraces: 


A. The sciences of inanimate things 


Meteorology, Mineralogy, Lithology, etc. 

B. The sciences of animate things 


(Physics and Chemistry are fundamental to both 
groups of sciences; Geology embraces portions of 
the subject-matter of both groups.) 

II. Subdivisions of Zoology. 

A. Morphology: 

1. Promorphology, which treats of general form; 

2. Anatomy; = the structure of parts; 

Gross = structure of organs and systems of 

Microscopic = (Histology, Cytology); struc- 
ture of tissues and cells; 

3. History of Development (structural stages): 

Individual = (Embryology, Ontogeny); 
Racial = (Phylogeny). 

B. Physiology: 

1. Physiology proper; = the functional relation of 
part to part and to the whole; 

2. Ecology; = relations of the individual to its 
whole surroundings. 

C. Distribution: 

1. In space = (Geographical Distribution); 

2. In time = (Palaeozoology, as revealed by fossils) . 

D. Classification, or the grouping of animals accord- 
ing to their likeness or kinship. 

E. Economic zoology considers all the points at which 
animals touch human welfare. 



13. Life. Life may be thought of in two somewhat distinct 
ways. It may be considered, first, merely as an expression 
for all the various activities of the organism, the sum of 
all the phenomena of its existence; or, second, as a force or 
form of energy from which the special modes of activity, as 
feeding, growth, motion, and thinking, arise. The latter is 
the more common use of the term, and yet the former is the 
only use of it which can be completely justified. Much of the 
activity of living things may be explained by reference to 
the ordinary physical and chemical laws. What we mean by 
this is that all the so-called vital activities depend on such 
physical facts as cohesion and adhesion, on the laws of fluids 
and of solution of solids in fluids; and on such chemical facts 
as the building up and tearing down of chemical compounds 
composed of just> the same elements that we find about us 
everywhere in the world. There are a great many biologists 
who think that there is nothing more in life than these physical 
and chemical processes and effects, and that we should be able 
to explain life and its activities if we knew all about the physics 
and chemistry of living matter. It is sure, however, that there 
are many vital phenomena which cannot be even remotely 
explained by what we now know of chemistry. Probably this 
will always be so. 

14. Living and Non-living Objects. Any one of us could, 
almost at a glance, tell whether an object is, or has been, alive; 
and yet it is not easy to describe just what it is that convinces 
us. There are two classes of differences between living things 
and things that have never been associated with life; differences 
(i) in make-up or organization, and (2) in powers. 



Organization includes form, size, organs or parts, as well as 
minute structure. While no two organisms are of exactly the 
same form, there are certain features that are very common. 
Living bodies are usually bounded by curved surfaces. They 
tend to be elongated in one axis more than in others; they tend 
to repeat certain of their parts; and they incline to be sym- 
metrical. They are limited in size, although some are very 
small and some are very large. They are differentiated, that 
is, they have organs, different kinds of structures at different 
parts of the body. As we shall see later they are made up of a 
substance called protoplasm, which is arranged in one or more 

Among the powers and activities of a living object may be 
mentioned (i) the power of changing food into its own substance, 
and thus of growth and repair; (2) the power of using this 
growth to separate off a portion and thus make a new individual 
like itself (reproduction); (3) the power to use some of the 
material of growth to develop energy of motion, heat, light, 
electricity, or thought, as the case may be; and (4) the ability 
by means of sensitiveness and the use of these various powers 
to adjust itself to very considerable changes in the environment. 
Every organism has all these qualities in some measure. No 
inorganic object has them. 

15. The Relation of Protoplasm to Life. Whatever life 
may be, in the last analysis, we never observe its manifestations 
except in connection with a substance called protoplasm, which 
is found both in plants and animals. Protoplasm does not con- 
tain any chemical elements which are not found in other than 
living materials. Notwithstanding this fact, protoplasm is 
different from any other known substance. It is more com- 
plex and more highly organized, as to its machinery, than any 
other chemical or physical compound whatsoever. Protoplasm 
has the power of growing by taking up and changing other 
non-living substances; but, so far as we know, it is never pro- 
duced except as the result of the growth and division of 
antecedent protoplasm. The protoplasmic or living material 
in an organism is normally composed of a number of unit- 


masses called cells (see Chapter III). These unit-masses of 
protoplasm are in some degree independent of one another, 
because normally each tends to form a wall about itself; and 
yet it is highly probable that the whole protoplasm of an ani- 
mal is physically continuous by means of delicate connections 
between the units. The life of the cells is not quite the same 
thing as the life of the organism to which they belong, for 
in animals composed of more than one cell a cell may die with- 
out involving the death of the animal. The protoplasm of the 
cell may also retain life for a time after separation from the 
living animal or after the animal as a whole has ceased to live. 
This is shown by the fact that cells may be taken from the 
body of young organisms and, if kept nourished under conditions 
similar to those of the body from which they are taken, will not 
merely live but will continue to divide and grow almost 
indefinitely. This has been shown to be true of several classes 
of cells. 

1 6. Protoplasm. While we describe protoplasm as the 
"physical basis of life" (Huxley), we no longer think of it as a 
constant or definite material with an exact composition. It 
is rather a complex mixture of substances, some of which are 
themselves very complex compounds. These substances in 
the mixture are continually bringing about changes in one 
another. It is inevitable that such a mixture of changeable 
substances should itself be most unstable and variable. It is 
agreed that there is much in common in all protoplasm, even 
in protoplasm as far- apart as that of plants and animals; and 
yet it is also true that the protoplasms of different animals and 
of different parts of the same animal are definitely different. 
This difference is ^hown by the difference in the work they can 
perform, as in muscles and nerve cells. 

Perhaps this is the most wonderful thing in life : that a sub- 
stance, so complex and so variable that it is not exactly the 
same any two moments in succession, should still be so constant 
that a small amount of it split off generation after generation 
should transmit all the essential qualities of the species to which 
it belongs, whether a paramecium or a man or an oak. 


17. Chemical Composition of Protoplasm. It is impossible to make a satis- 
factory chemical analysis of protoplasm, as it loses its characteristic powers and 
probably undergoes important chemical and physical changes in the act of analysis. 
The dead material thus obtained is no longer the substance with which we started, 
either as to its power or its structure. The experiment shows, however, that the 
substance is both chemically and physically unstable. By an analysis of the dead 
protoplasm, we find present several complex organic compounds, known as proteids, 
carbohydrates (starches and sugars), fats, ferments, pigments, etc. In addition to 
these are simpler inorganic compounds, as water and various salts. Doubtless 
some of these materials are food-substances on their way to form protoplasm, and 
others are the waste-products of protoplasmic disruption, ready to be cast out of 
the cell. The proteids are the most complex of all these substances and it is 
believed that protoplasm finds its real basis in these. 

The proteids are various in composition and properties, but agree in that their 
molecules contain carbon, hydrogen, oxygen, nitrogen, and sulphur, in proportion 
roughly as follows: C 53%, O 22%, N 17%, H 7%, S i%. Qarboji is thus the 
most important single constituent element. The white of egg, the fibrin of the 
blood, and casein in milk are examples of proteid. 

Carbohydrates consist of C, H, and O. The latter elements are always present 
in the ratio in which they are represented in water (H 2 0), 'e.g. CeHioOs. The 
starches, sugars, and cellulose, such as is found in cotton fibres, are illustrations. 

The fats contain the same elements as starch, but the percentage of oxygen 
in terms of the hydrogen is much smaller than in the starches. 

The ferments are complex organic substances which have the power of produc- 
ing important chemical changes in other substances without being themselves 
consumed. They play an important, but not thoroughly understood, r61e in the 
activities of the organisms, both within and outside the cells which produce them. 
The active principle of the digestive juices, as ptyalin and pepsin, are examples of 
ferments which have been extruded from the cells. 

Water (H 2 O) is very important in both the chemical and physical structure of 
protoplasm. It is very variable in amount, and the degree of activity of the 
protoplasm is roughly proportional to the amount of water present. Traces of 
inorganic salts, compounds of chlorine, potassium, sodium, calcium, phosphorus, 
iron, etc., are also found in solution in the water. 

Most of these substances cannot be considered as "living." The water and 
inorganic salts and starch cannot be. The starches and fats and urea are organic 
but not living, If any particular substances are alive it would seem to be the 
proteins. It may be, however, that life is the result of the intimate relations and 
interactions of all these various substances rather than a property of any one of 

1 8. The Physical Structure of Protoplasm. This varies 
much from time to time. On account of differences in the 
amount of water present, the consistency of protoplasm may 
vary from the quite fluid condition found in actively growing 
parts, to the very much more solid condition apparent in dry 
seeds and in the resting or encysted stage of some animals. 


In these latter instances the protoplasm eliminates a large per 
cent, of its water, forms a thick wall, and thereby becomes 
enabled to resist drouth and heat and cold as it could not possibly 
do otherwise. Under ordinary circumstances protoplasm ap- 
pears as a semi-fluid or gelatinous material. 

Concerning the architecture of protoplasm there is much 
diversity of opinion. It seems probable that this, like the 
chemical composition, is subject to considerable variation. It 
is certainly very complicated and represents several physic- 
ally distinct substances mingled in a very effectual and 
wonderful way. In some cases at least these take on the ap- 
pearance of a foam structure such as is obtained in an emul- 
sion of oil in water, or of air and water in a soapy lather. 
Whatever its form may be, it seems that there must be a close 
relation between the architecture and the great activity which 
protoplasm shows. It is physically as well as chemically 

19. Physiology of Protoplasm. The mass of protoplasm 
which we have called a cell, or unit, performs practically all 
the functions shown by the more complex organism. It has 
the power of feeding, of growth, of reproduction, of motion in 
response to stimuli, and of waste and repair. Even in the higher 
animals, made up of many of these units, the processes are 
performed, on last analysis, by the individual protoplasmic 
units of which the body is composed. 

20. Irritability. Owing to its chemical and physical in- 
stability, living protoplasm is constantly changing. These 
changes may be the direct result of internal or external con- 
ditions to whose influence the protoplasm may respond by a 
manifestation of energy greater than that involved' in the 
stimulus. This quality is called irritability. It further seems 
that changes may originate within the protoplasm itself, 
though this is much more difficult to demonstrate and may 
merely represent our ignorance of the processes occurring in 
the protoplasm. This power is called automatism. These are 
the most fundamental qualities belonging to protoplasm, and 
serve to make possible those which follow: viz., motion, assimi- 


lation, growth, etc. Protoplasm varies in the degree of irri- 
tability. In general it responds to stimuli most normally under 
those conditions which are most favorable to the ordinary vital 

21. Stimuli. All the disturbing forces or conditions, ex- 
ternal or internal, which tend to cause response in living pro- 
toplasm, are called stimuli. The principal stimuli are, 
chemically active substances, moisture, contacts, heat, light, 
electricity, and gravity. Inasmuch as irritability lies at 
the foundation of the various protoplasmic activities men- 
tioned below, all the natural causes which modify irritability, 
also modify, through it, the vital processes, such as motion, 
growth, etc. 

Light affects protoplasm profoundly. The direction of motion in protoplasm 
is largely determined by light. Light may either attract or repel protoplasm. 
Excess of light retards growth. Heat strongly modifies the rate of all the vital 
processes. There is an optimum temperature at which the protoplasm best 
performs its work. An excessive increase or decrease of this temperature produces 
a cessation of activity, a condition of rigor, and death. The fatal maximum 
temperature for ordinary animal protoplasm may be said to be about 45 or 50 C. ; 
the minimum, o, or below. ("Chemical agents may stimulate protoplasm in such a 
way as to attract or repel organisms. Paramecia, which are single-celled animals, 
may be seen to gather about an air-bubble, or at the margin of the cover-glass. 
They will retreat before an encroaching solution of certain salts. 

It is a most significant fact in this connection that protoplasm may become, so 
to speak, accustomed to a stimulus which has been long continued, so that it ceases 
to respond in the way it did when the stimulus was novel. Protoplasm may 
gradually be brought, for example, to endure and thrive at a temperature which 
would have produced death if suddenly applied. It is almost impossible to over- 
state the importance of this faculty in enabling organisms to survive changing 
conditions. Stimuli, then, may be said to be powerful in proportion to their 
suddenness and intensity. 

22. Assimilation. The process of changing food substances 
into protoplasm is called assimilation. It can be effected 
only by protoplasm. Such foods may be relatively simple 
substances or may be the complex protoplasm of other 
organisms. The protoplasm of the green leaves of plants has 
the power of utilizing the simple inorganic compounds, as 
oxygen, water, and carbon dioxid, in a larger measure than 
that of animals. Plants may build these up into foods, whereas 
animals must have the complex organised foods to start with. 


23. Growth and Reproduction. The result of assimila- 
tion is the addition of new molecules of complex organic 
matter among the molecules of the old. This produces growth. 
It is to be defined as increase in mass. If this continues in- 
definitely in excess of whatever may tend to destroy the pro- 
toplasm, the increase in size may lead to the division of the 

FIG. i. 







\ \ 


FIG. i. A diagram indicating the general cycle of events in the building up of inorganic sub- 
stances into living matter. Sunlight enables the chlorophyll of plants to build up water and carbon- 
dioxide (photosynthesis) into carbohydrates. Plants may use these together with nitrogenous com- 
pounds to form plant protoplasm and other proteids. Animals may by assimilation utilize these for 
growth, reproduction, and in doing work of various kinds. Work results in waste. Plants may 
again use these wastes. 

Questions on the figure. What are the three main courses open to plant sub- 
stances? In what sense are animals dependent on plants? For what? How 
important is photosynthesis in life? What is the real source of the energy of 
organisms? In what sense is the term cycle appropriate here? 

protoplasm. The parts may separate and lead an independent 
existence. Such is reproduction. In its simplest form it is 
merely growth beyond the limits of the individual. The cell 
cannot continue to grow indefinitely. Its size is limited by the 
necessity of physical support on the part of the soft protoplasm, 
and by the relation between the outer surface, through which 


the food must be taken, and the volume, which represents the 
mass to be fed. The surface increases as the square of the 
diameter, whereas the volume increases as the cube of the 
diameter. It is apparent that the nourishing surface does 
not increase as rapidly as the mass to be nourished, and in con- 
sequence the time will come when the nourishment possible to 
be absorbed will just nourish the volume, and growth must 
cease. This condition may constitute an internal stimulus to 
division. At any rate division furnishes a way out of the 
dilemma and allows a renewal of growth of the daughter units. 


FIG. 2. Streaming of Protoplasm in the .Amoeba. The forward motion of the granules takes 
place more rapidly in the centre of the pseudopodium (p) . Those at the margin fall behind those 
in the centre as the pseudopodium advances. 

Questions on the figure. Why may the amoeba readily change its form? Do 
its internal parts preserve a constant relation to each other? 

24. Contractility. A body of living protoplasm seems 
always to possess the ability to change its form in greater or 
less degree. This results in motion of parts or of the whole, 
and is called contractility. Movement or contractility is closely 
related to irritability, and results from the action of stimuli, 
external and internal, upon the complex protoplasm. It is 
made possible by the assimilation of food substances. These, 
in being broken down, furnish the energy shown in motion. 
The nature of the motion resulting from contraction differs 
somewhat, depending upon whether the protoplasm is en- 
veloped by a cell-wall or is naked. If without a wall, it may 
send out foot-like projections into which there passes a stream 
of granules, as in the Amoeba (see Fig. 2); if enclosed, the 



protoplasmic mass may rotate within the cell wall, or there 
may be narrow channels in which the currents move between 
banks of more stationary material. The latter motion is de- 
scribed as circulation. (Fig. 3.) 


FIG. 3. The circulation of protoplasm (p) in a cell of a stamen-hair of Tradescanlia. In the 
channels the granules move back and forth to the various parts of the cell. The remainder of the 
cell is filled with cell-sap ($) which in these cells is colored. 

Questions on the figure. In what respects are the activities of the protoplasm 
necessarily limited in this cell as compared with the condition in Amoeba;? Why is 
circulation an appropriate term? 

25. Demonstrations. The teacher should, if possible, demonstrate proto- 
plasmic motion to the students with a compound microscope of good magnification. 
The Amoeba will serve to illustrate the naked streaming motion; Paramecium, 
rotation; the hairs from the stamens of Tradescantia beautifully illustrate circula- 
tion. (There is a cultivated species which may be kept blooming in greenhouses at 
all seasons of the year.) Ciliary motion may be shown in several of the large 
Protozoa, or by living cells scraped from the esophagus of the frog. 

26. Dissimilation. Motion and the other responses which 
protoplasm makes to stimuli necessarily represent chemical or 
physical changes, or both, in the protoplasm. It is well known 
that complex chemical substances, such as are found in proto- 


plasm, can be made to yield energy when they are torn down 
into simpler ones by some element which has an affinity for 
some of the elements constituting the substance. The result 
of this action is, always, simpler and more stable compounds 
than the original, and therefore of less use in the further 
freeing of energy. This tearing-down process is the opposite 
of assimilation and is sometimes called dissimilation or katab- 
olism. Oxygen is one of the most important agents in nature 
for the freeing of energy by breaking down the complex chemical 
substances. It unites with the carbon and hydrogen particu- 
larly, and these unions are among the principal sources of 
energy which animals show. The process is called oxidation 
and is essentially the same thing that occurs when wood or coal 
is burned. The energy belonging to the wood by virtue of its 
chemical constitution is partly freed by the action of the oxygen 
in uniting with the carbon and hydrogen, reducing the wood to 
ashes, water, and carbon dioxid. In the stove the principal 
form of energy secured is heat; but in appropriate engines, 
locomotion and other forms of mechanical work, or light, or 
electrical energy may be secured by the oxidation. So in 
protoplasm, various types of energy may result from the tear- 
ing down of the complex substances. Among these are animal 
heat, motion, nervous energy and electrical energy. The living 
body with its protoplasm is really a mechanism, an engine. 

27. Secretion and Excretion. As a result of the constructive and destructive 
work already mentioned as characteristic of protoplasm certain substances, not 
themselves protoplasm, may be produced. If these products are of further use in 
the animal economy, they are usually described as secretions; if they represent the 
final reduction in the process of tearing down, they are called excretions. Such 
materials may be deposited either within the protoplasm or at its surface. In the 
latter case it may be deposited in a uniform sheet and produce a protective mem- 
brane (cell wall). The presence of such a covering to the protoplasm very materi- 
ally modifies all the elementary activities which have been described. 

28. Demonstrations. The teacher should make microscopic demonstrations of 
secretions and excretions: as starch grains formed in the leaves of plants; fat 
in adipose tissue; cell- walls in plants; crystals in plant cells (see Botanies); inter- 
cellular substance in cartilage or bone. 

29. Supplementary Topics for Library Work. Find and examine some of the 
classic definitions of life. Examine more completely the theories of protoplasmic 
architecture. In what ways would the presence of the cell-wall bring about 
modifications of the protoplasmic activities? Give an account of experiments 



showing the effect of some of the more important stimuli on protoplasm (as light, 
heat, electricity). What of the external conditions are so important as to merit 
the term "primary conditions of life"? Why may protoplasm be described as 
chemically unstable? Compare oxidation in the protoplasm with oxidation in 
ordinary combustion. 

30. Summary. i. Scientists are not agreed whether life 
is merely the action of the ordinary chemical and physical 
forces in connection with a peculiar substance, or represents 
these, guided by a type of energy of a higher order. 

2. Protoplasm, a chemical mixture of exceeding complexity 
and instability, is the "physical basis of life." Differences 
in various living things are probably due to differences in the 
chemical and physical structure of the protoplasm of which 
they are composed. 

3. Owing to the unstable character of the protoplasm it is 
readily acted upon and changed by external forces; and the 
various parts of the protoplasm act on each other in such a 
way as to produce a display of energy. The agents are called 
stimuli. Protoplasm responds to stimuli because of its irri- 
tability and contractility. These latter powers belong natively 
to protoplasm because of its physical and chemical composition. 

4. Protoplasmic matter and the materials which are de- 
stroyed in the production of energy are alike produced by the 
assimilation of food substances into new protoplasm. This 
is a most fundamental quality. 

5. Growth is increase of mass, following the formation of 
new substance by assimilation. The mere absorption of water 
also results in growth. Growth leads naturally to reproduction. 

6. Oxygen is one of the chief agents by which the unstable 
compounds in the protoplasm are made to release their energy. 
The breaking down of these compounds leaves unused mate- 
rials which must be excreted. Respiration, which is a term 
applied to the using of oxygen and the elimination of carbon 
dioxid, and excretion are thus seen to be protoplasmic func- 
tions immediately connected with its activity. 



31. Introduction. In studying the structure of organisms 
two methods are open to the student of to-day. He may 
begin with -the whole adult individual and by dissection he may 
reach a knowledge of the constituent parts, organs, tissues, 
cells. This, the analytic method, is the method of history and 
has given us the mass of details which we have at- present. 
On the other hand, it is possible to avail one's self of the 
results of such studies, to assume the unit of structure which 
is uniformly found, and, by a synthetic process, follow the 
building up of an organism from its elementary parts. This 
is the process which the development of the individual illus- 
trates. It has the special advantage of emphasizing the funda- 
mental unity of origin of the organs, and the likenesses of 
organisms, and gives the true significance of differentiation 
and development 

32. The Cell. Having discussed in Chapter IT the sub- 
stance in connection with which life manifests itself, it is 
necessary to recall the fact that the protoplasm of an organism, 
while connected in various ways, is separated by boundaries 
into unit-masses, each mass having the essential qualities of 
the whole. Each unit mass of protoplasm is called a cell. The 
cell is not to be considered as the ultimate unit of structure; 
it is itself, as we shall see, a group of bodies which are in turn 
composite. It is thus to be looked upon as an organized 

33. Cell Form. Cells, unhampered in the direction of 
growth, tend to assume a spherical form. Agencies, both in- 
ternal and external, as nutritive processes, tension, pressure, 
etc., may modify this in such a way that almost any form 
may be found: polygonal, flattened, elongated, fibrous, 
branched, etc. 




34. Size. While ordinary tissue cells are minute, there is 
great variation in the size of cells. Many single-celled in- 
dividuals are visible to the naked eye and egg-cells may be 
several centimetres in diameter; yet many tissue cells are less 
than .005 millimetre in diameter. Cells may be very much 

FIG. 4. 

FIG. 5. 




= a 

FIG. 4. Diagram showing the principal parts of the cell and something of the protoplasmic 
architecture as it might appear while living. , alveoli or spheres in the foam-work (see 18); c, 
centrosome; cy, cytoplasmic mesh work, containing granules; nu., nucleus; n, nucleolus; v, vacuole; 
w, cell wall. 

FIG. 5- Diagram showing principal parts of the cell as it appears when killed and stained. The 
protoplasm shows more of a mesh work (cy), the spaces representing the alveoli. /, formed sub- 
stance in alveoli. Other letters as in Fig. 4. 

Questions on figures 4 and 5. If these cells are in reality 25 /A in diameter, 
how much are they enlarged in the drawing? (jj. is .001 mm.). Identify the various 
structures referred to in section 35. 

extended in one or more directions without possessing great 
bulk. The outgrowths of nerve cells for example may attain a 
length of severaj, feet, as when the nerve fibres extend from the 
trunk to the tips of the toes. 

35. Structure. The. following parts are to be distinguished 
in the typical cell: (i) a general cell substance, partly living 
protoplasm, partly non-active matter both organic and in- 
organic; (2) usually a single highly differentiated nucleus 
which contains living protoplasm and is clearly demarcated 
from the protoplasm about it; (3) one or more specialized 
bodies known as centrosomes; (4) a cell wall or membrane 
(Figs. 4 and 5). 

The cell-substance or cytoplasm embraces that portion of 
the protoplasm outside the nucleus. This is apparently made 


up of a more dense portion which coagulates readily with certain 
reagents, and the more fluid cell sap which is composed of water 
with sugar, inorganic matter, and other substances dissolved 
in it. Suspended in the protoplasm and making a part of it 
may be starch grains, fats, crystals, and certain active bodies 
known as plastids, mitochondria, etc. 

36. The Nucleus. The usually single nucleus lies im- 
bedded in the cytoplasm and is ordinarily separated from it 
by a thin membrane. Nuclei vary greatly in shape, size, and 
degree of differentiation. While it is not always possible to 
find definite nuclei in all cells, it seems probable that all cells 
have nuclear material in one form or another at some stage 
of their history. The internal structure of the nucleus is 
equally as complex as that of the cytoplasm, having both living 
and non-living portions. It usually consists of a network 
of threads (chromatin) readily stained by certain dyes. In 
the meshes of this a less easily, stainable material occurs 
(achromatin) , a portion at least of which is active. One or 
more deeply stainable bodies, called nucleoli, usually occur, 
the real character of which is difficult to estimate. 

37. Centrosomes or Centrospheres. These bodies lie in 
the cytoplasm but are closely related to the nucleus, and ap- 
pear to have an important place in certain phases of cell activity 
(see "cell division," 41). The centrosphere is a clear space 
about the centrosomes. 

At such times the cytoplasmic elements radiate from the 
centrosomes in a very characteristic way (Fig. 8, c). The 
influence extends even into the nucleus and is accompanied by 
a rearrangement of the chromatic elements. The origin of the 
centrosomes is still a matter of disagreement. The centre- 
spheres are often spoken of as attraction spheres from the fact 
that they appear to exert an attractive influence upon certain 
portions of the protoplasm. 

38. Cell Wall. A cell membrane usually surrounds the 
protoplasm. It may be a non-living organic secretion, or may 
consist of metamorphosed or altered protoplasm in connection 
with such secretion. The wall is protective and supportive 



in function, and varies much in thickness, resistance, etc. 
Animal cells as a rule are not provided with such well de- 
veloped and resistant walls as are plant cells. 

39. Cell Functions. Since the cell is only a definite unit 
mass of protoplasm, its functions are in general those which 
have already been described as protoplasmic functions. They 

FIG. 6. 



PIG. 6. Modes of cell reproduction. A, B, and C, stages in the reproduction of the Infusorian, 
Colpoda, by the breaking up of the protoplasm to form numerous cells. A. encysted stage; B 
protoplasm escaping, spores partly formed; C, spores completely separated (adapted from Rhum 
bier); D, budding in Chlamydomyxa, a lowly Rhizopod. The bud is finally constricted off from 
the mother cell, b, bud; cie. t cell wall; m, mother cell; n, nuclear matter; s, spores. 

Questions on the figure. Compare the process and the results of the two modes 
of cell reproduction shown in this figure. Can you describe the fate of the 
"mother" cell in the two cases? 

are merely localized within the cell. The cell wall when present 
would naturally modify and limit in important ways, the more 
active protoplasmic functions, especially motion. In such cases 
the independent motion characteristic of so many cells must be 
accomplished by special devices. These frequently take the 
form of cilia or flagella, which are thin protoplasmic projec- 
tions penetrating the cell wall and used after the manner of 
oars. Locomotion of cells is not confined to single-celled or- 
ganisms, but is found in many cells of the higher animals and 
plants as colorless blood cells, sexual cells, etc., which have 


a distinct motion of their own. The muscle cells of higher 
animals possess the power of contraction and motion in a high 

40. Reproduction. The cell grows as a result of the nu- 
tritive processes and reaches the limits of size determined by 
its special conditions. The internal and external conditions 
together constitute a stimulus to the breaking up or division of 
the protoplasmic unit. This may occur (i) by the irregular 
breaking up of the protoplasm into numerous masses, each 
of which has the essential qualities of the whole (Fig. 6, A, and 
); (2) by budding, in which a process, or several processes, 
appears on the cell, develops into bodies like the original cell, and 
finally becomes separate from it (Fig. 6, D}\ (3) by division, 
in which there is a division of the original protoplasm into 
two essentially equal parts. In the last case neither of the 
cells can be considered the parent of the other. 

41. Cell Division. Cell division may be effected in either 
of two ways, (a) by direct or amitotic division, in which the 

FIG. 7. Direct cell division (Amce&a). A, active specimen with pseudopodia; B, becoming 
spherical preliminary to division; C, beginning of elongation and constriction; D, later stage; JB, 
daughter cells forming pseudopodia. ec. clear ectoplasm; en, granular endoplasm;/, food vacuole; 
, nucleus; ps t pseudopodium; v. pulsating vacuole. 

Questions on the figure. Why is this properly called direct division? What 
structures are divided? Are the resulting halves exactly or merely roughly equal, 
apparently? Do you see any possible gain to the organism in such a division as 


nucleus and cell merely constrict into two nearly equal parts 
(Fig. 7); and (b) indirect or mitotic division. The latter is 
the usual method and is very complicated. By means of it a 
very even division of the substances and structures of the 
nucleus, especially, seems to be secured. 

The more striking stages in the process as it usually occurs 
are outlined in the. text and figures which follow. The centro- 
somes and nucleus will be seen to be especially active. Such 
a description is only typical of the wonderful series of events. 
There are numerous variations from this in different organisms. 

1. In the quiescent or resting stage the structural elements 
are distributed in the way characteristic of the particular cell 
under examination (Fig. 8, A) . Most cells are in this stage when 

2. When division is about to take place, the chromatin 
elements in the network of the nucleus assume the appearance 
of a coil or tangle of thread (Fig. 8, B). The nuclear membrane 
often breaks up at this time. 

3. The centrosome divides and the halves migrate to op- 
posite poles of the nucleus, and from them as centres radiations 
pass into the cell body in all directions. Across the nucleus, 
from one centrosphere to the other, thread-like lines extend, 
producing the appearance of a spindle (Fig. 8, C, sp). In 
the meantime the coil of chromatin has been unraveled and has 
broken up into a definite number of pieces (chromosomes) 
which often form into V-shaped loops. After certain evolutions, 
under the influence of the centrospheres apparently, these loops 
come to lie in the equatorial plane of the spindle, the apices 
of the loops pointing toward the centre of the nucleus. This 
is called the astroid stage (Fig. 8, C). The process up to this 
point is known as the pro phase or preparation stages. 

4. Each of the chromatin loops next splits longitudinally 
into two. This is the metaphase or middle stage (Fig. 8, D). 

5. Each of these halves now begins to move toward its 
appropriate pole or centrosome (Fig. 8, E). As these half -loops 
leave the equator and collect about the poles they give rise to 
a double-star appearance or diastroid stage (Fig. 8, F). This 
is the anaphase. 


6, The loops of chromatin collected at each pole are re- 
constructed into a coil which then passes gradually into the 
resting stage at the new position, a membrane is formed, and 
the daughter nucleus is complete. The nuclear spindle dis- 

FIG. 8. 

FIG. 8. Indirect or mitotic division (diagrammatic); A, resting mother nucleus; B, coil stage, 
with the centrosomes separating; C, D (metaphase), and E, stages in the division of the chromo- 
somes (the cell wall is not shown in these three drawings); F, diastroid (anaphase) stage; G and H 
show the return of the daughter nuclei to the coil and to the resting condition, and division of the 
cytoplasm, and the formation of the dividing wall: c, centrospheres; cl, chromatin coil; chr, chro- 
mosomes; nu., nucleus; , nucleolus; sp, nuclear spindle; w, cell wall. 

Questions on the figure. What structures possessed by the original cell are 
divided in this process ? In what order ? Why is this termed ' ' indirect " division ? 
Which is the more common, the direct or the indirect? Can you see any special 
gain secured by this method? Describe the behavior of the nucleolus and the 
nuclear membrane by comparing this with other figures in reference books. 


appears, the radial appearance about the centrosomes, and 
even the centrosome itself, may disappear or become incon- 

7. Accompanying or following the last nuclear changes the 
cytoplasm may have become constricted into two masses, or 
separated by the formation of a wall perpendicular to the axis 
of the spindle (Fig. 8, G, H). The daughter cells may separate 
or remain united. These final stages are known as the telo- 
phase. There are other protoplasmic bodies in the cytoplasm 
which seem to divide, though not so accurately as the nucleus, 
into two sub-equal portions. These are variously named 
plastids, mitochondria, etc. The important point is that 
there seems to be a definite tendency to get equal division of the 
cell materials. 

Cell division is at the beginning of all the complexities 
of structure found in the higher forms of animals. Each sexually 
produced organism commences life as a single cell, from which 
the adult is formed by cell-division, and the clinging together 
of the daughter cells. 

42. Functions of the Nucleus and Centrosomes. While we 
can follow some of the externals of the various cell activities, 
the manner of their occurrence and their causes are in the 
greatest obscurity. We are not able to say just what part is 
performed by the different structures involved. It is hazardous 
to say that one structure is more important than another; yet 
it seems to be proven that the nucleus is quite essential in 
cells which possess nuclei, for the proper performance of even 
the ordinary nutritive functions. Some of the unicellular 
animals may be artificially mutilated in such a way that the 
lost parts may be regenerated and the normal form restored. 
A relatively small piece of the Protozoan, Stentor, for example, 
can reproduce the whole, if a portion of the nucleus be present. 
A much larger piece without nuclear material is wholly unable 
to regenerate lost parts, and even seems unable to control or 
exercise the ordinary assimilative functions. The phenomena 
of indirect cell division show that activity on the part of the 
centrosomes and nucleus precedes that of the cytoplasm. 


Experiments also show that the division of the cytoplasm may 
be checked or interrupted by external influences without 
interfering with the division of the nucleus. On the other hand 
nuclei separated from cytoplasm are incapable of continuing 
their functions. We are at least safe in saying that these three 
bodies, the centrosome, the nucleus, and the cytoplasm act as 
intracellular stimuli upon each other, and that all are important 
in the work of the cell. During nuclear division there is prob- 
ably increased interchange between the cytoplasm and nucleo- 
plasm. The breaking down of the nuclear membrane facili- 
tates this. 

43. Exercises for Library and Laboratory. The teacher should by all means 
secure preparations of properly stained cells showing the principal structures; also 
some of the stages of cell division (see Appendix: Laboratory Suggestions). 

What are chromosomes? In what respects and to what extent do nuclei differ? 
What is meant by the "cell-doctrine " ? Give an outline of its history. What are 
mitochondria? Compare the various series of figures in your library illustrating 
the stages of cell division. 

44. Summary. 

1. The cell may be considered as the unit of structure, and 
is to be defined as a "nucleated mass of protoplasm with or 
without a cell membrane." 

2. The cell may also be considered the unit of function, in 
the sense that it embodies all vital functions in epitome. 

3. The structure of the typical cell may be outlined as 
follows : 

(a) Cell body 

Cytoplasm living. 
Cytolymph non-living, fluid. 
Metaplasm non-living, solid. 

(6) Nucleus: 

Nucleoplasm living. 

Nucleolymph non-living, fluid. 
Metaplasm non-living, solid. 
[Protoplasm = Cytoplasm + nucleoplasm.] 


(c) Centrosome. 

(d) Cell wall. 

4. In addition to the general functions of protoplasm which 
cells possess we need to consider in connection with cells the 
additional functions : 

(a) Locomotion. 
(6) Reproduction. 

5. Reproduction of cells occurs by fragmentation, by 
budding, and by division. Division may be either direct or 

6. The following diagram, adapted from Flemming will 
serve to represent the stages in indirect division: 

One mother nucleus. Two daughter nuclei 

(a) Resting stage. Resting st 

(&) Coil stage. 1 =prophase Coil stage (f. 

) Astroid stage. ) Astroid stage (e. 

(d) Division of chromatin loops = metaphase (d. / 

7. The important effect of this complicated process is, ap- 
parently, to secure an equal division of the nuclear elements 
for the daughter cells. The cytoplasmic elements in the 
daughter cells may be strikingly unequal. 

8. The exact relations of the various structures in the cell 
are not known. They cannot be understood until the chemical 
and physical nature of living protoplasm is known. The 
cytoplasm, the centrosomes, and the nucleus seem to act as 
stimuli to one another, in assimilation, growth, and division. 



45. The Individual as a Cell -composite. In the simplest 
animals, as the Protozoa, the individual consists of a single 
cell, and the life history of the individual animal is such as 
has already been seen to belong to the cell (Chapter III). In 
such an individual one cannot speak of organs in the ordinary 
sense, for organs as we shall see are made up of cells bound 
together in the doing of certain work. Yet it is important to 
remember that there are none of the necessary duties of life, 
such as getting food, digesting it, breathing, moving, repro- 
ducing, and the like, which are not well done by these simple 
one-celled animals. The many-celled animals agree with the 
simpler ones in that they too start life as single cells appar- 
ently quite as simple as the one-celled animals themselves. 
When the cells divide, however, the daughter cells do not 
separate as in the Protozoa, but form a mass of cells by cling- 
ing together. Owing both to internal and external forces 
the cells in the mass do not long remain alike, but soon show 
such differences among themselves as serve as the basis for 
the great variety of structures found in the bodies of the higher 
animals. The change from the simple cell to the complex 
condition in the adult animals is not a sudden one, but takes 
place very gradually and the work which was formerly done 
by the single cell is divided up among the groups of different 
cells composing the body The division of the work to be 
done makes possible and necessary the specializing of certain 
cells to do each part of it, and the differentiation of structures 
makes it possible to do each separate task better than before. 
Thus division of labor and differentiation of parts go hand in 
hand as we pass from the simple to the complex animals. 



46. The Fertilized Ovum the Starting Point In speak- 
ing of the development of the adult animal from the simpler 
condition of the single cell it is necessary to remember that 
this cell, which has the power of giving rise to a complex in- 
dividual and is called a fertilized ovum, has a history that is 
very important. The fertilized ovum represents the union of 
two distinct cells, known as germ or sexual cells, which are 
ordinarily quite different in appearance and produced by dif- 
ferent kinds of individuals, males and females.- Both classes 
of cells may be produced by the same individual. This union 
does not produce a double cell, but the parts of each seem to 
fuse with those of the other in a very complete way. 

47. The Ovum. The germ cell produced by the female is 
known as the ovum, and is typically a spherical cell with 

FlG. Q. Types of ova. A, primitive amoeboid ovum of Sponge; B, semi-diagrammatic figure 
of spherical ovum of Sea-urchin in which the yolk is uniformly distributed; C, figure of a spherical 
ovum (such as may be found in some Worms or in the Frog) in which the yolk tends to collect at 
one pole, p.p., and the nucleus and protoplasm at the other, a.p.; m, micropyle; nu, germinal vesicle 
(nucleus); , germinal spot (nucteolus); y, yolk spheres. 

Questions on the figure. What are the points of agreement in these three ova? 
The chief points of contrast? What is the function of the micropyle? Is a 
micropyle always present in ova? Why are the poles of the ovum appropriately 
called active and passive ? 

abundant nourishment and inactive as compared with the 
male cell. It often has an especially well-developed cell- 
covering. Its nucleus is sometimes called the germinal vesicle 
(Fig. 9). The ovum must be distinguished from what is 
popularly known as an egg. The latter term is loosely used 
to describe the fertilized ovum more or less developed, to- 
gether with its nutritive and protective coats such as occur 


around the eggs of birds and reptiles. Ova differ very greatly 
in size. The largest are found among the birds. The ' ' yellow ' ' 
of these eggs represents the real size of the ovum. Variations 
in size are due not so much to a difference in the amount of 
protoplasm as to a varying amount of food or yolk in the 
cell. The food may be uniformly distributed throughout 
the ovum, mingled with the protoplasm, or it may collect at 
one pole, forcing the active protoplasm to occupy the other 
pole (Fig. 9, C). The yolk furnishes food to the young indi- 
vidual or embryo in its early development, that is, during cell- 
division, before it can get food for itself. 

48. The spermatozoon or male element is ordinarily in 
striking contrast to the female. It is typically very small* 


PIG. 10. Types of spermatozoa. A, from the round worm (Ascaris) with a cap, somewhat 
amceboid; B, from the Crayfish, with numerous projections; C, from Frog; D, from Sea-urchin.. 
h, head; m, middle piece; n, nucleus; t, tail or flagellum. The head is chiefly nucleus. . 

Questions on the figure. What are the chief points of similarity and dissimi* 
larity in these spermatozoa? How do they agree with, and how differ from, the 
ova in Fig. 9? How do they differ from the average cell? What parts of the: 
structure of typical cells are believed to be represented in the sperm cells? 

active, and with thin protoplasmic projections (Fig. 10). Struc- 
turally, the typical spermatozoon consists of a "head" piece,, 
a middle piece, and a ' ' tail ' ' or flagellum. The head is composed 
chiefly of the chromatic material of the nucleus. A delicate- 
covering of cytoplasm envelops the head and is drawn out into* 


the projection known as the tail (Fig. 10, D). The middle piece 
contains the centrosome of the male cell. 

49. Early History of Ova and Sperm. We formerly 
thought of ova and sperm as actually formed by the body of the 
parent. We now know that these germ cells are set apart early 
in the history of each individual, before the body is developed. 
They of all the cells retain their primitive undifferentiated 
nature. The body cells become different in appearance and 
powers, as nerve cells, muscle cells, bone cells, and the like. 
The body does not produce the germ cells ; it merely houses and 
nourishes them during their development. The body cells and 
the germ cells develop side by side, each influencing the other in 
their development. 

After the first putting aside of the primordial germ cells in 
the early life of the individual, the body cells have a period of 
rapid division and growth. During this time the germ cells are 
relatively quiet. As the parent animal becomes mature the pri- 
mordial germ cells enter upon a period of activity by which they 
produce such ova and sperm as are described in the preceding 
sections. This period of activity of the germ cells shows three 
stages: (i) a period of increase of the primordial germ cells 
(oogonia and spermatogonid) by division; (2) a period in which 
the last descendents of these divisions enlarge (becoming pri- 
mary oocytes and spermatocytes) ; and (3) a period of maturing 
or perfecting these (into ova and spermatozoa) . These stages are 
suggested in Fig. n. 

50. Maturation of Ova and Sperm. This third stage, the 
maturation or ripening of the germ cells, shows us some of the 
most remarkable happenings to be found in all biology. Both 
ova and sperm when ripe contain, as a rule, just one-half as 
many chromosomes in their nuclei as are found in the primordial 
germ cells, or in the body cells, of the species to which they be- 
long. Somewhere in the history of the egg and sperm there is 
therefore a reduction division of chromatic material in the nu- 
cleus. It does not always take place in just the same way, but 
the following will illustrate the process. The student should 



FlG. II. 



%? j' UNIVA 

/' l\ SPERMAT 














FIG. ii. A diagram of the germ cell cycle, suggesting the changes by which the primordial 
germ cells in the body give rise to the mature eggs and sperm; fertilization; cleavage; and the segre- 
gation of new primordial germ cells as the new body is developed. (See 40-52, 60.) The full 
number of chromosomes in the cells of the body is 2x. The half number found at certain stages is 
indicated by x. The large gastrula at the bottom of the figure is to illustrate the early differentia- 
tion of germ and body cells. 

Questions on the figure. Trace the changes of the chromosomes through the 
whole series of stages. The solid black is intended to emphasize the potential 
germ cells. Why are all the cells of the two and four-celled stages represented in 
black? Why are the chromosomes in the primary oocyte and spermatocyte said 
to be bivalent? What is meant by univalent? What does "continuity of germ- 
plasm" mean? 


FIG. 12. 












FIG. 12. A diagram to show the central place of the fertilized ovum, and the important stages 
leading up to it together with those following it. Compare with Pig. n. The male chromosome 
are figured black and the female light, throughout. Compare descriptive matter in text. 

Questions on the figure. In what respects is the maturation of the ovum and 
of the sperm similar? In what, different? What is the probable meaning of the 
pairing of the chromosomes (synopsis) in the early maturation stages. How does 
this first division of chromosomes differ from the ordinary divisions. Why may 
fhe polar bodies be called abortive eggs? What is accomplished by the union of 
the sperm and ovum nuclei? In cleavage the chromosomes from both parents 
seem to be distributed equally to all the cells of the body. What is the significance 
to this? 


follow closely the figures n and 12, in order to get the steps 

When the full-grown primary oocyte and spermatocyte are 
ready to divide at the beginning of the third stage, the chromo- 
somes in a given nucleus unite and fuse in pairs. Thus when 
division begins there is only one-half the usual number, but each 
chromosome is of double value (bivalent). As the primary cells 
divide into the secondary oocytes and spermatocytes, the bival- 
ent chromosomes behave as single chromosomes in ordinary 
nuclear division and split, and one-half of each bivalent chro- 
mosome passes to each daughter nucleus. In this way each 
secondary oocyte or spermatocyte contains one-half the 
usual number of single (or univalenf) chromosomes. By this 
device of first uniting and then separating whole chromosomes 
instead of splitting the single chromosomes into halves, as 
occurs in ordinary nuclear divisions, an actual reduction of the 
number of chromosomes is secured. Furthermore it may be, 
while these chromosomes are fused, there is an actual interchange 
and modification of the substance of each. 

The secondary cells divide once more in forming the mature 
eggs and sperm. In this last division the chromosomes simply 
split as they do in ordinary indirect nuclear divisions. 

In all these steps in the division of nuclear material the his- 
tory of ova and sperm is essentially the same. There are, 
however, very striking differences in the way the cytoplasm, and 
the cell as a whole, behaves. In the history of the sperm cells 
the protoplasm divides equally in both divisions, and thus each 
primary spermatocyte produces four small spermatids. Each 
spermatid gradually changes, without further division, into a 

In the divisions of the primary oocyte, on the contrary, one 
of the daughter nuclei enters a kind of bud in the wall of the cell 
and carries very little cytoplasm with it. It is clearly an abor- 
tive cell, and is called a polar body. The second nuclear division 
results in another abortive egg-cell or polar body. The first 
polar body sometimes divides into two. All the polar bodies 
finally disintegrate. Thus, since one of the nuclei has almost all 
the cytoplasm and the others almost none at all, there is only one 


mature well-nourished egg as the result of the two divisions. In 
the formation of the sperm there are usually four perfect cells 
arising from each spermatocyte. 

51. Fertilization. The union of a sperm cell with an ovum 
constitutes the act of fertilization. Often there is a special 
aperture (micro pyle) in the outer egg -membrane through which 
the spermatozoon finds entrance. Usually only one sperm cell 
gains admission to the interior of the ovum, whether by way of 
the micropyle or through the unmodified membrane. Changes 
normally occur in the membrane as soon as one sperm enters, by 
which all others are excluded. In eggs which have been kept too 
long or subjected to unfavorable conditions, the response of the 
membrane may not be so quickly effected and multiple fertiliza- 
tion may occur. Multiple fertilization occurs normally in some 
species. Such fertilizations may produce monstrosities. The 
sperm cell may enter the egg even before the polar bodies are 
formed; or it may enter after maturation is completed. It 
brings into the egg the nucleus, the centrosome and a very small 
amount of cytoplasm. It at once organizes itself as a second 
nucleus of the egg and is nourished by its substance. The sperm 
nucleus and the egg nucleus, each carrying one-half the full num- 
ber of chromosomes of the species, now draw together and organize 
into a new nucleus. Thus is formed the first segmentation nucleus, 
and the egg is fertilized. With the addition of the chromosomes 
in the male nucleus the fertilized ovum contains the same 
number of chromosomes as before maturation, which in each 
species of animals is a constant number. It appears that 
fertilization restores to the female cell essentially what it lost m in 
the process of maturation, and in addition stimulates it to active 
nuclear and cytoplasmic division as indicated in the next 

52. Segmentation or Cleavage. Following shortly upon 
fertilization, if conditions are favorable, ordinary mitotic nuclear 
division begins and the ovum divides promptly into 2, 4, 8, 16, 
etc., cells (blastomeres). In these divisions the chromosomes 
split, and one-half of each chromosome brought in by the sperm 
nucleus and one-half of each furnished by the egg-nucleus go to 


each daughter nucleus. This is continued in later divisions, and 
in this way every cell of the body gets its chromatic material 
equally from the father and the mother. The resulting cells be- 
come smaller and smaller with each division, since the whole egg- 
mass does not increase appreciably in size meanwhile. 

The first three cleavage planes are usually perpendicular to 
each other. Their position is much modified, however, by 
the presence of food or yolk substance in the egg. The yolk in 
general retards cleavage. If the yolk is in small quantity and is 
uniformly distributed through the egg, the blastomeres will be 
about equal in size (Fig. 13, A), and will continue to divide with 
practically equal promptness. If there is much of the yolk 
it is not likely to be uniformly distributed. Under the influence 
of gravity and internal forces, the yolk is likely to collect at the 
lower, and the protoplasm and nucleus at the upper, pole of the 
ovum (Fig. 13, B, C). The protoplasmic pole is known as the 
active or formative pole, and the lower as the passive or nutritive 
pole. The polar bodies are normally freed at the formative pole. 
Under these circumstances the blastomeres at the nutritive pole 
are larger and divide less rapidly than those in which the proto- 
plasm is in excess. If the yolk is excessive in amount that por- 
tion of the ovum in which it collects may be totally prohibited 
from dividing as will be seen in Fig. 13, C.D. 

53. Forms of Segmentation. The conditions suggested above give rise to the 
following classes of segmentation. 

A. Total segmentation. 

I. Equal: in which there is little yolk material, and that is well distributed. 
(Illustrated in most of the lower invertebrates and mammals.) Fig. 


II. Unequal: in which there is a moderate amount of yolk which accumulates 
at the passive pole. The cells at the active pole are more numerous and 
smaller than at the passive. (Illustrated in many mollusks and in the 
Amphibia.) Fig. 13, B. 

B. Partial segmentation. 

I. Discoidal: in which there is an excessive amount of yolk, with the 
nucleus and a small mass of protoplasm occupying a disc at the active 
pole. This disc alone segments, and the embryo lies upon the yolk. 
(Illustrated in the eggs of fishes, birds and reptiles.) Fig. 13, C. 
II. Peripheral: in which an excess of yolk collects at the centre of the ovum, 
with the protoplasm at the periphery. The dividing nuclei assume a 
superficial position and surround the unsegmented yolk. (Illustrated in 
the eggs of insects and other arthropods.) Fig. 13, D. 




PlG. 13. Cleavage and gastrulation as affected by yolk (not drawn to scale). The vertical 
rows A. B, C, and D represent different classes of ova. A, an ovum with little yolk; B, one with 
considerable yolk collected at the lower pole (P.p.fc C, one with a large amount of dense yolk crowd- 
ing the protoplasm to one side (a..) ; D, ovum with dense yolk collected at centre. The numerals 
(1-4) indicate stages in cleavage and gastrulation: i, ova; 2, 4-8 celled stages of segmentation; 
3, blastospheres, blastula stage; 4. gastrula stage, a, archenteron; a.p., active pole; bl, blastoderm ; 
bp., blastopore; ec, ectoderm; en, entoderm; ma., macrospheres; mi, microspheres; p.p., passive 
pole; s.c., segmentation cavity; y, yolk; y.c., yolk nuclei. 

Questions on the figures. What constitutes the difference between the active 
and the passive pole? Judging from the drawings and from your references to 
texts does gravity have any influence in determining the position of these? Your 
evidences? Which pole gives rise to ectoderm? Why does the food substance 
interfere with segmentation? What is the difference between the segmentation 
cavity and the archenteron? How does the presence of food substance modify 
the formation of an archenteron? 


54. Blastula and Morula. As cleavage continues the blasto- 
meres remain associated in a spherical mass. The individual 
cells project beyond the general surface not unlike the lobes 
of a mulberry, and for this reason this stage is called the morula 
or mulberry stage (Fig. 13, 2). By the growth of the cells and 
by the imbibition of water the morula may become a hollow sphere 
of cells (blastula) the central cavity of which is filled with fluid. 
The cavity is termed the segmentation cavity (Fig. 13, s.c.). 
Some animals, such as Volvox, never develop beyond the blastula 

55. Gastrula. In those eggs in which the segmentation is 
total, a next important step is the pushing in of that side of 
the blastula which corresponds to the original nutritive pole. 
The process is known as imagination, and the product as a 
gastrula (Fig. 13, 4). It takes place much as one might suppose 
one side of a hollow rubber ball to be dimpled or infolded by the 
exhaustion of the air within. The gastrula is to be described 
as made up essentially of two layers of cells, one external and 
called ectoderm, and one within called entoderm (Fig. 13, 4). 
The segmentation cavity may be wholly obliterated; in that case 
the entoderm and ectoderm come to lie in contact. The cavity 
of the invagination of the gastrula is the archenteron or embryonic 
digestive tract; the opening into it, that is, the mouth of the 
gastrula, is the blastopore (Fig. 13, bp). Such an organism as 
Hydra (see Fig. 81) may be looked upon as a permanent gastrula, 
somewhat modified in form. In morulas in which the seg- 
mentation cavity is small and the cells at the nutritive pole are 
large (Fig. 13, C, 4) this simple condition is much obscured, 
and invagination as described above becomes impossible. Nev- 
ertheless early in development the cells which produce the two 
primitive layers are to be distinguished, and their relations are 
always substantially as detailed. If the term gastrula is applied 
to these we have to say that they are formed in some other way 
than by ordinary invagination. 

56. Library Reference. Let students report briefly on gastrulation by over- 
growth (epibole), and by delamination. Compare the results attained by the 
various methods. Note what is constant in the methods and in the results. 


57. Germinal Layers. The ectoderm and entoderm have 
thus far been mentioned as the primary germinal layers of cells. 
Some of the Invertebrates have only these two layers, but in 

PIG. 14. Modes of forming mesoderm (diagrams modified from Whitman andJSelenka). 
A and B, special mesoblasts distinguishable early in segmentation (Annelid): A, surface view from 
active pole; B, sectional view of same, ec, micromeres destined to form ectoderm; en. macromeres 
destined to form entoderm; m, primitive mesoblast which produces the mesoderm. C, amoeboid 
mesodermal cells (c) budding from entoderm into the segmentation cavity (j.c.), in an Echinoderm. 
a, archenteron. 

FIG. 15. 


FIG. 15. Mesoderm formed by pouches from entoderm after gastrulation. A and B, early 
and later stages in formation of mesoderm and ccelom. a, primitive gut; bp., blastopore; cat, body 
cavity, formed from pockets of the archenteron; ec., ectoderm; en., entoderm; m., mesoderm;, 
body-wall mesoderm; m.sp,, visceral mesoderm; s.c., segmentation cavity. 

Questions on figures 14 and 15. Enumerate the three modes of mesoderm 
formation figured here. In which type may the mesoderm be identified most early 
in the embryonic development? By comparing with other texts determine in 
what groups of animals the mesoderm is formed as in Fig. 15. 

most cases a third mass of cells comes to be situated between the 
ectoderm and entoderm, from which important organs are de- 
rived. The third or middle layer (mesoderm) differs somewhat 
in its origin in the different groups of animals. It may originate 


(i) from the multiplication of a few special cells which, before 
invagination, in early cleavage stages, become distinct from 
those that are to form ectoderm and entoderm (Fig. 14, A and 
B, m)\ (2) by means of isolated, wandering cells budded from 
the other two layers, particularly the entoderm (Fig. 14, C, c); 
or (3) from entoderm, in the form of pouches or of solid buds 
of cells which arise from the walls of the archenteron and extend 
into the segmentation cavity (Fig. 15, m). In some instances 
there may occur a combination of these methods. 

58. Coelom. When the mesoderm develops by the last 
mentioned method, i. e., by the evagination of the wall of the 
primitive gut (Fig. 15, m), we see a pair of folds, or a series of 
pockets, the cavities of which are at first continuous with the 
archenteron, but later become separate from it and entirely 
surrounded by the mesodermic layers. The outer wall of the 
mesodermic pouches joins the ectoderm and forms a body wall, 
and the inner applies itself to the entodermal wall of the gut. 
The space between is the ccelom or body cavity. When the meso- 
derm arises as a solid mass, instead of a pocket, the body cavity 
is formed by the splitting of the mass into an inner and an outer 
portion. When the ccelom is formed by several pockets the 
cavities of these may ultimately coalesce, forming a single body 
cavity. Such a cavity is found in all the vertebrates and in the 
higher invertebrates, although it may become more or less ob- 
scured and modified in the adult. 

59. Differentiation of Organs and Tissues. We have al- 
ready in these three layers and their foldings the fundamental 
outline of that differentiation which is to give us the complex 
animal form found in the adult. From these layers, singly or in 
combination, all the tissues and organs of the body arise. The 
various layers become locally thickened, folded, or otherwise 
modified in form by rapid cell division, thus producing the begin- 
nings of organs. At a later date differentiation takes place 
among the cells, and tissues arise (see next chapter). In general 
each layer gives rise to such structures as its position and relation 
to the outer layers would suggest. This is especially noticeable 
in the ectoderm and entoderm. The ectoderm is more closely 


related to the outside world, and from it are produced the pro- 
tective and sensory structures. These include the outer portion 
of the skin and the hard parts often associated with it, and the 
whole nervous system together with the sensitive portions of the 
organs of special sense, as the retina of the eye. The entoderm 
is derived from the cells which contain, or at least are closely 
related to, the food originally stored in the ovum (Fig. 13), and 
it comes to lie in the interior of the embryo. It furnishes the 
lining of the adult digestive tract as well as the essential parts 
of the glands and other outgrowths arising from it. The me so- 
derm gives origin to the muscles and to the supportive tissues, 
to the blood vessels and blood. Many of the organs are made 
up of contributions from two or all of these germinal layers. 
Students must be referred to special textbooks on embryology 
for a more extended account of the manner in which the germinal 
layers give rise to adult organs. 

60. Summary of the Life -cycle and the Meaning of the 
Steps. It is important that the student bring together the 
essential points in the cycle of events that occur in the life 
history of an organism from one generation to the next. 

1. Fertilization. If we start with the mature egg and sperm 
ready to unite, we must recall (50) that each of these has lost, 
by the reduction division, one-half of the natural number of 
chromosomes in the species. There is an increasing body of 
evidence for the belief that the chromosomes bear the hereditary 
characters from one cell to another and hence from one generation 
to another. When the nucleus of the sperm unites with the 
nucleus of the egg in fertilization one-half of the chromosomes 
in the new nucleus thus comes from each of the parents. Fertili- 
zation may be said then to do two things: (a) it unites the 
substances carrying hereditary qualities from two parents (usu- 
ally); and (6) starts the development of the egg (Figs, n, 12). 

2 . Cleavage, and the Segregation of Germ Cells. The fertilized 
egg soon begins to divide (cleavage). The resulting cells may be 
much alike or may be very different in size and contents, from the 
very beginning. (See Fig. 13.) Most of these cells enter directly 
into the making of the adult body by the differentiation into 


entoderm and ectoderm, and later into nerve cells, muscle cells, 
gland cells, and the like. It has been clearly shown, however, 
for some species, that certain undifferentiated cells are sooner or 
later put aside, which do not take direct part in the development 
of the tissues of the body. By tracing the history of these cells 
through the various stages of embryonic development it is found 
that these are primordial or ancestral germ cells and that they 
ultimately produce the sperm and egg cells with which we are 
familiar. (See Figs, n and 12.) It is probable that something 
similar will be found to be true of animals generally. 

3 . The Parallel Development of the Body and the Germ Cells. 
From this time on two quite different things are happening in 
every normal body as it develops: (a) the body cells are multi- 
plying, growing, and differentiating to form the tissues and or- 
gans of the animal; and (6) the primordial germ cells, which 
have come from the same egg, and are cousins not descendants 
of the body cells are dividing and growing but not differentiating. 
During whatever time is necessary for the organism to develop 
its body tissues and organs, the primordial germ cells are rinding 
the permanent place they are to occupy in the body and are, 
in cooperation with certain of the body cells, forming sex glands, 
testes and ovaries. 

4. Sexual Maturity and the Perfection of the Ova and Sperm. 
In this parallel development of the germ cells and the body cells 
it is certain that they modify each other profoundly. The 
germ cells depend for their supply of food and oxygen, etc., upon 
the activities of the body cells. Undoubtedly the wastes of the 
body cells also influence the germ cells. However, it seems that 
the presence and products of the germ cells even more modify 
the growth of the body. This is especially true as the germ cells 
and glands approach maturity. In a number of species it has 
been shown that the development of certain parts of the body 
is very much changed if the maturing sex glands are removed 
early in life. In general it is believed that the differences in the 
bodies and the instincts of the male and female of a species are 
largely due to the effect of the development of sperm and ova 
within the bodies. 

In the last steps whereby the spermatogonia produce sperm 


and the oogonia produce eggs, there are two important happen- 
ings: (a) the union of the simple chromosomes of a nucleus, 
originally derived from the two parents, to form the bivalent 
chromosomes; and (b) the separation of these at the next division 
in such a way as to reduce the simple chromosomes to one-half 
the number usually found in the cells of the species (Figs. 12, 13). 

It will be recalled that the chromosomes found in any primor- 
dial germ or body cell are descendants of chromosomes that came 
in equal numbers from the egg and sperm nuclei when they first 
united. We have seen that every cell gets its part of every one of 
these chromosomes. It is believed, when these chromosomes 
unite in pairs in the act of maturing, that they do not unite in a 
chance manner, but that those pairing are corresponding chromo- 
somes coming originally from the mother and father. In other 
words this union is the final scene in the mixing of male and female 
qualities which began when the nuclei united in fertilization. 
The protoplast of ovum and sperm unite at fertilization; but 
the final union of the chromosomes of the ovum and sperm is 
delayed until near the time of the next union of sperm and ova. 
It is possible that these bivalent chromosomes may be exchang- 
ing material before their final separation. 

The reduction of the chromosomes to one-half their typical 
number is clearly a device which maintains the standard number 
when the ovum and sperm unite. If they were not reduced the 
amount of chromatin, and the accompanying hereditary effects, 
would be doubled at every fertilization. 

61. Summary. 

1. All the higher animals begin life as a single cell and reach 
their adult condition by a continuous series of divisions. By 
the growth and specialization of the cells . arising from these 
divisions the great complexity of the adult body is produced. 

2. This initial cell the fertilized ovum represents the 
fusion of two independent and unlike cells: the ovum (female) 
and the spermatozoon (male). 

3. Before the union (fertilization) occurs, the ovum reduces 
its nuclear material, by two successive divisions, to one-fourth 
its original amount and the chromosomes to one-half their 


original number, without a corresponding reduction of the cyto- 
plasm. The spermatozoon in its development undergoes a 
similar reduction of chromosomes. 

4. After the union of the male and female cells the fertilized 
ovum divides rapidly (segmentation or cleavage) forming a mass 
of cohering cells. The nature of these cells and of the mass 
depends much on the amount of yolk in the ovum and on its 

5. By processes which differ in different animals according 
to the nature of the segmentation, the cells become arranged 
with a layer outside (ectoderm), a layer within (entoderm), and 
from these a third layer or mass of cells lying between the other 
two (mesoderm). The entoderm bounds a cavity (archenteron) 
which communicates by a pore (blastopore) with the outside 
world. Within the mesoderm may be found a cavity (coelom). 

6. The ectoderm gives rise to the outer portions of the skin, 
its protective and sensory structures, to the nervous system, 
and frequently to the lining of the openings into the body. The 
entoderm lines the principal part of the digestive tract. The 
mesoderm gives rise to most of the other structures of the body. 

62. Suggestive Topics for Library Work. 

1. What suggestions have been offered as to the advantage 
of the addition of the male nucleus to that of the female in 
fertilization? Has a similar result ever been attained arti- 
ficially by means of chemical or other stimuli ? 

2. What explanations have been offered as to the signifi- 
cance of the process of maturation? Trace the maturation 
of the sperm cells more fully. 

3. What classification of ova do the text-books make? 
What is the basis of the classification? To what extent do 
eggs of different animals vary in size, shape, envelopes, etc. ? 
Give examples. 

4. Is there any explanation of the fact that there is such 
a difference in the amount of food substance in the eggs of 
different animals ? 

5. Trace out by reference to a text-book of embryology the 
principal changes by which the adult digestive tract is derived 


from the simple condition found in the gastrula (archenet- 
ron). What is the fate of the blastopore? How does the 
permanent mouth originate? 

63. Exercises for the Laboratory. 

The teacher should secure demonstrations of some of the 
smaller ova (as of the snail, fish, sea-urchin) for examina- 
tion with the microscope. Compare the ovum taken from the 
ovary of a hen with a new laid egg, noting especially the struc- 
ture of the latter. Obtain spermatozoa from the testis of a 
recently killed animal (as mouse, fowl, etc.), and examine with 
highest powers of the microscope. If possible secure permanent 
mounts of segmenting eggs of sea-urchins, showing the 2-, 4- 
8-celled stages. 



64. Two things of importance happen to the body as the 
organism develops from the simple condition of the ovum to the 
great complexity of structure in the adult : (i) the increase in the 
number of cells, which is quantitative in nature, and (2) the 
differentiation of cells, whereby the cells of the various parts 
become very diverse in shape, composition, and powers. This 
is a qualitative change. It is not yet fully known how much 
of the difference in the cells of the various tissues is due to 
qualitative differences in the daughter cells of a given division, 
and how much is due to external influences and the interrela- 
tions of the cells after division. We know, for example, that 
gravity acting on the food substance of the ovum before division 
does produce such differences among the daughter cells of the 
early cleavage stages as lead to results as diverse as ectoderm 
and entoderm. Doubtless there are also internal processes that 
tend to give rise to similar differences. On the other hand, it has 
been shown by experiment that, even as high up in the animal 
scale as the lowest vertebrates, the blastomeres of the two- or four- 
celled stage may be shaken apart and each develop into a small 
but perfect embryo. This experiment shows that up to this 
stage no specialization has taken place which limits the products 
that come from these cells. The blastomeres do not so develop 
after the 8- or i6-celled stage is reached, so far as is known. 
We are ignorant of the causes which determine that one cell 
shall develop into a muscle cell and its neighbor into a bone 

65. Tissues. A tissue is to be defined as a group of similar 
cells suited by their differentiation to the performance of a 
definite function. This differentiation affects the size, shape, 
and the interrelations of cells, and likewise the chemical and 



physical structure of the protoplasm, in such a manner as to 
cause great variation in their powers and activities. The 
chemical differences are especially shown in excretion and se- 
cretion, whereby various sorts of materials are deposited within 
and between the cells of the different tissues. The material 
deposited between the cells is known as intercellular substance. 
The intercellular substance differs much in character and 
amount. Both the cells and the intercellular substance are 
important in enabling the tissue to perform its work. In gen- 
eral if the tissue is active (as muscle) the cellular differentia- 
tion is the important point; if, however, the function is a more 
passive one, as support or protection, the nature of the inter- 
cellular substance rather than the cells determines its character 
(bone, connective tissue). 

66. Classification' of Tissues. From a physiological point 
of view tissues may be classed in one of two groups: vegeta- 
tive, and active. The vegetative tissues are those which per- 
form the more passive functions, as nutrition, protection, sup- 
port, etc. They resemble the plant tissues in their functions. 
The two chief classes of vegetative tissues are: epithelial or 
bounding tissues, and supportive or connective tissues. The 
active tissues may be looked upon as the characteristic tissues 
of animals. The muscular and nervous tissues belong to this 

67. Epithelial Tissues. This tissue is characterized by its 
primitive form, i.e., by its relative lack of differentiation, by 
the fact that it is the first to appear in individual development 
(ectoderm and entoderm in the gastrula), and by the absence 
of intercellular substance. It is a bounding tissue and consists 
typically of a single layer of cells, although several layers may 
occur. Epithelium bounds, by its own cells or their products, 
the outside of the body, the lumen of the digestive tract and 
its outgrowths, as well as the body cavity and the structures 
cpntained in it. 

68. Kinds of Epithelial Tissue. Located in a position 
superficial to the other tissues, epithelium is subject to a wide 
range of variation both as to form and function. Besides its 



primary work as a protective layer, the epithelium may have 
a glandular function, being favorably situated for the final 


FIG. 16. Various kinds of epithelial cells (semi-diagrammatic). A, columnar; B, cuboidal; 
C, pavement; D and E, ciliate (sectional views). In F is shown the surface view of pavement 
epithelium. cl. t cilia; cu., cuticula. 

Questions on the figures. For what different uses would you judge these 
variously shaped epithelial cells to be suited? Under what circumstances and on 
what surfaces would you expect to find each type? Compare with your reference 
texts and see if they are so found. Under what circumstances is a cuticula to be 
expected? Where would it be a disadvantage? What are cilia? 

- < FIG. 17. 


PIG. 17. Glandular epithelium, a, goblet or slime cells, unicellular glands; 6, similar cells 
which have become depressed below the surface, and empty their secretion through a duct. 

Questions on the figures. Are the glandular cells modified epithelial cells? In 
what respects do they differ from the cells about them ? 

elimination of products from the body. Since it is especially 
exposed it is the layer best adapted by position to receive those 
external stimuli which we know to play such an important r61e 


in the life of all organisms. The position of the epithelium 
also renders it specially liable to destruction. To compensate 
for this its primitive or undifferentiated character makes it 
particularly capable of regenerating portions of itself which 
may have been lost. Epithelium is often especially active also 
in the regeneration of other than simple epithelial structures, as, 
for example, the regeneration of nervous cells in a cut earth- 
worm. In close connection with this latter regenerative quality 

FIG. 18. 

FIG. 18. A. series of diagrams showing progressive stages in the development of a multicellular 
gland from an area of glandular epithelial cells. C and D show two somewhat different types of 
gland, the cup-shaped and the tubular, e. bounding epithelium; g, gland cells; d, duct; c t connect- 
ive tissue. 

Questions on the figures. How do the compound glands seem to arise from 
the simpler condition? What is the evidence that glands are lined throughout 
with epithelium? What is gained in the sinking of the glands below the surface? 

is to be considered the fact that the reproductive or sexual cells, 
by which new individuals are produced arise from an epithelium. 
The foregoing enumeration of functions suggests the physiolog- 
ical classification of epithelial bounding, glandular, sensory, 
and reproductive epithelia. The same layer may fulfill several 
of these functions at once. 


69. Bounding Epithelium. The ordinary protective epithelium may be made 
up of cells cuboidal in shape (Fig. 16, B), or columnar (Fig. 16, A), or much 
flattened (Fig. 16, Q. In extreme cases of flattening and hardening we have 
squamous epithelium, e.g., the outer cells of the human epidermis. Motile proto- 

FIG. 19. Sensory and muscular epithelium. A, sensory epithelium, from Worm, showing 
some of the epithelial cells (e) modified into sensory cells (s). B, epithelial cells from Hydra showing 
contractile or muscular processes at base (m). 

Questions on the figures. Is there anything to suggest that the sensory cells 
are modified epithelial cells? What are the principal changes which they have 
undergone as compared with the unmodified epithelium? 

FIG. 20. 

FIG. 20. Diagram of a portion of the ovary of Sea-urchin showing the eggs arising from the 
epithelium (reproductive epithelium) by constriction, e, epithelium; <?, ova in different stages of 

Questions on the figure. What is an ovary in its simplest form? Is the re- 
productive epithelium ectoderm al, entodermal, or mesodermal in origin, as a 

plasmic projections often extend from the free surface of the epithelium. Flagel- 
late epithelium (Fig. 79, D) has one such projection from each cell, whereas ciliate 
epithelium (Fig. 16 D, E) has numerous small ones. Cilia are more common in 

5 2 


the lower groups of animals, but are found even in mammals, in the moist internal 
passages, as in the nose, trachea, etc. 

Membranes bounding the body cavity are called serous membranes (endothe- 
lium). The lining of the digestive tract is described as a mucous membrane. 

Epithelial cells often secrete upon their outer surface a layer of material 
(cuticula}, which serves to protect the cells beneath and the organism as a whole 
from external influences (as the covering of the cray-fish). From the epithelium 
arise various outgrowths, as scales, hair, feathers, and the like. 

70. Glandular Epithelium. The ordinary columnar or pavement epithelium 
may here and there present cells or areas of cells which are specially active in 
producing and pouring out on their free surface certain materials, called secretions. 
In its simplest form the gland or secreting surface may consist of a single cell, as 
the goblet or slime cells (Fig. 17, a). 

Such a cell may become much enlarged and sink below the general level of the 
epithelium, retaining in the meantime a narrow connection with the exterior (Fig. 
17, &). Multicellular glands represent areas of such cells which have sunk below 

FIG. 21. 

FIG. 21. Section through ovary of a young Mammal (modified from Wiedersheim) . The eggs 
(o) are seen to be formed from the epithelium by a process somewhat more complex than in Fig. 18. 
c, connective tissue of ovary; e, epithelium;/, follicle of epithelial cells in which the ova ripen; o, ova 
in different stages of ripeness. 

Questions on the figure. In the ovary of the mammal what additional service 
does the epithelial layer render the ovum after its formation ? Is it apparent that 
there is anything gained by the sinking of the ovarian follicles into the tissue of the 
ovary, instead of escaping immediately, as in Fig. 20 ? 

the surrounding surface, forming a tube- or flask-shaped cavity, which may become 
very much branched. Glands with such branched ducts are described as compound. 
They consist of numerous final secretory sacs communicating by ductules with a 
common duct or outlet to the surface. Transitional conditions between the 
simple secretory epithelium and the compound gland may be seen in Fig. 18. 

71. Sensory Epithelium. In the lower animals there may be found here and 
there over the surface of the body modified epithelial cells, which are specially 


capable of being stimulated by contact or other stimuli to which the organism may 
be exposed. Likewise in higher forms we find highly specialized areas of sensitive 
cells, which can be shown to belong primarily to the epithelium. These are the 
end organs of special sense, as touch, sight, and the like, and they get their special 
value from their connection with what will be described presently as the nervous 
tissues of the central nervous system. The sensory cells are typically spindle-like 
or even hair-like in form, often extended as fine fibres at the inner end, whereby 
connection is established with the nerves (Fig. 19, A). 

72. Reproductive Epithelium. The sexual cells, both male and female, arise 
from epithelium, ectodermal, entodermal, or, as is usually the case, mesodermal. 
The budding of the sexual epithelium, in the development of the germ cells, sug- 
gests the formation of glands (Figs. 20, 21). The sexual cells often develop at the 
expense of the epithelial and other cells about them. 

73. Supportive or Connective Tissues. This class of tissues 
embraces the bulk of the non-active tissues in animals. They 
vary much in appearance and structure, agreeing in little except 
in their mesodermic origin, their passivity, and in the preva- 

FlG. 22. 

FIG. 22. Cellular Connective Tissue, showing large vacuoles, v, in the protoplasm. 

Questions on the figure. Would you say that these cells are of a high or a low 
order of differentiation? Why? Is there any intercellular substance? Where is 
tissue of this kind found? (See reference texts.) 

lence of intercellular substance. The intercellular substance 
gives the distinctive character to the connective tissues, the 
cells having a relatively unimportant place after the produc- 
tion of the intercellular substance. The general function 
of the supportive tissues is to bind and sustain the more active 
tissues in their relations to the body as a whole. The classifica- 
tion of supportive tissues is based on differences in the intercellu- 
lar substance. This may be fluid (as in blood) or solid (as in 
bone) ; it may be homogeneous (as in some forms of cartilage) , or 



fibrous; it may be almost wholly organic, or very largely inor- 
ganic. The principal classes are cellular connective tissues, ge- 
latinous connective tissue, fibrous connective tissue, cartilaginous 
tissue, and osseous tissue. 

FIG. 23. 

FIG. 23- 

Gelatinous connective tissue, showing stellate cells (c), epithelium (), the gelatinous 
intercellular substance (s), and the intercellular fibres (/). 

Questions on the figure. What seems to be the relation of the epithelial layer 
to the tissue below it? What classes of cells are found in the gelatinous tissue? 
What is their origin? What is the nature of the intercellular substance? Are the 
fibres cellular or intercellular? 

74. Cellular or Vesicular Tissue forms an exception to the general rule of abun- 
dant intercellular substance. It is an embryonic tissue, a forerunner of the more 
permanent tissues, and is chiefly interesting from that fact. The cells have 
large vacuoles or vesicles which are enveloped by a thin layer of protoplasm (Fig 
22). It is found in the notochord of vertebrates. 

75. Gelatinous tissue has a matrix of intercellular substance enveloping stellate 
cells, the radiating projections of which serve to connect them. Fibres are often 
developed in the matrix. This tissue is abundantly found in the jelly-fish (see 
Fig. 23). 

76. Fibrous connective tissue has in its ground substance a rich supply of 
fibrils variously arranged. The cells or corpuscles are often elongated and 
branched. If the intercellular fibres cross, running in various directions, a loose 
yielding tissue results, as in the ordinary connective tissue about the muscles and 
nerves (Fig. 24, A); if the fibres are parallel the tissue naturally becomes more 
compact. There are two types of the more compact sort differing in the quality 
of the fibres. The latter may be white and inelastic, as in tendons, or yellow and 
elastic. Fat is frequently deposited as spherical drops of oil (Fig. 24, B) in the cells 
of connective tissue. 

77. Cartilage. In cartilage the intercellular matrix is much firmer than in 
those tissues already described. It may appear homogeneous as in rib cartilage 



(Fig. 25, A) ; or it may contain numerous fibres which give coherence and elasticity. 
The cells are usually rounded except where they have been flattened by mutual 

FIG. 24. 

FIG. 24. Fibrous connective tissues. A, ordinary connective tissue found binding muscle 
and nerve fibres, showing the fibrous intercellular substance. The cells (c) are never conspicuous 
in this tissue. B, adipose connective tissue showing fat-laden cells among the fibres (/). o, oil 
droplets in the cells. 

Questions on the figures. In these two types of tissue which element gives 
special character to the tissue, the cells or the intercellular substance ? How would 
the deposition of large drops of oil in the cell affect the activity of the cell? Why? 
Why are fatty deposits less hurtful amid connective tissue than elsewhere in the 

FIG. 25. 

FIG. 25. Cartilage. A, Hyaline cartilage; B, fibrous cartilage. In the latter a large portion 
of the intercellular substance is conspicuously fibrous. The cells occur in pockets (p) in the matrix;, 
f, intercellular fibres. 

Questions on the figures. What are the points of similarity and of difference 
in the two types of cartilage? In what manner do the multicellular pockets arise? 
What is the nature and origin of the intercellular substance in each case? 

pressure, and usually occur in pockets in the matrix. Cartilage is bounded on its 
free surfaces by a fibrous membrane, the perichondrium. This membrane assists in 
the growth of the cartilage. There are no blood capillaries in cartilage. 


Salts of lime may be deposited in the intercellular substance, giving it some of 
the qualities of bone. 

78. Osseous or Bony Tissue. These tissues are found only in vertebrates, and 
are the most complicated of the supportive tissues. The firm matrix which is 
secreted by the bone cells consists of a mixture of organic substance and inorganic 
matter, especially the salts of lime. The cells with their fine filamentous branches 
occur more or less regularly between thin plates or lamella of the bony material. 
A cross-section of one of the long bones shows the typical condition. The perios- 
teum is a superficial fibrous membrane about the bone, well supplied with blood 
vessels. Its inner layer of cells is capable of producing bone. Within this is a 
region of firm bone, in which a series of lamellae are parallel with the surface of 
the periosteum. Between the lamellae occur the spaces (lacuna) occupied by the 
bone-cells which have been left behind as the matrix was deposited. Deeper in 

FIG. 26. 

FIG. 26. Bony Tissue. A, portion of cross-section of a bone, the upper portion of the figure 
representing the outer surface of the bone, just beneath the periosteum. The open spaces, h, are 
Haversian canals; I, lacuna, occupied in life by bone cells. The minute canals through the bone 
connecting the lacunae are canaliculi. B, a portion of one Haversian system much magnified, h, 
Haversian canal, containing artery (a), vein (n), lymphatic spaces, nutritive cells; c, canaliculi; I, 
lacurse; la, plate of bony intercellular substance. 

Questions on the figures. How does bone compare in appearance and structure 
with the other supportive tissues? What is the really living part of bone? How 
is its intercellular substance laid down? How are the cells in the bone nourished? 
How do they come to lie in the solid bone? What changes occur in this type of 
tissue with age? What is the function of the Haversian canal? 

the bone the lamellae and cells are in concentric layers about the numerous blood 
vessels (occupying spaces known as Haversian canals) which penetrate the bone, 
chiefly in a longitudinal direction. The included bone-cells communicate with 
each other and with the blood vessels by processes which occupy minute canals 
(canaliculi) in the intercellular substance (Fig. 26). Within this region and imme- 
diately surrounding the central cavity of the bone is often a mass of spongy bone 
in which the regularity of arrangement of the cells is lost. Bone may be formed 
by replacing cartilage, or wholly independent of it. 



PIG. 27. Blood corpuscles (amphibian), c, colored corpuscles, flatwise and in profile; I, colorless 

corpuscles (leucocytes). 

FIG. 28. 

FIG. 28. 

Blood corpuscles (human), c, colored; I, leucocytes. 
rows with the sides in contact. 

The red cells tend to collect in 

Questions on figures 27 and 28. Compare by means of the figures, the text 
and reference books the colored and colorless corpuscles of these two types of 
vertebrates and note the differences. In what other respects do the colored cells 
differ from the white? Which are the less highly differentiated? Reasons for 
your view? Why are the colorless corpuscles also called phagocytes? 

Dentine and enamel, though differing in structure from bone, are to be looked 
upon as belonging to the same class of tissues. They differ chiefly in the fact that 
no cellular elements are included in the secretion. They are thus harder and denser 
than bone. 

79. We find all stages of transition between the more sim- 
ple and more complex supportive tissues, and it may be seen 
furthermore that there is a fundamental embryological sequence. 
In the development of the organism the simpler connective tis- 
sues give place, by transformation or substitution, to the more 
complex. The cellular connective tissue of early life is replaced, 
for example, by cartilage, and this may be transformed into 
bone in adult life. 


80. Nutritive Fluids. The body fluids known as blood and lymph are frequently 
classed among the supporting tissues, the fluid portion being regarded as the inter- 
cellular substance and the corpuscles as the cells. They differ however from the 
ordinary tissues in the important fact that the intercellular substance is not pro- 
duced by the cells. In the vertebrates these cells are of two kinds, the amaboid 
or colorless and the colored. Both kinds occur in the blood; the colorless alone are 
found in the lymph. The colored corpuscles are relatively numerous and are disc 
shaped. Regarded as cells they present a series of degenerative changes which 
results in a loss of the distinctively protoplasmic character, by the substitution 
of certain proteid substances, one of which haemoglobin is notable for its affinity 
for oxygen. The degeneracy may go to the extent of the entire loss of the nucleus, 
as in the mammals. The colorless cells have the power of independent motion 
such as is found in the amoeba, and may ingest solid particles of food. The body- 
fluids of the invertebrates contain as a rule only colorless corpuscles, and are there- 
fore more like the lymph of the vertebrates. When their blood is colored it is 
usually from pigment in the plasma or fluid portion of the blood. In addition to 
the cells the blood carries a rich supply of proteid and other substances for use in 
the tissues, and of waste products in process of removal from the body. 

81. Muscular Tissue. The remaining tissues are charac- 
teristically active. Muscular tissue by its contractility has the 
power of producing movements of the parts to which it is at- 
tached. This contractility of muscle may be looked upon as a 
specialization, and a limitation in direction, of the power of 
contraction which we have seen to be resident in all living proto- 
plasm. Muscular tissue differs somewhat in structure and 
degree of differentiation in various animals, but in general agrees 
in the presence of elongated fibres which are to be considered as 
modified cells or parts of cells. The contractile muscle substance 
is, in part at least, a plasmic product rather than mere proto- 
plasm; yet it differs from the intercellular substance of the tis- 
sues already described in that it is deposited within rather than 
among the cells. 

Two stages in the differentiation of muscular substance are 
to be noted: (i) the fibres may be plain, in which case we find 
elongated, contractile single cells without conspicuous external 
differentiation (Fig. 29); (2) cross-striate fibres, which always 
show conspicuous differentiation of parts in each fibre as seen 
under the microscope. The plain fibres are characteristic 
of sluggish animals, and those parts of animals whose muscular 
action is least prompt in response to the nervous stimuli (e. g., 
digestive tract in vertebrates) . The cross-striated fibre usually 



represents several incompletely separated cells, or a multi- 
nucleate condition of a much-grown and metamorphosed single 
cell. In both classes the fibres are made up of numerous minute 
strands or fibrillcs which in the plain muscle are homogeneous 
throughout, but in the cross-striated are made up of alternating 
segments of lighter and darker optical appearance (Fig. 30, B). 

FIG. 29. 

FIG. 29. Plain muscle fibres, n, nucleus of muscle cell; p, undifferentiated cell piotoplasm; p' , 
the differentiated contractile portion of the cell. 

Questions on the figure. What are the two principal portions of these cells? 
How do very young muscle cells compare with older ones in the relative amount 
of these portions in the cell? Which is the more highly differentiated portion? 
Where are such tissues found in the animal body ? Why are muscle fibres elongated ? 

The undifferentiated protoplasmic remnant is often very small 
in amount, and is collected about the nucleus (Figs. 29, 30). It 
may be at the surface of the fibre or in the center, enveloped by 
the contractile matter. A thin membrane (sarcolemma) binds 
the fibrillae into fibres. The fibres are bound together by 
strands of connective tissue into bundles, and of these bundles 
the muscle is made up. 



FIG. 30. 

FIG. 30. Diagram of nervous and cross-striate muscular tissue, showing the mode of connec- 
tion between nerve fibres and muscle fibres. A, nerve cell (g) connected with muscle fibre (mf.) 
by nerve fibre (./.). The muscle fibre (m./.) is composed of numerous fibrils (/) which are made up 
lengthwise of alternating discs of lighter and darker substance. These fibrils are shown more 
highly magnified in B and C. In B the fibril is uncontracted; in C it is contracted. D, nerve fibre 
more highly magnified showing a, axis; m, medullary sheath; and s, Schwann's sheath; ax., axon; 
d, dendron; n, node; n.m., nerve-muscle plate. 


82. Origin of Muscle Tissue. In those animals in which a true mesodermis 
wanting, the epithelial cells may develop, at their inner extremity, contractile 
roots, either plain or striate (Hydra, Fig. 19, B). These cells may wholly lose 
their epithelial quality and position and become entirely muscular. In the higher 
animals this is very much modified by the early appearance of separate mesoderm 
from which the whole muscular system is derived. 

83. Nervous Tissue: its Functions. The nervous tissues 
are in close relation on the one hand to the sensory epithelium 
and on the other to the muscular tissue. Through the former 
they receive the stimuli from the outside world; by means of 
their connection with the latter they are enabled to effect re- 
sponses. The reception of stimuli, the transmission of the results 
of stimulation, and the initiation of appropriate responses con- 
stitute the fundamental work of nervous tissue (Fig. 30, A, D). 
In some of the lower Metazoa the same cell may do all these tasks. 

84. Structure. The principal elements of nervous tissue 
are the nerve-cells (ganglion-cells) and nerve-fibres. The cells, 
which are the centres of nervous activity, are usually large with 
conspicuous nuclei. The fibres are, in their essential parts 
merely outgrowths of the ganglion-cells. These outgrowths are 
of two sorts: the dendron, which is a much, and irregularly, 
branched structure ; and the axon, or nervous fibre proper. The 
ganglion with its dendrons and axons make up a neuron. It is 
believed that the whole nervous system even in the higher ani- 
mals is merely a system of neurons in connection. The impulses 
can go in only one way : into a cell by way of dendrons and out of 
it by way of axons. From the axon of one cell they may pass 
to the dendron of the next; but not in the opposite direction. 
Each cell may have one or more processes arising from it. These 
fibres may pass just as they arise from the cells, without special 
structural modification, to their connection^. Such are called 
non-medullated or gray fibres. There are usually however one or 
more protective sheaths formed about this essential axis: (i) 

Questions on Fig. 30. What are the principal points of contrast between the 
plain and the cross-striate muscular fibres? Enumerate the principal regions of 
the nerve cell figured. How does it differ from a typical cell in form? What are 
the principal parts of the nerve-fibre (D)? What are the supposed functions of 
these various portions ? Why is it necessary for nerve cells to be in connection 
with other kinds of cells? What are the differences between the contracted and 
uncontracted muscle fibre (B and C) ? What is meant by a neuron? 


the medullary sheath, consisting of a framework filled with a 
fatty material, surrounded by (2) Schwann's sheath, a homoge- 
neous sheath with occasional nuclei along its course (Fig. 30, D). 
Fibres possessing the medullary sheath are called medullated 
or white fibres. 

A nerve cell together with its processes is called a neuron. 
The whole nervous system may be considered as made up of such 
units, which connect with each other by the delicate terminal 
branches of the outgrowths. See Fig. 36. 

85. Origin of Nervous Tissue. Nervous tissue always arises from the ectoderm 
of the embryo, so far as we know. In some of the lower forms of animals, as the 
Coelenterata, the nervous cells may be derived individually from the epithelium. 
In such instances they have a close connection with those muscle elements which 
are also of epithelial origin (see 82). In the higher forms the origin of the nervous 
matter from the ectoderm is somewhat less direct but essentially similar. The 
connection of the nervous centres with the muscles and glands, etc., in the higher 
animals is a secondary condition and is the result of the growth of the nerve fibres 
from the centres toward such organs. . What directs their growth to the right place 
when the fibres begin to grow, we do not know. 

86. Summary. 

1. The individual becomes complex by the increase of the 
number of cells, and by their differentiation. 

2. A tissue consists of a group of similar cells with their 
products, which are adapted to the performance of special work 
or function. 

3. Tissues differ morphologically in respect to the form, ar- 
rangement, and structure of the cells, and in the amount, 
arrangement and consistency of the intercellular substance. 

4. Physiological differentiation accompanies the morpho- 
logical, the division of labor becoming very complete in the 
higher forms. The physiological value of a tissue may depend 
either upon the cells or the intercellular substance. 

5. Tissues may be classified as follows: 
A. The vegetative or passive tissues. 

I. Epithelial: 

function: protection, absorption, secretion, 

sensation, reproduction, etc. 
kind: pavement, columnar, ciliate, glandular, 
sensory, muscular, reproductive, etc. 


II. Supportive or connective: 

function: binding, support, protection, 
character : abundant intercellular substance, 
form: vesicular, gelatinous, fibrous, cartilagi- 
nous, osseous, nutritive (blood and lymph) r 
B. The active tissues. 

III. Muscular: 

function: irritability, especially to nervous 
stimuli ; contraction in a definite direction. 

form: plain and cross-striate (depending on 
the differentiation of the contractile sub- 

IV. Nervous: 

function: reception of general stimuli, 
transmission of impulses, interpretation, 
and the initiation of appropriate responses, 
form: central cells (ganglion) with fibrous 

branches (axon, dendron). 

6. The epithelial tissues arise from ectoderm, entoderm and 
mesoderm; the connective tissue, from mesoderm; the muscu- 
lar, chiefly from mesoderm; and the nervous tissue, from 

87. Exercises for the Laboratory (these may be made as 
extensive as time and facilities will allow). 

i. Temporary demonstrations of the simpler tissues should 
be made by the teacher or pupil, by teasing out with needles 
small portions of the appropriate material in a drop of water on 
the slide. 

(a) Blood. Compare that of earth-worm or insect, frog, 
man. Place a drop of fresh blood on the slide, and cover. 
Examine at once. The teacher should have a permanent prepa- 
ration of the blood of the frog, stained to show nucleus of 

(b) Epithelium. Mesentery of cat; film shed from skin of 
frog kept a few days in captivity; cells scraped from the eso- 
phagus of a recently killed frog. 


(c) Connective Tissue. Found surrounding muscle, i.e., 
lean meat. Compare tendon. 

(d) Muscle. From wall of stomach, from heart, from skele- 
tal muscles. 

(e) Nerve Fibres. Small portion of nerve of frt>g or cat. 

2. The teacher should secure permanent mounts of sections 
of cartilage, bone, and tooth showing dentine and enamel. 
Properly stained preparations of glandular tissue, of nerve cells 
and their branches, and of reproductive epithelium (see appen- 
dix), will greatly assist the pupils in securing an accurate idea of 
these tissues and their work. 




88. Protoplasmic Functions. It has already been stated 
that in protoplasm reside the fundamental powers belonging to 
living things. Through its agency all the vital processes are 
performed. Through the activity of its ferments, foods undergo 
changes that prepare them to be used in the manufacture of 
protoplasm and other complex cell-substances. It alone has the 
power to assimilate or build up these foods into the living state. 
In protoplasm occur the oxidation and other chemical changes 
which result in the manifestation of energy, as heat, motion, 
light, etc., accompanied by the formation of waste products 
which are to be eliminated. The power of receiving and 
responding to surrounding influences, which we have called irrita- 
bility and contractility, is likewise a power of protoplasm. Out of 
these arises the possibility of organisms becoming adapted to their 
surroundings. It is not yet possible certainly to localize all these 
functions within the individual cell, although it seems probable 
that in some degree even such protoplasm as is found in the 
Amoeba has localized functions. In many single-celled animals 
there is a considerable localization. 

89. Division of Labor. As the protoplasmic units, i.e., the 
cells, increase in number by cell division and form large masses, 
they are no longer subjected to the same influences, and are not 
equally favorably situated for the performance of all the original 
functions. The protoplasm of all cells retains the power % of 
using food and of building up their own substance, but we find 
certain activities largely given over to special groups of cells; 
e.g., secretion is specially noticeable in some, contractility in 
some, and irritability in others. This division of labor, is 
accompanied by a corresponding differentiation of structure 
which constitutes an adaptation to the special work to be done, 

s 65 


and is of great advantage. We have described these structure- 
groups as tissues (see Chapter V) . 

90. Organs. The tissues which have been described are 
never independent, but are associated with each other in the per- 
formance of a common function, to form an organ. In each 
organ there is usually a principal tissue which determines its 
function (as muscular tissue in muscle, or the glandular tissue 
in glands), and one or more accessory tissues for support or con- 
trol (as connective or nervous tissue in the organs mentioned) . 
To accomplish some of the activities, in the higher animals 
especially, several organs of a similar kind must work together. 
These are sometimes spoken of collectively as systems of organs, 
e.g., digestive system, circulatory system, and the like. 

91. Classification of the Systems of Organs and Func- 
tions. The work that needs to be done by an organism may be 
considered under the following heads: (i) metabolism in- 
cluding digestion, circulation, assimilation, respiration, and 
excretion; (2) protection and physical support; (3) growth; 
(4) reproduction; (5) movement; (6) sensation. Eight sys- 
tems of organs may be distinguished by which this work is done. 
They are (i) the digestive; (2) circulatory; (3) respiratory; (4) 
excretory; (5) skeletal and integumentary; (6) reproductive; 
(7) muscular, and (8) nervous. 

92. Metabolism (Nutrition). Metabolism embraces two 
sets of processes, (i) constructive or anabolic, known as assimi- 
lation, and (2) destructive or katabolic. By constructive we 
mean all the building-up processes in the organism which 
result in the storing of food and energy, in growth, repair, and 
reproduction. We class as destructive all those processes by 
which the complex cell substances are broken down or rear- 
ranged, and energy set free, leading to change of temperature, to 
nervous or muscular action, to secretion and excretion. In the 
higher animals the nutritive process is a very complicated one 
and demands the cooperation of numerous organs. It embraces 
the ingestion or taking in of food; the digestion of food; its 
absorption from the digestive tract into the body fluids blood 


and lymph; and its transportation in these systems, which is 
made necessary by the fact that digestion is confined to a special 
region. It likewise includes the further absorption of these 
materials from the blood and lymph by the cells for whose bene- 
fit all the preceding work has been done; the assimilative process 
within the cell whereby the food material is made into proto- 
plasm or other complex cell-products; the reception and trans- 
mission of oxygen, by the combining power of which (oxidation; 
see 26) these complex substances are broken down into simpler 
ones useful, useless, or hurtful to the animal economy. Finally, 
the elimination of the products of this oxidation or burning is a 
necessary part of the nutritive process. If the material elimi- 
nated from the cell is of further use the process is known as 
secretion, if not, excretion. It is undesirable to attempt to make a 
sharp distinction between excretion and secretion. Most so- 
called secretions are really excretions from the point of view of 
the protoplasm which produces them. 

93. The Digestive System. The simplest condition of 
the digestive tract is found in the gastrula (archenteron, Fig. 
13, 4) or in Hydra (Fig. 81). Here there is only one cavity 
in the body and the food is taken up immediately by the cells 
needing it. A simple modification of this condition is seen 
in Fig. 31. A still more complicated condition is shown in 
Fig. 93. In this form which we may take as the type, the 
digestive tract is a tube, running through the body, lined with 
its own epithelium and is separated from the body wall by 
the coelom or body cavity. The tube itself may have any 
degree of complexity, but consists essentially of (i) an an- 
terior portion (stomodczum) lined with ectoderm, (2) a pos- 
terior portion (proctod&um) also lined with ectoderm, and 
(3) a middle portion (mesenteron) lined with entoderm. The 
stomodaeum or mouth region is usually supplied with devices 
for the capture and ingestion of the food. The mesenteron 
is the true digestive region. It is supplied with cells which 
secrete materials which act upon foods in such a way as to 
render them capable of being absorbed through the entodermic 
cells into the body cavity, or into that special portion of it known 


as the circulatory system. Pouches and outgrowths from the 
wall of the mesenteron are of common occurrence. These 
serve to increase the glandular or secreting surface, the absorbent 
surface, and also to retain the food longer in contact therewith 
by retarding its passage through the canal. The removal of 
the digested food from the canal may be effected by absorption 
or by the active engulfing of food by the entodermal cells, much 
as is done by the amoeba. 

c v 


FIG. 31. Stenostoma (after Hertwig). In this Turbellarian the digestive tract (d.t.) is a blind 
sac. st., boundary of stomodseum and mesenteron; c, cilia; g, ganglion (brain); g' t ganglion of a 
new individual which is being formed by fission; o, mouth; o', mouth of new individual in process 
of formation; w, excretory system. 

Questions on the figure. How much of this digestive tract is lined with 
ectoderm? Which portion with entoderm? Is there a proctodaeum? What are 
the evidences that the worm is in process of division? Compare this digestive 
tract with those in Figs. 81, 87, 95, 101. 

94. The Respiratory System and Function. In addition 
to its other food requirements, all protoplasm, in proportion 
to its activity, must have free oxygen. This is obtainable from 
the air or from the oxygen dissolved in water. Oxygen, being 
a gas, must enter the system in a somewhat different way from 
that by which fluids and solids are ingested. It is best obtained 
by absorption through moist, thin-walled membranes. Such 
surfaces, in connection with which blood vessels are usually 
found, constitute the respiratory organs. Any exposed sur- 
face meeting these requirements may serve as such. The 
general surface of all animals is respiratory in some degree. 
In the more complex animals, however, special additional 


6 9 

organs must be provided. This may be effected by thin out- 
growths of the body surfaces, which are especially adapted 
to water forms and are called gills or branchics (Fig. 32); or a 
similar increase may be attained by pits or ingrowths of the 
body surface, suited to get oxygen from the air. Such are 
called lungs or trachea (Fig. 33). Carbon dioxid, a gaseous 
waste product resulting from the union of oxygen with carbon 

FIG. 32. 

FIG. 33. 

b. c. 

FIG. 32. Diagram illustrating gills or branchiae, b.c., cavity in which the body fluids crciu- 
late; br., branchial filaments which are merely much thinned out-pocketings of the body wall (IP); 
ex, the external medium water in which the oxygen is dissolved. 

Questions on the figure. What are the essential features of gills as suggested 
by this figure? Why are they better suited to water than to air? 

FIG. 33. Diagram illustrating lungs or tracheae, b.c., the cavity in which the body fluids 
circulate; /, the walls of the lung, which are much thinned in-pocketings of the body wall (a>); ex., 
the external medium usually the atmosphere in which the oxygend is foun. 

Questions on the figure. What are the essential features of lungs as suggested 
by the figure? Why are such organs better suited to aerial than to aquatic life? 
In what respects are gills and lungs better than the mere body- wall for the exchange 
of the gases ? 

which takes place in the tissues, is ecnomically eliminated by 
the same organ which admits the oxygen, inasmuch as the en- 
trance of one gas is not retarded by the outward passage of the 
other. This double process constitutes respiration, although 
the latter half is also appropriately described as excretory. The 
surface devoted to the exchange of the gases and the special 
devices necessary to renew the air or water make up the respira- 
tory system. The respiratory organs are frequently associated 
with the anterior or posterior end of the digestive tract. As in 


the case of other necessary substances, the blood is the vehicle 
by which oxygen is distributed from the gills or lungs to the parts 
of the body needing it. The student will realize that this is 
only the first step in respiration. The real respiration takes 
place in the protoplasm of the individual cells. 

95. The Circulatory System and Function. In such con- 
ditions as are shown in Fig. 81, there is no circulatory system. 
The digested food is merely distributed from cell to cell. In 
animals in which the digestive apparatus is well developed, 

FIG. 34- 

FIG. 34. A scheme to represent the circulation of the blood, in its essential features. The 
arrows indicate the course of the blood, a, arteries; aur., auricle or receiving portion of the heart; 
d, digestive tract; c. d., capillaries of the digestive tract; c.r., capillaries of the respiratory organs; 
c.s., capillaries of the system; va., valves; ve, veins; vt. t ventricle. 

Questions on the figure. What portions of the apparatus are necessary to 
secure circulation? Which secure the real objects for which the circulation 
exists? Why are valves essential ? What common work occurs in the three classes 
of capillaries figured above? What special type of work is characteristic of each of 
the three? 

some device becomes necessary for the distribution of the food. 
The body cavity with its contained fluids may do this work as 
in Fig. 31. Usually, however, when the mesodermal layers be- 
come well developed, there arises in connection with it a series 
of branching tubes containing special fluids, blood or lymph. 
These tubes by their ramifications connect the digestive sur- 
faces with the various parts of the body. Some branches like- 
wise extend to those special surfaces where the oxygen of the 
external medium may be had. Naturally then the complexity 


and the special structure of the circulatory system depend 
largely upon the position and degree of development of the 
digestive and respiratory organs. In order to secure the neces- 
sary motion of the fluids contained in the tubes, the walls of 
the latter are supplied with muscular fibres, and contract more 
or less rhythmically. If the motion is to have a definitely con- 
tinuous direction, as is ordinarily the case, valves are usually 
so placed that motion in the opposite direction will be im- 
possible. The (one or more) contractile regions are called 
hearts; vessels conducting blood from the heart are arteries, 
those carrying blood toward the heart, veins. In the region 
where the vessels are smallest and have very thin walls, the 
exchanges between the blood and the other tissues occur. This 
is the region of capillaries. The blood system has capillaries 
in the walls of the digestive tract, in the respiratory organs, 
in the kidneys, in the liver, and in and about all the tissues re- 
ceiving a direct blood supply. The capillary region is that for 
which the rest exists; it is the physiologically important part 
of the system. Fig. 34 illustrates the arrangement of parts 
found in a common type of circulatory apparatus. 

96. Demonstration. Circulation of blood in tail of tadpole; in the web of the 
foot of a frog; or in the fin of small fish. Distinguish veins and arteries. Notice 
behavior of corpuscles in passing through small capillaries. Compare rate of flow 
in vessels of different size. 

97. The Excretory System and Function. Beside the car- 
bon dioxid eliminated from the blood in the lungs or gills, other 
waste products of oxidation are to be removed from the tissues 
where they are produced. Important among these are the nitrog- 
enous wastes, urea and uric acid. In organisms in which 
there is no regular blood system, these waste products may be 
carried directly from the tissues to the surface by a system of 
tubes beginning as capillaries. In the majority of animals the 
canals (nephridia) pass from the body cavity to the exterior. 
These are seen in a simple condition (Fig. 3 5) in the segmented 
worms. For a more unified condition see unsegmented worms 
(Figs. 88, 89). The kidneys of higher forms are considered 
to be derived from these. In the higher animals the kidneys 


have a special blood supply and the waste products are extracted 
from the blood while it is in the kidney. 

98. The Skeletal System and its Functions. The cells of 
the body frequently excrete from themselves materials which, 
while no longer of use to the protoplasm, are not entirely re- 
moved from the body by the blood and serve important passive 
functions. These excretions or secretions may surround and 
protect and give rigidity to the cell itself (e.g., cell- walls; 
shells in the single-celled animals), may bind the softer cells to- 

FIG. 35- 

PIG. 35. Diagram of a nephridium (simple kidney tubule) of a Segmented Worm. b., b'., 
blood vessels; c, ccelom; d, duct of the nephridium; e, external opening; cf, ciliated funnel opening 
into ccelom; gl., glandular or secreting portion; s, septum; W, body wall composed of longitudinal 
muscle fibres, circular fibres, and epithelial layer; w, wall of gut. 

Question on the figure. Judging from its relation to the coelom, to the blood 
vessels, and to the outside world what would seem a reasonable function for the 
nephridium ? 

gether into a resistant tissue (intercellular substance in bone, etc.) , 
or may be secreted at the surface of the organism as a whole 
(cuticula in insects and shell in mollusks). The hard parts 
serve primarily for the support and protection of the softer 
tissues. Incidentally they come to serve a very important use 
as points of attachment for muscles. The skeleton may be 
external (the integumentary skeleton, or eocoskeleton) as in 
crayfish, or internal, as the endo skeleton of vertebrates. In 
many instances both kinds of skeletal structures may occur 
simultaneously, yet it is usually true that if the exoskeleton 
is well developed the endoskeleton will be poorly represented. 
Each has important advantages and limitations. To allow 


motion as the result of muscular action the skeleton, if rigid, 
evidently must be in segments and jointed. The articulation, or 
joint, is as important for motion as the harder parts. Clearly 
the muscles, to do any work on the skeleton, must attach on 
opposite sides of the joint. 

99. Growth. There are no special organs of growth, yet 
growth is one of the most immediate and important manifesta- 
tions of the nutritive process. Growth is to be defined as in- 
crease in volume or mass and may result from either or all of 
three processes: viz. (i) absorption of water, (2) formation of 
protoplasm and the multiplication of cells, and (3) formation of 
non-protoplasmic cell-products, either within or among the cells. 

The rate and character of growth are modified by such ex- 
ternal conditions as temperature, light, quantity and quality 
of the food supply, etc. Growth does not continue indefinitely. 
Its continuance is determined by the relation of the anabolic 
to the katabolic processes in the body. The time conies in the 
life of every complex organism where the income no longer 
equals the outgo, and growth must cease. Later still the wear 
is not made good by the income, and death results. Just what 
causes organisms to cease growing we do not know. 

100. Reproduction and the Reproductive Organs. Since 
individual organisms are limited both with regard to growth 
and length of life, it is apparent that a given class of forms 
cannot continue, unless some method of originating new in- 
dividuals be found. This production of new individuals by 
the instrumentality of the old is reproduction. In many of 
the lower animals this is merely a growth process, "growth be- 
yond the limits of the individual." In the single-celled ani- 
mals reproduction means the formation of the protoplasm into 
two or more masses, by dividing into two equal parts (divi- 
sion), by breaking into a large number of sub-equal portions 
(fragmentation), or by budding (Chapter III, 40). In bud- 
ding there is the formation of a local outgrowth which ulti- 
mately attains the size and character of the parent. In division 
the resulting individuals cannot be distinguished as parent and 
offspring. Such reproduction, involving only one parent, is 


asexual. It usually occurs when the adult size of the animal 
is attained. It is not confined to the Protozoa or single-celled 
animals, but may occur in several Invertebrate groups in which 
(Hydra, Fig. 81) there is not a high degree of specialization. 
The budded individual or offspring may in such cases consist 
of one cell or of many. In addition to the internal stimulus af- 
forded by the attainment of normal size, external conditions 
such as diminished food supply, temperature changes, etc., in- 
fluence the process of non-sexual reproduction. 

101. Sexual Reproduction. It seems for some reason that 
even in the one-celled animals the method of reproduction by 
division cannot be continued indefinitely without some ill effects 
to the organism. In many Protozoa there is at certain times a 
union of two individuals, either temporarily or permanently, 
accompanied by exchange of nuclear material or by a fusion of the 
whole protoplasm. After a period of rest and the coming of fa- 
vorable conditions of life, division begins again with renewed 
activity. Something similar is seen in the more complex animals 
the Metazoa. After a period of cell divisions, by which the 
individual body is built up, the majority of cells, as muscle or 
nerve cells, appear to lose their power of dividing, and even the 
less differentiated cells which we have described as the ova and 
sperm, which arise from the primordial germ cells ( 49), are in- 
capable of continuing the division necessary to produce a new 
individual until they have been stimulated by union with each 
other (or by some artificial means). Such unions of cells, to 
form by later divisions a new individual, are called conjugation 
or fertilization, and the new individual which results is said to 
arise by sexual reproduction. The uniting cells may be similar 
(as in Pandorina), in which case the union is isogamous. More 
usually the cells are different and the union is heterogamous. 
In the latter case the cells are called ovum and sperm (Chapter 
IV) and are usually formed in different individuals, though 
very often the same individual may give rise to both classes of 
cells (hermaphroditism) from different regions of the germinative 
epithelium, or in the same organ at different times. The 
special organs in which the ova are produced are called ovaries. 


The sperm cells are formed in testes. The individuals (that 
is, the male and female) producing the different classes of cells 
are often very different in other respects also. This is known 
as sexual dimorphism (Chapter VII, 149). 

1 02. Practical Exercises. Compare the males and females of the various animal 
types with which you are familiar. In what groups of animals does non-sexual 
reproduction occur? Give the gist of Geddes and Thompson's theory as to the 
origin of sexuality. What is the view based on the discoveries of Mendel? Com- 
pare any other theories available to you. What are the conceivable advantages 
and disadvantages of the asexual method? of the sexual? of hermaphroditism ? of 
sexual dimorphism? 

103. Movement and the Muscular System. The desira- 
bility of motion in animals arises from the necessity of seek- 
ing food and of escaping unfavorable influences. These con- 
ditions constitute the most imperious stimuli to which the or- 
ganism is subject. We have already seen (20, 24) that the 
fundamental irritability and contractility of protoplasm make 
this response possible in the simplest conditions. In somewhat 
higher forms, specially developed protoplasmic fibrils appear, 
such as cilia, or the fibrils in the -stalk of Vorticella, in which 
the power of contracting is strikingly manifest (see Figs. 68 
and 70). While this is found in Protozoa, it is much more 
clearly shown in the muscular tissue (Fig. 30) of still higher 
animals. Locomotion varies in efficiency in different animals 
not merely on account of varying muscular structure but in 
accordance with the arrangement of the hard parts to which 
the muscle fibres are attached, and the nature of the medium 
which must be penetrated. Many aquatic forms, though free- 
swimming in their early stages, may become attached and give 
up the power of locomotion in the adult condition. Such 
attached or poorly moving forms ordinarily secrete an external 
shell or covering into which they can withdraw for protection 
(e.g., barnacles, many polyps). They must depend upon cur- 
rents in the water to supply them with food . They are frequently 
able to produce the currents by the motion of parts of the body. 
The majority of active movers have hard parts which serve 
as levers to which the muscles are attached. The parts of the 
skeleton, which may be either external to the muscles or sur- 
rounded by them, articulate with one another by a hinge or 


movable joint, as illustrated by vertebrates or insects. In 
some forms without a conspicuous skeleton, as the earthworm, 
there is a der mo-muscular wall surrounding a fluid-filled cavity. 
Locomotion is effected in these by the alternate use of the 
longitudinal and circular fibres, changing the relative position 
of the parts of the body. The special appendages, particularly 
the paired appendages, are important motor organs in nearly all 
actively moving animals. 

104. Sensation and Sensory Structures. In a simple bit 
of protoplasm it is manifest that the differences between the 
living matter and the outside world are greater than the struc- 
tural differences between the parts of the protoplasm itself. 
Thus we would expect the stimuli arising from the action of 
environment upon the living material to be among the most 
important experienced by the organism, and that the superficial 
protoplasm by virtue of its irritability (see also 20) would most 
promptly feel and respond to such stimuli. The changes thus 
instituted will be felt sooner or later to the remotest parts of the 
cell mass. This transfer of the effects of a stimulus through a 
longer or shorter distance introduces us to a second nervous 
function, internal irritability of protoplasm or conductivity. 

105. As an organism increases in the number and variety 
of its cells, the specialized structures need to be more com- 
pletely bound to one another. It becomes necessary not only 
that they receive impulses from such parts as are favorably 
situated for the reception of stimuli, but that a degree of co- 
ordination of the interrelated parts be secured, in order that 
just such response shall be made as will best meet the needs 
of the organism. This power of coordinated response to ex- 
ternal stimuli makes it possible for an organism to become suited 
to its environment. 

1 06. In the higher forms, the work above described de- 
mands five classes of structures (see Fig. 36) : (i) end or sense 
organs, which are specially sensitive to stimuli of different orders, 
as mechanical (touch), chemical, ethereal (light), etc.; (2) con- 
ductive tracts (afferent nerves'), which connect (i) with (3) cen- 



tral nervous structures (ganglion-cells) where the impulse is re- 
ceived and suitable responsive impulses are originated; (4) con- 
ductive tracts (efferent nerves), which make the work of the 
central organs of value by carrying an impulse which produces 
corresponding activities in (5), some form of actively related 
cells, muscular, glandular, or nervous. It is readily apparent 
how increase of volume and differentiation of the other parts will 
make necessary a more complicated nervous system. The spe- 
cial arrangement of these parts of a complete system differs very 

FIG. 36. 


PIG. 36. Scheme showing the essential relations of the parts of a nervous system: i, the sensory, 
end organ (epithelial); 2, afferent nerve tract; 3, central nervous cells (ganglia); 4, efferent nerves 
leading to 5, muscle, gland, etc. g, ganglion cells; gl., gland; m, muscle fibre; ./., nerve fibre; s.e.. 
sensory epithelium. 

Questions on the figure. What seems to be the function of the various parts 
or elements in this scheme? Your reasons for your view? 

much in various animal groups, yet it may be said that there is 
a progressive accumulation of the central nervous matter at the 
anterior end of the body as we ascend the scale of animal life. 
When this concentration is well advanced the mass of nervous 
matter is called the central nervous system which always includes 
the brain. The nerves passing to and from the central part and 
their endings, taken collectively, are described as the peripheral 
nervous system. 

107. Arrangement of the Central Nervous System. The 

ganglion cells composing the nervous system may be so scat- 


FIG. 37. 

FIG. 37. Diagram shoving arrangement of the nervous matter in Starfish, c, ganglionated ring 

about the mouth; o, mouth; r.n., radial nerve in each arm. 

FlG. 38. The nervous system of the Clam, from the dorsal aspect, c, anterior; o, mouth; e.g., 
cerebral ganglia (brain); p.g., pedal ganglia; v.g., visceral ganglia. 

Questions on Fig. 37. Describe in your own terms the way in which the 
principal nerve elements are arranged in the starfish? Compare it with those 
which follow. In what respects similar? In what unlike them? 

FIG. 39. 

FIG. 40. 

FIG. 39. Arrangement of the nervous material in the anterior end of an Oligochete Worm, 
seen in profile. That part of the body wall nearest the observer is supposed to be removed, a, 
anterior; b.w., body wall; g, dorsal ganglia (brain); g', ventral chain of ganglia; n, nerve ring around 
the pharynx; o, mouth; p, pharynx. 

FIG. 40. The central nervous system in a Leech. Lettering as in Fig. 39. 

Questions on figures 38, 39, and 40. How is the nervous matter related to the 
digestive tract and to the animal as a whole in all of these figures ? Compare with 
figures in other texts. Is there any apparent correlation between the form, 
symmetry or segmentation of the animal and the arrangement of the nervous 
material? Can you state your conclusion as a law? 


tered through the superficial layers as scarcely to deserve the 
name ''central" (Hydra). The nerve cells may be arranged 
in a ring about the mouth, or gullet, with or without addi- 
tional bands of nervous tissue, containing cells, passing radially 
from it (as in echinoderms and some ccelenterates ; see Fig. 
37). In the higher invertebrates this process of concentration 
continues and the ganglionic cells are collected into two or more 
ganglia connected by nerve fibres (commissures). Usually 
a pair of ganglia occurs in the region of the mouth, and dorsal 
to it (e.g., clam, Fig. 38). In segmented forms, as the earth- 
worm and crayfish, there is also a series of ganglia connected 
by fibres, ventral to the digestive tract. This chain is in turn 
connected with the dorsal ganglia by a loop of fibres passing 
round the esophagus (Figs. 3 9 and 40) . In vertebrates the cen- 
tral nervous system consists primarily of a tube with nervous 
walls the spinal cord which may be specially enlarged and 
thickened at the anterior end to produce the brain (Fig. 172). 
From the various parts of this cord the nerves take their origin, 
and run to all parts of the body. 

1 08. The Peripheral Nervous System: Sense Organs. 

We know by experimentation that in the lowest animals even, 
or for that matter, in protoplasm, certain external conditions 
produce definite responses or changes. We also know that 
these external happenings and their responses, in our own 
case, are accompanied by certain sensations, as touch or taste. 
By inference, both from the nature of the response and from 
the structure of organs, we reach the conclusion that the lower 
vertebrates and higher invertebrates experience sensations in 
some degree similar to our own. The classes of possible stimuli 
have already been mentioned (21). Those producing in us 
definite sensations are: simple contact stimuli, producing the 
sensation of touch and pressure; vibratory contacts, giving rise 
to hearing and temperature sensations; gravity, giving us our 
sense of position in respect to the pull of gravity; chemical ac- 
tions, making possible sensations of taste and smell; ethereal 
vibrations, producing the sensation of light. In the lowest 
forms of animals there are no specialized organs for the reception 


of particular stimuli, and in such cases it is reasonable to infer that 
the distinctness of the sensation cannot be very great. In 
almost all animals, however, certain areas are specially suited 
to be stimulated by special stimuli. 

109. Touch. There are two principal ways by which contact stimuli are 
received among animals. Fibres of the central nervous system may pass to the 
skin and end among its outer layers as free nerve terminations, or these fibres may 
become intimately united with one or more of the cells of the epithelium. The 
most common of the tactile organs in vertebrates are of the first class. Where the 
stimulus reaches the nerve through a nervous epithelium, the epithelial cells often 
have special developments such as hairs, bristles, and the like, whereby the 
possibility of contact with external objects is increased (Fig. 19, A, s). The 
appreciation of changes in temperature is also associated with the general skin 

no. Chemical Sense (including taste and smell). It is impossible for us to 
distinguish between taste and smell in the lower animals. Indeed it is with 
difficulty, in some instances, that we separate the sensations obtained from the two 
sources even in our own case. Almost all animals seem to have some power of 

FIG. 41. 


FIG. 41. Statocyst in a Mollusk. n, nerve; o, statolith; s.c., sensory cells in, wall of statocyst. 

(After Claus.) 

Questions on the figure. What immediately stimulates the sensory epithelium 
in this case? What kinds of general agencies might be supposed to produce the 
necessary motion for this purpose? What is the present view of the function of 

appreciating the chemical condition of the medium in which they live. In aquatic 
animals the chemical sense organs may be distributed over the surface of the 
body. In the higher animals they collect more and more at the anterior or mouth- 
end of the animal, with manifest advantage to the animal. In the higher land 
forms, especially the vertebrates, the organs of the chemical sense come to lie in or 
about the mouth and nose, the beginnings of the digestive and respiratory tracts 
respectively. These senses are specially related to the testing of food and the 
medium in which the animal lives. For this purpose their position in the mouth 



and nose is especially favorable. The senses thus far enumerated seem among 
the earliest developed in the animal kingdom. 

in. Hearing and Equilibrium Sense. It is by no means certain that the lowei 
animals possess the ability to appreciate those vibrations in matter (air, water, 
etc.), which arouse in us the sensation of sound. There are in several groups of 
such animals organs, the structure of which would suggest that they might receive 
vibrations of the medium in which they live. In their simplest condition they 
consist of a sac (otocyst or statocyst) derived from the ectoderm and lined by an 
epithelium containing sensory cells. From these cells sensory hairs extend into 

FIG. 42. 

FIG. 42. Antenna of Male Mosquito (Culex pipiens). By J. W. Folsom. 

Questions on the figure. Compare with the antennae of a female (see Fig. 65). 
What are the differences between the head of the male and female mosquitoes? 
What is believed to be the function of these plumose antennas? What are the 
evidences for this view? 

the cavity (Fig. 41). The cavity contains a fluid which may support one or more 
solid particles (statoliths} . With the vibration of the medium the whole would be put 
into vibration, but the inertia of the contained fluid and statoliths would cause the 
latter to strike against the hairs and thus serve as stimuli to the sensory cells. 
Late researches tend to prove that these structures are organs enabling the organism 
to appreciate the pull of gravity and movements in the water rather than to hear. 
In higher forms the ear becomes immensely more complex, but the general condi- 
tions both of origin and structure appear to be much as described for the statocysts. 
That is to say, the final sensory cells are ectodermal in origin, but now line a sac 
deeply imbedded in the tissues of the skull. In some of the lower animals, as insects, 



there are also external vibratile hairs and vibrating membranes which are auditory 
(Fig. 42). 

112. Sight. There are three distinct facts to be noted with respect to visual 
sensation in the higher forms of animals: the perception of light, the perception of 
color (i. e., light of different wave-frequency) and the formation of images of ex- 
ternal objects. It has already been seen (21) that protoplasm is sensitive and 
responsive to light without any special organs. The simplest visual organs found 
in multicellular animals consist merely of epithelial cells containing pigment in 

FIG. 43. Diagrams showing some of the stages in the increasing complexity of the simple eye 
in Invertebrates. A, simple pigment spot in epithelium having nerve-endings associated with 
pigment cells (as in some medusae); B, pigment cells in a pit-like depression (as in Patella}; C, with 
pin-hole opening and vitreous humor in cavity (as in Trochus); D, completely closed pit, with lens 
and cornea (as in Triton and many other Mollusks); E, pigment area elevated instead of depressed, 
lens of thickened cuticula (as in the Medusa, Lizzta); F, retinal cells more highly magnified, ep., 
epidermis;/, nerve fibre; /, lens; op, optic nerve; p, pigment cells; r, retina; v.h., vitreous humor. 

Questions on the figures. What changes take place in the sensory epithelium 
in this series of figures? What is gained by such a depression as occurs in B? 
What purpose is served by the pinhole and the vitreous humor of C? Describe 
the change from C to D. What is gained? What may be the function of the 
pigment? Compare texts. In what respects does E differ from the other types? 
What two types of cells are figured as belonging to each retina? What constitutes 
the retina? 

which changes are wrought apparently by the action of light (Fig. 43, a). These 
changes affect the nerve fibres associated with the pigment cells and thus the 
central nervous organ. Such eyes are capable only of giving knowledge of the 
intensity or, if properly constructed, of intensity and direction of the light and do 
not form an image of external objects. There are several types of image-forming 
eyes in the animal kingdom. The most familiar of these is the "camera eye" of 
vertebrates, so called because it illustrates the principles made use of in the 


construction of the photographic camera. In this there is a lens or body which 
refracts the rays of light in such a way that all the rays passing from a point of the 
object are brought to a focus at a point on the retina. Another type of image- 
forming eye is the compound eye of insects and Crustacea (Fig. 44). These are 
made up of a large number of eye elements each structurally complete in itself 
whose separately formed images must nevertheless be joined in order to form a 

FIG. 44. 



JL n 

PIG. 44. Diagram illustrating the compound eye of arthropods. A, the whole eye shown in 
section; B, one of the eye-elements (ommatidium) more highly magnified, c, cuticular facets; ep, 
epidermis; /, group of cells forming lens-like body; n, optic nerve fibres; o, optic ganglia; p, pigment 

Questions on the figures. In what way is the independence of each ommatidium 
secured? In other words what is to prevent the light which comes in obliquely 
from passing from one ommatidium to another? In what conceivable way is a 
general image obtained from these various partial views? What groups of animals 
possess eyes of this sort? Compare the diagram B with the figure of the complete 
ommatidium of the lobster (Fig. 127). 

The degree to which the color-sense is developed among lower animals is very 
uncertain. The simplest animals may respond differently to light of different 
colors, but this is a very different thing from saying that they possess the color- 
sense in the meaning that we give to the word. 

To summarize, the essential part of the eye is the sensitive layer known as the 
retina. The other parts of the complex eye-structure serve the purposes of shutting 
out the light except from certain directions; of focusing the light admitted in such a 
way as to increase its intensity and form an image on the retina; of adjusting this 
apparatus to objects at different distances; of nourishing and supporting the more 
important portions of the apparatus; and of moving the eye so as to take into view 
different portions of the surroundings. Some of the various grades of complexity 
of eye-structure in the invertebrate series beginning with a pigment spot and 
ending with a complete lens-eye, are shown in Fig. 43. 


113. Analogy and Homology. In comparing various ani- 
mals we find that they may do the same work with organs that 
arise in very different ways, which, however, because they are 
adapted to perform similar tasks, look somewhat alike. Such 
structures are said to be analogous (as the wing of a bird and the 
wing of a butterfly). In other cases organs that originate in 
the same way may have been used so differently as to have a 
very different appearance, as the various "legs" of the crayfish, 
or the wing of a bird and the arm of man. These, notwithstand- 
ing their superficial differences, are said to be homologous because 
of the fundamental equivalency of structure. 

FIG. 45- 

FIG. 45. Diagrams illustrating two stages in the development of the vertebrate eye. A, 
showing the relation of the ectoderm, the brain vesicle, and the optic vesicle. The right side of the 
figure shows a later stage than the left. B, later stage, showing the lens, eye-ball and retina in 
position, b.v., brain vesicle formed by the invagination of the ectoderm (ecf.); I, lens; mes., meso- 
dermal tissue; o.n., optic nerve; o.s., optic stalk; o.v., optic vesicle, a portion of the brain vesicle; 
r, retinal layer; v.h., interior of eye-ball which comes to contain the vitreous humor. 

Questions on the figures. Which portions of the eye are derived directly from 
the ectoderm? Which indirectly, i.e., from the brain? Which portions seem of 
mesodermal origin? By following the invagination by which the retina is formed 
do you find any suggestion of an explanation of the fact that the sensitive portion 
of the retina (rods and cones, Fig. 175) is directed away from the light? Refer to 
some work on the embryology of the vertebrates for more complete series of figures. 

114. Differentiation and Reintegration of Parts. The chap- 
ter thus far should have given the student a good appreciation 
of the degree to which the work of a complex organism is divided 
up among the organs and of the differences found in the organs 
themselves because of this. We all know that this differentia- 
tion makes for efficiency. There are two sides to this, however. 


In becoming suited to doing one kind of work the cells cease, to 
be able to do other kinds. The various cells thus become mutu- 
ally dependent. Unless they can be brought to work together 
suitably there will not be efficiency. Division of labor is only 
one-half of efficiency. There must also be a reintegration of 
these special parts into a higher kind of oneness. It is this that 
determines the state of the individual made up of these special- 
ized parts. There are three phases of this reintegration. 

a. Physical Reintegration. This is done by the cement be- 
tween cells, by the connective tissues, the skin, the skeleton, 
and similar tissues. When the fertilized egg first divided, this 
cement held the cells together. It is only in this way that a 
multicellular animal could arise. In the mature body the bil- 
lions of cells are further bound together by these connecting tis- 
sues into a physical unity that makes it possible that they work 

b. Nutritive Reintegration. This physical bond would mean 
little but for a deeper one. In the differentiation of the cells , 
while all must have food and oxygen and all produce wastes 
which they must get rid of, the various cells of the body do differ- 
ent things. They have somewhat different needs and produce 
different kinds of waste products. Some cells absorb the foods 
for all; others absorb oxygen. Some eliminate carbon dioxid 
from the system; others eliminate the urea and other wastes 
for all the cells. It follows therefore that all these cells must be 
connected in a nutritive way. This is done by the body fluids, 
including chiefly the blood and lymph. 

In addition to the work of lungs, digestive epithelium, the 
skin and kidneys referred to above, with which all the active 
cells of the body must be connected in this chemical interchange 
the liver, the spleen, the sex glands, the thyroid, the adrenals 
and other groups of cells take from or add to the blood sub- 
stances that vitally change it. Very often these special sub- 
stances poured into the blood by particular organs, as the testes 
or the thyroid, will be carried all over the body by the blood and 
will produce profound effects on the well-being of other groups of 
cells far removed from them. For example, it seems to be a 
product of the sex glands which gets into the blood and produces 


growth in the voice box and vocal cords in man. If the thyroid 
is diseased or removed, so that its product is no longer poured 
into the blood, profound changes take place in certain other 
structures in wholly different parts of the body. If the healthy 
thyroid tissue of another individual is grafted at any point in 
the body of such a diseased individual the latter will become 
normal again. These organs are believed to secrete substances 
(hormones) into the blood which control activity elsewhere. 

Indeed we are coming more and more to understand that 
every cell in the body changes the blood in some degree for all 
the other cells of the body. It takes from the blood as it passes 
and at the same time pours out into the blood the by-products 
of its own living. We must think of the blood as more than 
just a means of connecting lungs and intestine and kidneys and 
the general system. The life of every cell in the body is con- 
ditioned by the make-up of this medium whose composition is 
determined by every cell in the body. We are just beginning to 
appreciate its full meaning; but this chemical integration of the 
body is profoundly important. 

c. The Coordinating Reintegration. In addition to the phys- 
ical and chemical unity, the parts of the body must work in 
harmony. When food is eaten the digestive glands must be- 
come active. When the body exercises the breathing must 
become more rapid. When an organ acts it must have an in- 
creased blood supply. When we undertake to walk, scores of 
muscles must contract just enough and in the right relation to 
other muscles. When external conditions operate we must be 
able to do the right thing. This harmony of operation among 
the parts is coordination. Harmony between the action of the 
organism and the outside conditions is adaptation. The nervous 
system reintegrates these numerous diverse units of which we are 
made, and makes them work together. All the active cells of 
the body are connected by way of the nervous elements. A part 
of the work of the hormones is also of this higher coordinating 
value. It has been found by experiment that the pancreas, for 
example, is brought to secrete at the proper time by substances 
in the blood absorbed from the digestive tract rather than 
through nervous action purely, as we formerly thought. 


115. Summary. 

1. Division of labor and differentiation of structure proceed 
together as the individual develops. All tissues retain the 
power of using food, of oxidation, of eliminating useless prod- 
ucts. Other functions incidental to these may be relegated to 
special cells or tissues. 

2. Associations of tissues to accomplish a more or less definite 
work are called organs. Organs of a similar kind are collectively 
known as systems of organs. 

3. The principal functions of animals and the organs or 
systems performing the work may be classed as follows: 

Function System 

(a) Metabolism Nutritive. 

(6) Support and protection. . . Skeletal, and integumentary. 

(c) Growth 

(d) Reproduction Reproductive. 

(e) Motion Muscular, in connection with 


(/) Sensation Nervous, including sensory epi- 

4. Metabolism or nutrition embraces the following processes: 
(a) Ingestion of food (including oxygen), 

(6) Digestion, 

(c) Absorption (from the digestive tract, and at every other 
cell wall), 

(d) Circulation, 

(e) Assimilation = anabolism, 

(/) Dissimilation = katabolism (including respiration), 
(g) Secretion and excretion (of waste matter including carbon 

The processes in (a), (b), (c), and (e) are anabolic, i.e., add 
to the resources of the body. Those in (/) and (g) are katabolic, 
i.e., tend to destroy the materials, develop energy, and eliminate 
waste. Circulation contributes to the accomplishment of both 

5. The supportive skeletal structures may be internal, or 
external, or both. They may arise as a secretion of the super- 


ficial cells of the body, or consist of a mixture of cells and in- 
ter-cellular substance. Their nature and arrangement are 
profoundly important in determining the distribution of the 
other more active organs. 

6. Growth and reproduction are the outcome of the nutri- 
tive processes. Growth is increase in mass; reproduction is the 
production of new individuals from old. Reproduction always 
involves cell division and may be asexual or sexual. The latter 
normally involves two parents. In it two cells, which may be 
either similar or dissimilar, must unite before development will 

7. The nervous and muscular systems are closely related 
in function. Their united work is to receive, coordinate and 
respond to the external or internal stimuli affecting the animal. 
The structures to receive stimuli (end organs) are largely super- 
ficial; the coordinating and controlling parts (central organ) are 
deep-seated, thereby securing protection and a central position; 
the muscular system must have definite relations with the hard 
parts upon which it acts. Thus arises the necessity of connec- 
tives or nerves between the various portions. 

8. The sense organs represent areas of the epithelium which 
are peculiarly adapted to the reception of some one of the forms 
of stimulus to which animals are subject, supplemented by a 
more or less complex apparatus which serves to intensify or 
modify the original stimulus. The sense organ determines the 
kind of stimulus which may be received, but the central nervous 
organ determines the nature of the sensation which results, and 
the response. 

9. If cells and organs become differentiated, and specialized 
in the division of labor, it becomes doubly essential that they be 
brought together again in a complete unity if there is to be an 
efficient individual. This reintegration of diverse parts is (i) 
physical, by means of connective and supportive tissues; (2) 
chemical and nutritive, by means of blood and lymph; and 
(3) cooperative, by means of the nervous system. 

10. Organs with different origin which by reason of similar 
function have come to look alike are said to be analogous. 
Organs with similar origin and structure, even though they may 


appear differently because of their differing functions, are 

1 1 6. Topics for Investigation, in Library and Field. 

1. What are the advantages and the disadvantages of divi- 
sion of labor and differentiation? Illustrate your views very 

2. Illustrate the variety of foods used by different animals 
with which you are acquainted. Classify the animals you know 
on the basis of their food preferences. 

3. Compare the ways in which animals known to you cap- 
ture and prepare their food for swallowing. What special 
structures arise in connection with this function ? 

4. Do animals have any power of storing food within the 
body for future use ? Compare with plants. 

5. Compare gills and lungs as to general form and arrange- 
ment and see in what ways they appear to you to be suited to 
their particular media, i.e., gills to water and lungs to the air. 
Why might not the conditions be reversed ? 

6. What seem to you to be the comparative advantages and 
disadvantages of the exoskeleton and endoskeleton. 

7. Devices to accomplish locomotion in animals known to the 
student. Find as many variations as possible. 

8. Select four animals, as diverse as possible, representing 
each of the following conditions of locomotion : through the air, 
through the water, on the earth, and through the soil. Compare 
the problems which each must solve, and the organs by which 
the work is accomplished. 

9. Compare known animals as to rate of locomotion. Do 
you find a satisfactory explanation in any case ? 

10. Let the student attempt to prove that the dog experi- 
ences the same sensations which we have. Hold him rigidly to 
his evidence. 

11. Report on the general differences between the eyes of 
insects and of vertebrates, with a statement of their structure 
and the work done by each. 

12. In what way could the statocysts possibly act as organs 


to enable the animal to appreciate its position in space, and thus 
maintain its equilibrium ? 

13. What are the simpler facts connected with the process 
of absorption or osmosis of dissolved substances in the body ? 

14. Find in text-books of chemistry a fuller account of the 
process of oxidation and why it results in a liberation of energy. 

15. Demonstrate how a biconvex lens forms an image of 
objects. Why inverted ? 



117. We have seen in the preceding chapters how the work which an organism 
must do is divided among its parts, and that the parts become specialized in con- 
nection with this division of labor. This complexity which is known as organiza- 
tion is, in any animal, the result of forces both within and without the animal, and 
expresses the adaptation of the internal structures to each other and to external 
conditions. The simplest organism known is thus organized. Organization is 
merely more evident in the more complex organism. In addition to the fact of 
the organization and heterogeneity of structure it is easy to see, after examining a 
number of animals, that these different parts are not thrown together without 
some definite order. For example, the ordinary vertebrates move with their long 
axis horizontal, and possess certain organs that we always expect to see at the 
anterior end; their appendages are arranged in a definite way in relation to the 
long axis. The parts of the starfish are arranged according to a different but 
equally definite plan. All consideration of the general plan according to which 
animals are constructed may be called Promorphology. The fundamental plan 
may be similar in groups of animals which are otherwise very different, because of 
similar external conditions and similar modes of life. 

118. Definition of Sections. In trying to express the plan of structure in 
animals it is convenient to have in mind certain planes to which we can refer the 
parts. A section perpendicular to the main axis of an organism or of an organ is 
called a transverse or cross section. The longitudinal median section separating 
the body into right and left halves is a sagittal section. A longitudinal section, 
perpendicular to the sagittal and separating the dorsal (or back) and ventral (or 
belly) portions of the body is described as a frontal section. An animal is said to 
be symmetrical with regard to any of these planes when the parts severed by the 
plane are essentially similar. 

119. Axiality. As an organism grows from its small beginnings in the fertilized 
ovum, or from a spore in the simpler forms, the new materials may be added more 
or less uniformly so that a mere increase in size results; or growth may take place 
more rapidly along some radii than along others, making it depart from its original 
spherical form ; or materials or organs of one kind may occur along one radius and 
different ones on another (as in Fig. 48). These lines of special growth and develop- 
ment are called axes. We may investigate them as to their number, their space- 
relations to each other, and the likeness or unlikeness of the two ends or poles of 
each axis. 

120. Types of Symmetry. It is desirable to distinguish the following types: 
i. In a spherical organism in which no differentiation is apparent (as in a 

simple spherical cell, or blastula, Figs. 13 A, 3; 46) any plane passing through the 
centre divides it into symmetrical parts and all the axes are essentially equal 



In such a case there may be said to be an infinite number of similar axes, and the 
poles of each axis are similar. This may be described as universal symmetry. 

FIG. 46. 

FIG. 47. 

FIG. 46. Spherical cell (resting stage of Amoeba) illustrating general or universal symmetry. 
Any plane passing through the centre will divide it into two essentially equal portions. 

Question on the figure. What prevents this animal being a perfect illustration 
of universal symmetry? 

FIG. 47. Amoeba in active condition. Entirely unsymmetrical. 

FIG. 48. 

FIG. 48. Aclinomma aster acanthion, a Radiolarian with a limited number of specialized radii 
(axes), symmetrically arranged about the centre. A, whole animal with portion of two spheres of 
shell removed. B, section, showing relation of the protoplasm to the skeleton, n, nucleus; p, 
protoplasm; sk., skeleton. From Parker and Haswell. 

Questions on the figures. In what way does this species differ in symmetry 
from Fig. 46? How many specially developed axes appear to be present? By 
how many planes may the organism be divided into essentially equal portions? 

2. An organism may be wholly asymmetrical, without any definite form, the 
axes being without regular arrangement. (Amoeba in its active stages, Fig. 47, 
some Sponges.) In other instances the form may be definite and axes developed; 


but the structure is such that no plane will separate the animal into symmetrical 
parts (Paramecium and many other active Protozoa; see Figs. 68-71). 

3. As a variation of the universally symmetrical condition seen in I, a limited 
number of axes may become distinguishable from the others by some specialization 
of structure (Fig. 48). These special axes are similar and their two poles are alike. 

4. Starting again from the undifferentiated spherical form, one of its numerous 
similar axes may come to differ from all those perpendicular to it by increased or 
diminished length, or by a difference in construction. This special axis is to be 
known as the principal axis. The poles of the principal axis do not usually remain 
alike. Perpendicular to this principal axis one may select an indefinite number of 
subordinate axes which are essentially similar to one another. The poles of each 
subordinate axis are alike. Such a condition is realized in the simplest gastrulae 
(Fig. 13; A 4, B 3, C 2). Any plane including the principal axis will divide such 



ab. o. 

b 1 

FIG. 49. Diagram of Medusa, illustrating radial symmetry. A, viewed from the oral end of 
the principal axis; B, a section along the principal axis and through one of the subordinate axes ao 1 : 
o, ab. o, the oral and aboral poles of the principal axis; a, a 1 , and b, b 1 , the similar poles of the two 
chief subordinate axes. 

Questions on the figures. Are the poles of the oral-aboral axis alike or unlike? 
How many clearly differentiated secondary axes are there? What would be the 
appearance of a section midway between aa l and bb l ? Would the resulting halves 
be symmetrical? Compare this condition with the definition of radial symmetry 
in the text. Find other illustrations of radial symmetry in the figures of this 

an organism into two equal halves. In general external appearance a hen's egg 
would illustrate the type. This is the least differentiated form of what is known as 
radial symmetry. 

Two important variations from this simple condition of radial symmetry are 
found in the animal kingdom: 

(a) Special organs, such as those of locomotion and the like, may be developed 
about the principal axis. These usually come to be arranged in a limited number 
of the planes which may be passed through the principal axis. Considered from 
the point of view of the subordinate axes this means that there are a limited number 
of special axes (Fig. 49, aa l and bb l ) perpendicular to the principal axis (Fig. 49, 



o-ab.o) instead of an indefinite number as in the former case. These special sub- 
ordinate axes are usually 3, 4, or 5, or some multiple of these numbers. The num- 
ber however may be reduced to two in which the four poles are all alike. Many of 
the medusae, coral polyps, and some echinoderms illustrate this type of symmetry. 
(b) A further variation of (a) is seen in the fact that in some animals, otherwise 
similar to those described in (a), one of these special axes perpendicular to the 
principal axis comes to differ from the other. The two poles of each of the sub- 
ordinate axes are essentially alike, but are unlike the poles of the other subordinate 

FIG. 50. 

ab. o. 

Pic. 50. Diagram of the sea-anemone, illustrating another type of radial symmetry. A, 
cross section; B, longitudinal section. Lettering as in Fig. 49. c, the chamber between the mes- 
enteries (m). 

Questions on the figures. How does the symmetry of the anemone compare 
with that of the Medusa? Express the difference clearly in terms of the axes and 
their poles. Are aa l and bb l strictly similar axes? Do their planes divide the 
animal into halves? Are the four halves thus obtained equivalent? In B what 
difference in the position of the section will account for the differences on the right 
and left side of the figure? 

axis. This arrangement is found in the sea-anemone (Fig. 50). The differences 
between aa l and bb l are not usually so great that we cease to speak of the form as 
radially symmetrical. It is of importance to know that in the radially symmetri- 
cal animals the principal axis, whether longer or shorter than the subordinate 
axes, is normally perpendicular to the substratum on which the animal rests, or to 
which it is attached. 

5. If such an animal as was last described were to have its principal axis 
horizontally placed, with one of its two subordinate axes vertical and the other 



horizontal, and were maintained in this position, it would likely happen that the 
formerly similar poles of the new vertical axis would become unlike, because 
subjected to different influences. These poles are known as the dorsal and ventral 
poles. The poles (right and left) of the other transverse or subordinate axis would 
remain similar, as they are subjected in the long run to similar conditions. This 
gives us the condition, found in all the higher, free-moving animals, known as 
bilateral symmetry. It consists, to recapitulate, of (i) a main axis (antero-posterior 

FIG. 51. 

FIG. 51. Diagram of the cross section of a fish, showing the bilateral symmetry of the parts: 
dv, dorsoventral axis; rl, right-left axis. a. p., anterior appendage; b.c., body cavity; ch, notochord; 
d.f., dorsal fin; g, gut; h, heart; A. a., haemal arch; m, muscles; n.a., neural arch; sp, spinal cord; t.c- 
vertebral column. 

Questions on the figure. In what respects is the symmetry as shown in this 
cross section different from that shown in the cross section of sea-anemone? Com- 
pare carefully, and express your conclusions in terms of the axes and their poles. 
Find other figures in this book illustrating bilateral symmetry. 

axis) usually horizontal and with dissimilar poles; (2) a transverse axis, usually 
vertical (dor so-ventral axis) with dissimilar poles; and (3) a transverse axis perpen- 
dicular to the other two, horizontally placed, with poles alike (right-left axis). 
Such an animal has only one plane (the sagittal) by which it may be divided into 
symmetrical halves (Figs. 51, 31, 101). 


121. Antimeres. There is a striking tendency among organisms to repeat or 
duplicate organs or parts. This we have seen in the occurrence of similar rays 
about the main axis, in radially symmetrical animals like the starfish. Parts thus 
repeated are known as antimeres. The term is also applied to the right and left or 
paired halves of bilaterally symmetrical animals. 

122. Metameres. When the parts or organs are repeated in a linear sequence 
along the main axis, as in the segments or rings of the Earthworm, the arrangement 
is described as segmental or metameric. Metamerism may be shown both by the 
internal and external structures. The vast majority of the elongated, bilaterally 
symmetrical animals are segmented. In the higher Vertebrates it is not manifest 
externally, but is shown in the vertebrae, the nerves, etc. 

123. Appendages. Nearly all the animals, whatever the fundamental sym- 
metry may be, have appendages of one kind or another for locomotion, capture of 
food, protection, respiration, and the like. These outgrowths from the body may 
be generally distributed over the body surface (as cilia in some Protozoa and free- 
swimming larvae) ; or radially arranged, often about the mouth, as in many radi- 
ally symmetrical animals (Figs. 49, 50, 81) ; or in a right and left series in bilaterally 
symmetrical animals (Figs. 131, 137, 191). The paired appendages of bilateral 
animals may be attached dorsally, laterally, or ventrally, as determined by the 
uses they serve. They may be uniformly distributed along the axis, one or more 
pairs to each metamere, as in some arthropods, or be confined to special segments in 
certain regions of the body, as in the higher arthropods and the vertebrates. 

124. Practical Exercises. Let the student find illustrations, from the animals 
with which he is acquainted, of paired appendages with dorsal attachment; with 
lateral; with ventral. What is the work to be done by each? Does their position 
appear to be of advantage in the performance of it? Find likewise animals in 
which the appendages are clustered at the anterior portion of the body; some in 
which only posterior appendages are found, or at least are better developed than 
the anterior. Does the arrangement seem in any way related to the habits and 
surroundings of the animal? 

125. Specialization of Metameres. In the lowest segmented animals, as 
worms, the metameres are much alike in external form, in their appendages, and 
in the contained structures. In the adult insects this becomes less true, and the 
various segments are specialized for particular duties. The segments in the head 
region become very different from the body segments. The same is even more 
true of the higher vertebrates. This progressive differentiation of a distinct head 
is one of the most remarkable facts to be noted in animal development. Accom- 
panying the specialization of groups of segments in various parts of the body we 
often see the complete fusion of such similar segments for the better performance 
of their common work (as in the head of insects and vertebrates and the thorax in 
insects and Crustacea). 

126. Formation of New Segments and Regeneration. In many of the animals 
in which the segments seem of nearly equal value there is a more or less continuous 
formation of new segments. By this process the organism increases in length and 
in the number of its segments, and frequently, with the aid of a somewhat similar 
process, produces two individuals by division, as in many worms. Such a pro- 
ceeding necessitates the formation of a new head or tail in each of the daughters, 


by a segment which, in the mother, was a body segment. When such an animal 
is artificially cut in two, each half may reproduce segments like those which have 
been removed from it. This is known as regeneration. Naturally this ceases to be 
possible in animals in which the segments become more highly specialized; yet 
even in the highest animals some power of replacing lost tissue or even lost organs 
remains (as in healing of wounds, formation of a new tail by lizards, etc.). It is 
recognized as a general law that in making these repairs or healings the newly 
formed material tends to restore the symmetry possessed at the outset. 

127. Summary. 

1 . Promorphology treats of the ground plan in accordance with which the parts 
of animals are combined. 

2. Symmetry relates to the possibility of passing one or more planes through 
the animal and obtaining similar portions on either side of these planes. 

3. If no such division is possible the organism is described as without symmetry. 
If each of three mutually perpendicular planes separates the animal into equivalent 
portions, it may be described as universally symmetrical. If no plane transverse 
to the main axis can divide the animal into symmetrical parts, and two or more, 
which split the animal along the main axis, are capable of doing so, we describe the 
form as radially symmetrical. When there is only one such plane capable of 
separating the body into equal parts we have the condition of bilateral symmetry, 
which represents the highest condition of development, that of active animals. 

4. Antimeres are parts of animals repeated on different sides (two or more) of 
the main axis of the body. 

5. Metameres are parts repeated in the main axis, i.e., one behind another. 
The successive metameres may be almost entirely alike (homonomous) , or they may 
become much differentiated in the performance of diverse functions (heteronomous). 

6. From the main trunk of animals special appendages often appear. They 
usually adapt themselves to, and accentuate, the fundamental symmetry of the 
organism. They may therefore be asymmetrically placed, or uniformly distributed 
over the entire surface, or along the radii, or in pairs as in the bilaterally sym- 
metrical forms. There are typically one or more pairs to each metamere, though 
this number may be much reduced. Paired appendages, in series, are regarded as 

7. Many animals have the power of restoring by growth parts or segments 
which have been lost (regeneration). In the lower segmented forms this power 
is closely associated with the power of increasing the number of new segments in 
an uninjured animal. In heteronomously segmented animals both these powers 
are less manifest. 

128. Topics for Investigation. 

1. Determine the nature and degree of symmetry in (i) the sponge of com- 
merce; (2) skeleton of starfish; (3) crayfish or grasshopper. 

2. What is the final criterion by which you determine which is the anterior and 
which the posterior end of an animal? Justify. 

3. Find among animals of your acquaintance instances of difference between 
the dorsal and ventral surfaces as to color, form, etc., and see if you can discover 
any possible advantage resulting therefrom. 

4. What degree of difference have you ever noticed between the right and left 
halves of the body in various animals. Is perfect bilateral symmetry ever found ? 



5. Can you assign any reason for the location of the sense organs at the anterior 
end of the body? In a given individual? In the race to which the individual 
belongs? (Distinguish between cause and advantage). Do you think they occur 
here because it is anterior, or is it anterior because they occur here? 

6. Why are vehicles (made by man) bilaterally symmetrical? t 

7. Express in tabular form the differences between the poles of the three axes 
of bilateral animals, and what you can gather as to the causes of the differences. 

8. For what kind of life does bilateral symmetry fit the animal? 

9. For what kind of life does radial symmetry seem suited ? Verify by illustra- 

10. To what extent do animals seem able to regenerate lost parts? Trace 
out the conditions in various groups, as far as your reference library will allow. 

11. Is there any apparent limits to the numbers of metameres which animals 
may possess? What degree of constancy or variability in this particular can you 
discover in various individuals of the same species? 



129. The Individual and its Environment. We have thus 
far considered the mature individual as the end for which the 
various developmental processes exist. It has been seen that 
the individual becomes complex as its parts grow and become 
differentiated to do the work necessary to the well-being of the 
animal. In this differentiation the parts become dependent 
upon each other, and in the healthy state they work harmo- 
niously among themselves. It is now necessary to pass from the 
consideration of these internal structures and relations in order 
to consider the individual animal as a unit in its relations to 
everything about it, that is, to its environment. This term 
includes not merely the inanimate materials and conditions 
surrounding an organism, but in addition all living things both 
plant and animal which directly or indirectly influence it. The 
environment of no two animals is the same, nor is it the same for 
any given animal for two moments in succession. This con- 
tinual change in the environment leaves its impress on the struc- 
ture and habits of all organisms. Every individual is thus re- 
lated to its own environment from day to day; in addition to 
this, all the individuals of any generation, owing to the facts of 
reproduction, are also to be considered in the light of the con- 
ditions to which their parents have been subjected from the 
remotest time. The study of the individual in its relations to 
its environment brings the student face to face with many very 
important problems. No department of zoology is more 

130. Heredity. One does not study organisms very long 
without being impressed with two things: first, that there are 
remarkable similarities among them, even among those little 
related ; and second, that there are interesting differences among 



even those whose kinship would entitle them to great likeness. 
There is always a disposition among students to feel that the 
likenesses are due to internal causes and that the unlikenesses are 
due in some way to the varying external influences. In other 
words the former are thought to be due to heredity and the latter 
to the environment. 

Characteristics which animals receive from the union of the 
germ cells are described as hereditary. We ascribe the fact that 
the fertilized hen 's egg produces a fowl and a frog's egg a frog to 
the action of heredity. No less is the repetition in the child of 
minute parental peculiarities of feature and form a fact of 
inheritance. While these likenesses are due to the action of the 
internal forces of heredity , it must not be deemed that heredity 
is a purely conservative influence in the life of the organism. 
The offspring of two parents may inherit qualities entirely 
different from their parents and thus present differences 
among themselves due solely to inheritance. The offspring 
may also present such a mingling of the qualities inherited 
from their ancestors as to possess characteristics decidedly 
new to any of them. Thus likeness to parents and unlike- 
ness to parents may equally be due to heredity. It at once 
perpetuates old qualities and introduces variations. 

It was formerly considered that all the characteristics which 
parents possess are equally subject to inheritance, but it is now 
denied by many biologists that the qualities which a parent 
acquires in its own lifetime, as the result of its own actions or of 
the environment, are capable of being transmitted. In the 
light of what was said about the parallel development of prim- 
ordial germ cells and body cells (49), this is just another way 
of saying that changes that come to the body cells may not spe- 
cifically change the germ cells so that they will later rise to the 
same sort of a change in the body cells which arise from them. 
It is unquestioned, however, that qualities received by inheritance 
from the preceding generations are, under favoring circum- 
stances, capable of being transmitted to the following. 

131. The Bearers of Heredity. It follows from the fact 
that the adult organism is produced from the union of the male 


and female elements that these two cells are in some way en- 
dowed to carry the parental qualities. They are all that pass 
from the old generation to the new. There are strong evidences 
that the chromatic elements, or chromosomes, in the nuclei of 
the male and female cells are the most important material struc- 
tures by means of which transmission is effected. The chromo- 
somes of the fertilized ovum are contributed equally by the male 
and female elements, and they are the only structures in the 
sperm and ovum which are apparently equal in amount. This 
taken in connection with the fact that one parent does not seem 
to have any more power than the other, on the average, to influ- 
ence offspring, furnishes a basis for the belief that the chromo- 
somes are the physical basis of heredity. 

Many good students of the subject believe that the cytoplasm 
is, equally with the chromosomes, a carrier of heredity qualities. 
They think that cytoplasm is responsible for those qualities 
that both sexes have in common. That is, the cytoplasm con- 
serves the character of the species. The chromosomes are held 
to be more responsible for those qualities in which the individuals 
of a species differ. Recent investigations seem to show that 
the male and female chromosomes retain their distinctness in 
large measure and are equally distributed to all the nuclei of 
the developing embryo. 

132. Library Exercises. The student may increase his knowledge of the facts 
of heredity by endeavoring to find answers to the following questions. What is 
atavism, and what explanations have been offered for it? Do the male and female 
seem, as a rule, to have equal power of transmitting their individual characteristics? 
Cite some facts tending to show that the nucleus is especially concerned in trans- 
mitting parental qualities; that the chromosomes are instrumental therein. What 
are the essential features of the old " perf ormation " hypothesis to account for the 
fact that an adult similar to the parent springs from an egg? Examine some of 
the principal theories of inheritance: Darwin's "pangenesis;" Brooks' modifica- 
tion of it; Weismann's "continuity of germ-plasm," etc. What is Mendel's law 
of inheritance? 

133. Variability. Notwithstanding the fundamental like- 
ness existing between parent and offspring, and among the off- 
spring of common parents, no two individuals even among the 
lowest animals are exactly alike. This fact of variation is only 
less fundamental than the fact of likeness. Variation among 


animals appears to depend upon two sets of considerations: 
(i) the physical and chemical instability of the protoplasm of 
which animals are so largely composed, and (2) the diversity of 
the environment in the broadest sense. Through the interaction 
of these two influences, even if all individuals were alike at the 
start, it would only be a question of time until the offspring 
derived from them would present noteworthy differences. 
Such differences would tend to increase with the lapse of time. 
This is the more true in proportion to the degree in which varia- 
tions are capable of being transmitted under the influence of 
heredity. So far as we can tell organisms may vary to any 
amount if we give time enough. 

134. The Part Played by the Environment in Producing 
Variation while not completely understood must be recognized 
as very real. Even though much stress must be put upon the 
hereditary complexity and instability of protoplasm as the 
source of variations, it is evident that the external conditions 
serve as stimuli to produce the changes on the inside. For 
example, it is a matter of common observation that the quantity 
and quality of food greatly influence not merely the rate of 
growth of the body but the size and quality of the adult organ- 
ism as well. Life would be impossible without food, oxygen, 
water and suitable temperature. Any variation in these con- 
ditions at once has its effect upon the organism. Experiment 
shows that the varying degrees of salinity of the water may be 
accompanied by striking individual differences of form in certain 
marine animals. Caterpillars of certain butterflies placed in 
boxes lined with differently colored papers develop pupae with 
colors harmonizing with those of the boxes containing them. 
Colors in various animals are intensified or changed by special 
foods or by changed temperature. In general it may be said 
that changes in any of the conditions important to animal life 
produce some change or variation in the body of those animals 
subjected thereto. Since this is true, it becomes inevitable that 
the various individual animals on the earth are differentiated 
from each other somewhat as was seen to be the case with the 
cells and tissue of which the individual itself is composed. The 


following paragraphs trace out some of the ways in which this 
differentiation of individuals takes place, the relations of the 
various organisms to each other and to the environment. 

135. The Struggle for Existence. All animals (with a few 
possible exceptions in those which possess chlorophyll) depend 
ultimately upon green plants for food, those which live on other 
animals no less than those which use plant food directly. Only 
a limited amount of vegetation can be supported by the earth 
without cultivation. The number of animals therefore which 
can find a livelihood on the earth is in turn restricted. There is, 
however, no such limit of the powers of reproduction, either 
among plants or animals. Any pair of organisms if unchecked 
could in a very few years supply descendants enough to populate 
the earth up to its full powers of support. That they do not 
thus multiply at a geometric ratio is due solely to the influences 
at work to destroy these descendants. Any group of organisms 
will hold its own when, on an average, a pair of individuals can in 
a lifetime bring to maturity another pair to take their place. 
More than this means conquest of new territory; less than this, 
the extinction of the group. When we recall that all organisms 
have this unlimited power of reproduction, it is easy to see that a 
time must soon come when a struggle for food and a foothold on 
the earth is inevitable. The struggle would be more intense 
between those organisms which demand the same kind of food, 
that is, among kindred. This is the fundamental struggle. It 
would be complicated by the fact that some groups of animals 
prey upon others, and that the primary conditions of life, as 
water, temperature, etc., are subject to striking changes. These 
facts tend, by just so much as they destroy individuals, to 
relieve the struggle within the species, and to introduce new fac- 
tors which give great variety and interest to the life problems of 
animals. There is nothing more certain than that this struggle 
has occupied organisms practically from the beginning, and all 
our explanations of present conditions must take note of the 
fact. All the important structures and activities of animals are 
modified by this competition for a livelihood. 

136. Library Exercises. The student should be invited to make real to him- 
self the possibilities of a geometrical increase as applied to organisms. Take 


the known rate of increase (that is, the total number of descendants in an 
average lifetime) of a number of common animals and determine the possible 
living descendants in a specified time. Find references concerning infusoria, 
insects, fish, man. Have you any observations relating to the reality of the 
struggle for food among animals? 

137. Natural Selection. In spite of this power of reproduc- 
tion we see that, on the average, individuals do not increase. The 
earth is no more thickly inhabited by animals today than it has 
been for countless ages. The proportions of, different animals 
vary now and again, but that is all. Out of a family of one 
hundred young individuals striving for a foothold, no two of 
which are alike, ninety-eight will be destroyed. Which will 
survive? Barring accidents beyond the powers of any of the 
individuals to resist, those will survive which possess or acquire 
some quality, structure, or habit, suited to the struggle in which 
they find themselves. This may be a matter of strength, of 
speed in eluding enemies or capturing prey, of specially acute 
senses, of a tendency toward concealment, or any one of a 
thousand things calculated to fit an organism for a special place 
in life. It is not necessary to suppose that these elements of fit- 
ness exist in striking degree at first. The struggle is so intense 
that even the slightest handicap may mean the destruction of 
the individual. This elimination of the weaker individuals 
results in what has been called natural selection through the 
"survival of the fittest." The hereditary qualities thus pre- 
served in the individual are, if inherited, subject to transmission 
by heredity; and by the continuous action of natural selection 
and heredity through a long series of generations these elements 
of fitness are believed to accumulate, and thus animals become 
better and better adapted to their surroundings. 

138. Artificial Selection. Since man has been on the earth 
he has been a most potent factor in the environment of the other 
animals. He has helped in the elimination of animals hurtful to 
his interests; has domesticated others which he has deemed 
useful, thus rendering their environment highly artificial and 
removing from them the struggle for existence in certain meas- 
ure. For natural selection he has substituted a conscious selec- 
tion of such organisms as are best suited to his needs or fancies, 


and has allowed these to reproduce, eliminating the others. 
This artificial process, which obtains results more rapidly than 
the natural, has given rise to the various breeds, strains, or 
races of dogs, horses, cattle, fowls, etc. By means of this selec- 
tion the habits and dispositions of the domestic animals have 
been improved as surely as their structure. Their power of 
self-support, however, has been so materially diminished that 
some of them could not succeed in finding a living in the wild 
state under ordinary circumstances. 

139. Practical Exercises. Are there any domesticated animals whose species 
is represented in the wild state? Compare the habits and general structure of 
some of the domesticated animals with that of their nearest kin among wild species. 
How many species of domestic animals can you enumerate? From what groups 
do they come ? Trace the history and results of the domestication of some of the 
common animals, as fowls, pigeons, cats, dogs, etc. Have any strictly American 
species been domesticated ? 

140. The Adaptation of Animals to their Environment. 

There are two distinct questions of importance to be con- 
sidered in connection with this subject: (i) the necessity of the 
adjustment of organisms to their environment, and (2) the 
means by which this adaptation takes place in the individual and 
becomes fixed in the species. It is clear that the limited food 
supply and the unlimited powers of animals to reproduce result 
in a struggle for food among the animals at any time occupying 
the earth (135). This struggle is not merely among the animals 
in question, but is in reality between every organism and its 
whole environment. Extremes of heat and cold, drouth and 
famine, and numerous changes in the conditions of life make it 
absolutely necessary that the individual shall have some power of 
adapting itself to what is permanent and what is changeable in its 
environment. What are the means then by which animals that 
are not completely in accord with their surroundings may become 
so ? There are two possible ways in which this may come about. 
The animals may migrate to regions where the conditions are 
naturally more favorable to their well-being, that is, to regions for 
which they are already adapted. As a matter of fact this is 
known to be a common occurrence. Animals often disperse 
from their old centre of multiplication under the influence of 


hunger or unfavorable local conditions. They are often assisted 
in these dispersals by such natural agencies as winds, currents of 
water, and by other animals. If the migrating forms succeed in 
finding new regions suited to their needs, there results a condi- 
tion of adaptation between organisms and their respective envi- 
ronments, but without any active change in the characteristics of 
the organism. The environment itself is subject to continual 
change and there are too many barriers in the way of universal 
migration for this to be accepted as a complete explanation of 
the widely observed adjustment of animals to the conditions 
which surround them. 

Again animals may become suited to their environment by 
variation, without migration. There is no question that this 
also occurs and that it is the more important factor of the two. 
It has been shown (133) that all animals are variable. Students 
of biology have suggested two important ways in which varia- 
tions may give rise to a harmony between the organism and its 
surroundings. This result may take place through natural 
selection, which eliminates the unfit (137). According to this 
view the organisms naturally tend to vary. The changing en- 
vironment stimulates this tendency to variation. Out of a 
thousand individuals of similar parentage there will be numer- 
ous slight differences of structure and physiological qualities. 
Some of these will be more, and some less, favorable to the en- 
vironment. In the struggle those will be eliminated which 
for any reason are strikingly unsuited to the environment. On 
the other hand, those animals whose variations are most in 
accordance with the local condition will persist and propagate 
their kind, tending through heredity to pass on to their off- 
spring the qualities which enabled them to adjust themselves 
to their surroundings. Thus there will be a gradual, ever- 
increasing adaptation in the whole species of which they are a 
part, by natural selection. If, however, only inherited qualities 
are transmitted, and the fluctuations of body produced by use 
and environment cannot be transmitted, natural selection could 
not act, by way of the acquired characters, in improving a 

Occasionally there occurs in offspring, from the action of the 


environment or from other causes, a sudden and considerable 
change from the parent type. Such a product is known as a 
"sport." It is quite possible that natural selection may seize 
on such and if in a favorable direction preserve and increase 
them. In such cases adaptation might take place with great 
rapidity, instead of in the gradual way described above. 

In the second place it has been argued that the immediate 
effect of the environment on the organism and the efforts of the 
organism to respond to the stimuli of the environment produce in 
the organism just such definite variations as will tend to fit it for 
its surroundings. In other words the majority of the variations 
brought about by a given external condition are definite and 
naturally in the direction to meet the necessities of the case. For 
example, cold stimulates the surface cells of the body of an 
animal. The immediate response of the nervous and nutritive 
processes in the organism are such that the surface cells take on 
greater activity and produce materials at the surface of the body 
which tend to protect the animal from the ill effects of the cold. 
This is an individual variation. To become effective in making 
the species better adapted to the environment these results must 
be handed down by inheritance to the next generation. If 
this can take place this theory would go a long way toward 
explaining how adaptations arise. There is, however, very 
great doubt whether such adaptations acquired in the life of 
an individual can be transmitted to offspring. If this cannot 
occur we are thrown back upon natural selection as the prin- 
cipal explanation thus far offered to account for the progressive 
adaptation of animals to the environment. There is no rea- 
sonable doubt that natural selection is such an explanation. 
So far as we know either small or large variations may be se- 
lected if they are only inherited variations and useful ones. 
To what extent it is assisted by other factors is at present un- 
certain. It will be assumed in the following pages that it is one 
of the most important known factors in producing adaptation. 

141. Classification. Since the environment is not the same 
at any two places on the earth and there is an accumulation, 
from generation to generation in animals, of those features 


which tend to bring them into harmony with their different 
environments, it is inevitable that the animals themselves come 
to be very diverse, no matter how similar they were at the 
outset. In the discussion of them it therefore becomes neces- 
sary to devise some means of expressing the degree of likeness 
and unlikeness among the great number of individual animals 
existing on the earth. This may be done by means of an ap- 
propriate classification. The differences of structure and func- 
tion may be superficial or fundamental, but it must be remem- 
bered that all these differences are in some way the outcome 
of the history of the organisms, and that the likenesses are 
signs of kinship, or of similar history, or both. The grouping 
or classifying of organisms has two objects: (i) convenience, 
that is, to make future work easy ; and (2) to express the results of 
past study. Insomuch as the first motive may predominate the 
classification may be artificial, that is, may bring together ani- 
mals that are really not closely related, though possessing a 
superficial resemblance. The grouping together of bats and 
birds on the ground of their power of flying, or whales with fishes 
because of their habitat, would illustrate such a classification. 
In proportion as classification takes in all the facts known with 
regard to animals and expresses the relationship of forms classed 
together, it is said to be natural. Every classification is in some 
measure artificial since we do not know all the facts concerning 
the structure or history of any organism. 

142. Terms Used in Classification. From what has been 
said concerning the power of multiplication in animals, the re- 
sulting struggle for existence, the variability, and the elimination 
of those whose variations are not suited to the various environ- 
ments into which the offspring migrate, it will be readily under- 
stood that even the descendants of a single pair of organ- 
isms will come in time to be noticeably different in form, size, 
color, and the like. The individuals of a given region will usu- 
ally be more like each other than like their cousins who have 
been subjected to some other kind of environment. There is 
thus a need of terms to express the degree of difference which, 
through these influences, finally characterizes the descendants 


even of common ancestors. Such groups of forms are usually 
known as varieties or subspecies of the original type from which 
they all sprang. Thus in the human race while all men are con- 
sidered as belonging to one common type and possibly derived 
from the same human ancestors there is enough difference be- 
tween the American Indian and the Caucasian to make it neces- 
sary to distinguish them as different varieties. Many of our 
widely distributed animals as the dog, the horse, the common fox 
have varieties which are readily distinguishable. 

When the causes which produce varieties have been at work 
long enough to eliminate the intermediate forms which are often 
found connecting the varieties, and to secure a close adaptation 
of the varieties to the environment, the term species is applied to 
what were formerly called varieties. Species thus merely rep- 
resent the further progress of individual differentiation and adap- 
tation to the different modes of life which give rise to variation in 
individuals that is, to varieties. A species of animals may 
again split up by the action of the forces mentioned (and other 
conditions which have not been mentioned) into new varieties 
and finally into new species. It is believed that the present 
diversity of animal and plant life has come about from a much 
more limited number of kinds of ancestors by a method essen- 
tially such as that described above. The student will realize 
that in nature there are only individuals. There is really no 
such thing in nature as a species. This is purely, a mental con- 
cept of our own. 

Varieties of the same species usually cross freely. Their 
offspring are usually fertile. The individuals of different spe- 
cies as a rule cross less freely and when they do cross their off- 
spring are called hybrids. Hybrids are often sexually infertile. 

The genus is related to species somewhat as the species to 
the varieties which compose it. A genus embraces those kin- 
dred species which show a high degree of relationship among 
themselves. The characters which serve to distinguish differ- 
ent genera are more fundamental than those by which we 
recognize varieties or species, and argue a more extended time 
in the differentiation of genera than is required for species. 

Other terms, as families, orders, classes, phyla, are used to 


denote the still more extensive and comprehensive divisions 
of the animal kingdom. 

143. Illustration of Classification. The domestic cat has 
many varieties or breeds, as the maltese, manx, tortoise-shell, 
etc. On the other hand, the wild-cat, the tiger, the leopard, 
the lion have numerous points of structural likeness to the 
domestic cat, and are said to be species belonging to the same 
genus (Felis). The genus Felis and others less common are 
placed together in the family Felidcz. These with the members 
of the dog family and others constitute the order Carnivora 
(flesh eaters), and similarly for the higher groups in the diagram 

Kingdom Animalia (Protozoa, arthropods, chordates). 
Phylum Chordata (fishes, birds, mammals). 

Class Mammalia (carnivora, ruminants, bats, man). 
Order Carnivora (dogs, wolves, cats, etc.). 
Family Felidae (cat family). 

Genus Felis (cat, lion, tiger, etc.). 

Species Felis domestica (with its numerous 
varieties) . 

The name of an animal is its generic name followed by its 
specific name as above. The variety name is added when there 
are distinct varieties. 

144. Relation of the Individual to the Species. The vari- 
ous types of animals produce their offspring in numbers pro- 
portional to the difficulties encountered in bringing the young to 
maturity. In the most favorable circumstances many more 
are produced than can survive. In cases where enemies are 
numerous millions of eggs may be deposited in order to secure 
a single adult. Nature is thus said to be lavish in her waste 
of individuals in order that the species may be continued and 
improved in its adaptations. These surviving descendants, 
generation after generation, have become, through natural 
selection, more and more suited to their surroundings. This 
means that the production of many individuals, a large num- 
ber of which never reach maturity, secure the development of 


a small aristocracy which propagates the type. The species 
is related to its individuals something as the individual is to 
the renewed and changing cells of which it is composed. Spe- 
cies are not constant, but even the most fixed must undergo 
change or extinction when confronted by new conditions. 
Species however are less variable than the individuals composing 
them because the species represents an average condition 
of all its individuals. Adaptation to environment is the great 
problem which every animal must solve. Those which do solve 
it successfully constitute the species. It is needful then to 
consider next those characteristics of structure, habit, or in- 
stinct whereby a species of organisms becomes successfully ad- 
justed to its surroundings. In a broad sense all the organs 
which were outlined in the preceding chapters are adaptations : 
the digestive organ and process, to the nature of food; the 
nervous system and the special senses, to the external stimuli; 
the lungs, gills, and skin to the need of oxygen, and the like. 
In contrast to adaptations of this kind we now consider as 
adaptations those more special modifications of fundamental 
structure by which a species becomes more suited to some lim- 
ited habitat or to some special mode of life which is of signal use 
to it in the struggle for existence. 

145. Classification of the Principal Types of Adaptation. 

A. Adaptations primarily in relation to the inorganic en- 

B. Adaptations primarily related to other organisms. 

I. Among animals of the same species. 

1. Friendly and social, 
(a) Mating. 

(6) Parental care of young. 

(c) Organic colonies. 

(d) Social and communal life. 

2. Competitive: for food, mates, etc. 

II. Among animals of different species. 
i. Friendly and social. 

(a) Commensalism. 
(6) Symbiosis. 


2. Competitive. 

(c) The predaceous habit: adaptations for offense 
and defense. 

(d) Parasitism. 

146. Special Adaptations to the Inorganic Environment 

These embrace such special structural devices as hair, feathers, 
the blubber of whales, which enable the body to maintain its 
temperature despite the condition of the medium. The habits 
of burrowing and hibernation, the winter migrations of many 
animals^ especially birds, are examples of instinctive adaptation 
to cold. The same end is obtained by man by artificial clothing, 
by houses, and by the use of fire which has been one of the most 
important instruments in his progress. Rotifers, infusoria, and 
some other animals have become capable of retaining life during 
thorough drying, and of resuming activity on the return of mois- 
ture. Adaptations to locomotion in different media, earth, air, 
and water; to climbing; to stationary life, belong to this group. 
These are only a few of the many instances of adaptations of 
organisms to the 'materials and the forces about them. It is 
easy to see that some of the adaptations are of life and death 
importance, and without them the species would become extinct. 
It is believed that these qualities of the organism arise in a way 
something like this: owing to the irritability of all protoplasm, 
the prevalent external factors as heat, light, gravity, moisture, 
and chemically active substances must produce some change, 
that is, some response on the part of the organism. Those or- 
ganisms in which the response is not in accordance with the best 
adjustment to the special environment are less likely to survive 
in the struggle for existence. Those which do survive and propa- 
gate their kind because of their favorable responses to these 
stimuli are, by reason of these facts, more and more likely in 
succeeding generations to possess those habits and structures 
suiting them to their surroundings. 

147. Practical Exercises. Find other instances which seem to indicate adapta- 
tion either in structure or habit to special features in the environment: as adapta- 
tions to prevent undue evaporation in a dry climate; adaptations to warm condi- 
tions; to drouth; to the use of special plants as food; to light; to gravity. Illustrate 
from observation and by library reference the types of adaptations cited in the text 


above. Is the power of sleeping an adaptation of any value? Among what 
animals is it found ? Find instances in which useful adaptations have become 
useless and even hurtful from changed conditions of life. 

FIG. 52. 

FIG. 52. Young Opossum (Dideiphys virginiana) photographed from life by J. W. Folsom. 

Questions on the figure. Of what conceivable value to the animal is the 
prehensile tail? In what other groups of animals is the tail prehensile? What are 
the habits of the opossum ? How is this species distributed on the earth ? Where 
are other marsupials found ? 

148. The relations of animals of the same species to one 
another is an interesting mixture of competition and coopera- 
tion. In the higher forms the parents instinctively make great 
personal sacrifices that the offspring may be cared for; the off- 
spring on the other hand struggle with each other for this 
parental provision. In the classification offered (145) it should 
be remembered that both friendly and competitive habits and 
structures are always represented in the same individual. 


149. Mating Adaptations. One of the most striking forms 
of individual variation is seen in the differences between the 
sexes of higher animals. The male and female are often so 
widely different in form, size, color, and other qualities, that 
naturalists have classified them as belonging to different species 
and yet it is very manifest that, though different, the sexes are 
closely adapted to each other. In the lower types of animals 
the sexes are frequently represented in the same individual. In 
such cases the elements often mature at different times. An 
individual is thus alternately male and female. This is re- 
garded by many as being the primitive condition, the separa- 
tion of the sexes being accomplished by the repression, so to 
speak, of one or the other sex in each individual. Some think 
that the temperature and the amount and quality of food have 
something to do with the proportion of males and females which 
are produced. So sexual dimorphism in some species may be 
in some measure a response to external conditions and presents 
every evidence of being an advantageous adaptation to the con- 
ditions of life. On the other hand, it is believed by many that 
the sex of most organisms is determined by conditions in the 
egg and sperm that unite. In other words it is thought that 
sex is inherited, and cannot be changed by external conditions. 
The very union of the sperm and the ovum, whereby two cells 
lose their individuality in one, with a renewal of powers and the 
mingling of the qualities of two parents, must be looked on as an 
adaptation of the very highest moment to the animals in which 
it first appeared and to their descendants. So too are the won- 
derful internal tendencies that cause the definite unions and 
separations of chromosomes in the sex cells. The chemical 
attraction which the female cell exerts on the motile sperm cell 
is a special adaptation to accomplish this union. Furthermore 
it is undoubtedly true that many of the color markings, notes, 
motions, and the like in which the male and female animals 
differ are recognition marks whereby the presence of one sex is 
made known to the other. In some animals in which sexual 
fertilization normally occurs, the ova may develop in the absence 
of sperm (parthenogenesis) . This may have arisen as an adapta- 
tion to temporary scarcity of males. This view is in some cases 


supported by the additional fact that parthenogenetic eggs 
produce male individuals wholly, or in excess. 

150. Practical Exercises. What is the difference in the notes of the male and 
female of the American quail which would serve as recognition marks? Mention 
other cases of sexual dimorphism which appear to you to serve a similar end. 
What evidences have we that the mingling of sperm and ovum results in a re- 
juvenescence? in the introduction of greater variation? Show how these are 
important as adaptations in the struggle for existence. In what groups is partheno- 
genesis found ? Give details of the facts in several cases. 

151. Reproduction and Care of Young. The very rate of 
reproduction is an adaptation to the severity of the struggle for 
existence experienced by the animals of a given species. Those 
forms with few enemies and abundant food usually need to pro- 
duce only a few young in order to maintain their place. Others 
less favored in these regards, as many insects, the lobster, the 
salmon, must reproduce thousands of young in a lifetime. Simi- 
larly the length of the reproductive period and of life becomes 
an adaptation to the same end. It is clear from these facts 
that any device which the parent may adopt likely to bring a 
larger percentage of the young to maturity will make for a 
saving in the necessary birthrate. This husbands the parental 
resources and conduces to the efficiency of the individual and 
of the species. It must not be supposed that parental care is 
confined to the higher animals. In its most elementary condi- 
tion it takes the form of food stored in the egg, and in depositing 
the egg in a safe place for hatching. After hatching it takes the 
form of supplying food, or protection, or both. Cephalopods, 
fishes, and birds have a large amount of food substance stored in 
the egg. Many animals, as the clam, some fishes, some reptiles, 
and the mammals, retain the eggs in special portions of the 
body until development has well begun. The flies lay their 
eggs in the decaying matter which the young use as food. The 
solitary wasps seal theirs up in nests with the food (dead or 
wounded spiders or insects) on which they are to develop. 
Other insects bore into the tissues of living plants and deposit 
their eggs, about which "galls" or masses of abnormal vegetable 
tissue are developed. The ichneumon fly deposits its eggs in 
the body of some other animal. Thus we see an immense num- 



FlG. 53- 

FlG. 53. Galls on oak, cynipid (Holcaspis duricoria). Natural size. Photo by Folsom. 

FIG. 54. 

FIG. 54. Galls on elm, produced by an aphid, Colopha ulmicola. Natural size. Photo by 

J. W. Folsom. 


her of adaptations useful to the organism have been developed 
in connection with the egg-laying habit. After such provision 
the majority of animals leave the young to care for themselves; 
but many higher forms take further pains to protect and train 
their offspring during the course of their development. The 
care which the birds and mammals give their young is a matter 
of common observation. It takes the form of food, of special 
homes, as nests, burrows, dens, etc., and of the personal ser- 
vices of the parents, who will often protect the young from their 

FIG. 55- 

FIG. 55- Galls on hackberry leaf, produced by a fly (Cecidomyiida). Natural size. 
Photo by Folsom. 

Questions on figures 53, 54, 55. What does the gall represent from the point 
of view of the plant? From the point of view of the insect? What seems to 
cause the undue vegetable growth? Find other galls in nature and try to find 
what type of insect is responsible for them? In what ways may one hope to 
determine this fact? 

enemies at the risk of their own life. Similar care is shown by 
some insects, especially the social forms, such as bees, ants, and 
the like. The lobster carries its young on its abdominal ap- 
pendages for months after hatching. The lower invertebrates 
are practically destitute of these later care-taking instincts. 

It is interesting to notice that animals differ very much in 
their helplessness at hatching or at birth. The young of the 
reptiles, or the duck, or the chicken are relatively well developed 
at hatching, and are very soon able to run about and feed (pre- 



cocial). The young of the song birds, as the thrushes, swallows, 
etc., are wholly dependent on the care of the parents for a con- 
siderable time (altricial). In the herbivorous mammals, as the 
sheep and cattle, the young have the use of their limbs in a short 
time after birth. Among the carnivorous forms, as the cat and 
dog, the young are more helpless. In the human species the 
period of helplessness is longest and consequently the necessity 

FIG. 56. 

FIG. 56. Nestling Marsh Hawks (Circus cyaneus). From Year-Book. Department of 


Questions on the figure. What are the nesting and breeding habits of the 
marsh hawk? Are the young precocial or altricial? 

of parental care greatest. In general, the longer period of 
parental protection accompanies the development of more com- 
plex and highly organized instincts, and intelligence. The 
lengthened period of dependence, while a burden to the parent 
in one sense, is an advantage to it in the saving in number of 
offspring, and serves to benefit the species, not merely by keep- 
ing the offspring alive until they may reproduce, but in the 
greater development of such parental instincts as gentleness, 
self-sacrifice, and the like. In the human race it has given rise 


to the home and family, which we regard as the real basis of 
modern society ; and such social organization in turn has been a 
most powerful factor in the progress of the human species. 
Death and the length of life must also be considered as special 
adaptations. This differs in different species very widely. 
Life in general, where natural selection acts, will be the period 
of youth, plus the period of fertility, plus the time necessary 
to rear the latest offspring. For the species, the death of the 
individual becomes an advantage at the completion of this 
period, and this fact is sufficient to insure that death will nor- 
mally occur at this time. 

152. Practical Exercises. Add instances of parental care which have fallen 
under your own observation, and give a statement of the facts in the case. Com- 
pare the mammals with which you are acquainted, in this regard. Compare the 
condition of the young of the robin, the quail, the blue-jay, the pigeon as to 
maturity at hatching. Do any animals of your acquaintance reproduce more than 
once in a year? Why is one reproductive period per year a common adaptation. 
Compile statistics concerning the longevity of various animals, and its relation to 
size, to reproductive period, and to the time demanded to reach the adult stage. 

153. Colonies. In some of the lower groups of animals, 
as the polyps and jelly-fishes, in which the reproduction by 
fission or budding is prominent, the newly formed individuals 
remain for a longer or shorter time in association with the parent 
or with each other. These units which otherwise might be 
separate individuals are originally connected and often come, 
by the continuation of the process, to form immense masses, 
as in the coral. Such organic associations are called colonies, 
Colonies rarely occur in animals in which the organs are highly 
specialized. Very often the individuals become specialized for 
the performance of a special portion of the work, and thus we 
get several quite differently constructed individuals within the 
colony (polymorphism, Fig. 87). The whole colony may then 
behave somewhat as an individual, the polyps taking the place 
of organs (Siphonophora) . Colonial animals are almost always 
attached to fixed or floating objects. These polymorphic indi- 
viduals are closely adapted to each other in structure and divi- 
sion of labor; and the colonial habit in general, even where there 
is no division of labor, is a successful device whereby limited 
areas are completely occupied by the members of a species (as 


in the case of the branching corals) where the single polyps would 
be practically helpless. The arrangement of the polyps on the 
common skeleton and the rate of growth of the different polyps 
are beautifully adapted to the best use of the currents of water 
by which the food and oxygen are conveyed. 

154. Library and Museum Exercise. What phyla of the animal kingdom 
supply instances of colonies? Trace different degrees of polymorphism. In 
what different ways do the individuals occur on the common stock? Show how 
the relative rate of growth of the differently placed individuals determines the 
ultimate form of the colony as a whole. 

155. Social and Communal Life. Animals of the same 
species often become associated even when there is no organic 
connection between the individuals. The association may be 
temporary or permanent. The bond in these cases is not phys- 
ical, but instinctive and psychical. In its simplest form this is 
merely a matter of gregariousness such as is seen in the schools of 
fishes or flocks of birds, which are apparently brought together 
at certain periods by a common instinct or by common needs. 
A step more intimate is the banding together of predaceous 
animals as wolves or vultures, or pelicans, for mutual help in 
finding or capturing the prey. Corresponding to this, on the 
part of their victims, we find the herding of the bison, of deer, 
and their allies for protection, whether by fighting together or 
by the stationing of sentinels to give notice to the feeding herd 
of the approach of danger. In still other forms, notably among 
such insects as the bees and ants, there is a very intimate and 
permanent union in social life. This is usually associated with 
the instinct of home building, and thus a high degree of division 
of labor with its great advantages becomes possible. This is 
carried to such an extent that often polymorphic individuals 
result, much as in the organic colonies. In such cases it is clear 
that the individual life comes to be bound up in the success of 
the community. Such forms usually exert great care for their 
young and develop a relatively high order of "intelligence." 
The principal social forms are the ants, of which there are more 
than two thousand species; some of the bees and wasps; the 
termites, or so-called white-ants; beavers; some monkeys and 


156. Library Exercise. Make a report on the social life of the honey-bee, 
including the following points: the home; the kinds of individuals, their origin, 
and their work in the community; their food and its preparation; mode of caring 
for the young; swarming and its significance. Make a similar report concerning 
some species of ant. Find facts concerning the following topics: "ants' cows"; 
slave-making among the ants; army ants; the agricultural ant. 

157. Competition among animals of the same species is not, 
for the most part, of a personal character except in the case of 
the struggles of the males of polygamous animals. The ordinary 
struggle for existence among them is merely that of food-seeking, 
where all possess the same organs and habits but in varying 
degrees of excellence. Those which have the greater strength, 
hardiness, or intelligence are more likely to get their portion of 
food at the expense of the weaker, and thus to propagate their 
qualities. Sometimes, however, animals live directly at the ex- 
pense of their own species. Young spiders before escaping from 
the cocoon in which they are hatched devour each other, thus 
instituting an acute phase of the struggle for existence in the 
place of the protection prepared by parental care. Many fishes 
are known to devour their own young. We have all had occa- 
sion to wonder what becomes of the small frogs in a box contain- 
ing large ones. 

The struggle between the males for the possession of the 
females has resulted in the development of many interesting 
adaptations. The struggle may take the form of actual combat 
in connection with which organs of offense and defense are 
found. Such are the horns, tusks, spurs, manes, and even the 
excessive size of the males as compared with the females. Mani- 
festly the same qualities which make a male a formidable rival 
to another are likely to be of service to himself, his mates, and 
his young, and thus to the species, in protecting them from the 
attack of their enemies among other species. The competition 
between males is not all of this stressful kind, however. It is 
believed by many naturalists that, in those instances where 
simple mating rules, those males with the most striking colors, 
pleasant voices, and winning ways displace their less favored 
rivals and thus tend to accumulate by natural (sexual) selection 
the adaptations of this class. 


158. The individuals of one species of animals may often 
be practically indifferent to the presence of those of other species. 
Their relation is simply that of competing for the general food 
supply and thus assisting in the elimination of the unfit in all 
species. They may graze in the same pasture, swim in the same 
pool, or even be parasitic on the same host, and have no other 
relation. From this as the simplest relationship we may pass 
by gradual stages to the most intimate friendships and the most 
bitter antagonism. Every species is indifferent to some and 
hostile to other of the species which surround it; and man is no 
exception to the rule. It is a perversion of manifest fact to 
pretend that all animals are of some use to man. 

159. We have seen that the individuals of a given species 
are engaged in a struggle among themselves for the means of 
subsistence, and that in certain cases they form communities 
or colonies a kind of organic corporation in order to meet 
more successfully the demands made upon them by their en- 
vironment. Similar partnerships may be formed by animals 
of different species. The simplest of these associations are 
known as commensalism or " mess-mateism," in which the degree 
of dependence and mutual advantage is perhaps not very great. 
As instances may be cited the occupancy of the same burrows 
by the prairie dog and a species of owl; the attachment of 
barnacles to whales and sharks; the hundreds of species of 
other insects which live in the nests of ants; the lodging of 
fishes and other animals in the body-cavity of some of the 
large tropical sea-anemones or among the tentacles of some 
of the Hydrozoa. Each member of the association can live 
without the other, but for some reason they often occur to- 
gether. The way in which species of rats and mice follow 
man and occupy his habitations perhaps may be considered 
as illustrating a similar condition. 

1 60. Symbiosis. Under this term are included even closer 
relationships between members of different species, where there 
seems to be a distinct advantage accruing to both members of 
the partnership sufficient to account for it. The relation 
of the ants to the aphides or plant-lice which they capture may 



be so described. The aphides, although captives, are nour- 
ished, often at great expense of labor to the ant, on the food 
which they most prefer, and in return the ants use the sweet 
secretions of their bodies as food. Certain hermit-crabs, 
whose habit it is to occupy gasteropod shells as a home into 
which they insert the soft posterior part of the body, cultivate 
friendly relations with a sea-anemone which becomes attached 
to the shell, often with the active help of the crab. The 
anemone is supposed by some to conceal the hermit and to help 

FIG. 57. 

FIG. 57. Hermit-crab in the shell of a Gasteropod. After Morse. 

Questions on the figure.^-What structural adaptations has the hermit-crab 
to this mode of life? What conceivable gain has such a habit? What animals 
are cited as symbiotic with the hermit-crab ? 

protect it by means of its nettling cells, and in return is carried 
about to fresh fields, and enjoys a portion of the food broken 
up by the strong pincers of the crab. Observers have claimed 
that the crab offers choice morsels of food to its companion. 
When the crab by reason of its growth needs a new home it is 
said to transplant the anemone thereto. These must be looked 
upon as very remarkable adaptive instincts. Symbiosis is 
probably more common between animals and plants than 
among animals. The most interesting of these latter are seen 
in the so-called "ant-loving" plants, in which the plant pro- 



duces special homes or special foods for the ants, and the ants 
in return protect the plant from the ravages of other leaf- 
cutting ants or hurtful insects. Certain sea- anemones possess 
unicellular algae imbedded in the cells of the entoderm. These 
algae derive their nourishment from the wastes of the animal 
tissues and supply oxygen and possibly other matter to the 
cells in which they lie. The close relation between the struc- 

FIG. 58. 

FiG. 58. Argynnis cybele on thistle. Natural size. Photo by Folsom. 

Questions on the figure. For what purpose does the butter fly visit the 
thistle? What special adaptations does the butter fly possess for this mode of life? 
What is the gain to the thistle from the visits ? 

ture and instincts of insects, on the one hand, and the form of 
flowers, their products and needs, on the other, illustrates a 
symbiotic adaptation which has long attracted students both 
of botany and zoology. See Fig. 58. 

161. Library Studies. Make a report concerning the various myrmecophilous 
plants. Accumulate all the supposed instances of symbiosis which your library 
records. Lichens, among plants, are considered to illustrate symbiosis. How? 


162. The Preying Habit. The effects of this habit are 
stamped upon the structure and activities of both the pursuer 
and the pursued. It is in this relation that nature is indeed 
"red in tooth and claw." While in general the same organs 
and habits which are of value in the capture of prey are useful 
in the defense of the possessor, it is possible to find a series 
of adaptations of an offensive character and others more 
specially of defensive value. The curved claws and sharp 
teeth, the stealthy approach, the sudden spring, and the great 
agility of the one are met by the timidity, the keen senses, 
the fleetness of the other. We can see that these defensive 
adaptations must keep pace with the offensive else the prey 
would be exterminated, which would entail no less surely the 
destruction of their enemies than if these should lose their 
power of capturing their prey. 

163. Adaptations for Protection. In addition to the alter- 
natives of fighting or fleeing, the animals which are preyed 
upon have very interesting and effective qualities that make 
for safety. Many forms, as the Crustacea, have permanent 
outer coverings; most mollusks have a box arrangement into 
which they can retire when threatened by attack; others by 
burrowing or otherwise come to occupy obscure corners in 
nature where enemies find it difficult to follow. Forms as 
widely different as the earth-worm, mole, and the chamois find 
safety in retirement. 

This hiding-theme may be wrought out in ways almost 
equally effective by what is called protective resemblance. By 
this is meant that the animal becomes less easily distinguished 
from its environment because of its color, or form, or both. 
This resemblance may be to some particular object, or merely 
a general harmony of color with the surroundings. As illus- 
trative of the latter head we may cite the quail among the 
dead leaves and grasses, the sober-hued lizard on the logs, the 
green caterpillars or tree-toads among the leaves; the tawny 
color of desert animals, the white fur of arctic forms, the trans- 
parency of many marine animals. Indeed the great majority 
of animals show some traces of resemblance to the surround- 


FIG. 59. 

Nestling Mourning Doves (Zenaidura macroura). 
Year-Book, 1900. 

From U. S. Dept. Agriculture 

Questions on the figure. Is there anything suggestive of protective markings? 
What are the nesting habits of the dove? What character of nest is constructed? 

FIG. 60. 

FIG. 60. A sea-horse, Phyllopteryx eques. From Eckstein. 

Questions on the figure. Compare this figure of sea-horse with figures of 
other species and note the chief difference between them and the typical fishes in 
external characteristics. What about the figure suggests protective resemblance? 
At what point does the tail of the fish end ? 



ings, since concealment is alike advantageous to the predaceous 
and to their prey. In some instances there is the ability to 
change color with changing environment, as in the tree-toads, 
the chameleon, and in some fishes. This is not chiefly by the 
direct action of the light on the pigment cells but by reflex 
action of the nervous system stimulated through the eyes. 

Many other animals become inconspicuous by reason of a 
resemblance to special objects. It is among the insects that the 
most numerous illustrations of this are found. The walking- 
stick insect appears as dead twigs when not in motion. Many 
butterflies resemble leaves when at rest. A noted instance is 
Kallima which is a large species, conspicuous when flying be- 

FIG. 61 

FIG. 61. Walking-stick insect (Diapheromera veliei) on twig. Natural size. By J. W. Folsom. 

Questions on the figure. To what group of insects does this belong ? Do you 
see any reason to suppose that it illustrates protective resemblance? 

cause of blue and orange patches on the upper surface of the 
wings. The wings are folded when at rest and the lower sides 
are colored and marked so like a dead leaf that the deception, 
is very complete. The larvae of some of the geometrid moths, 
often called "measuring- worms," are remarkably like the 
twigs on which they crawl, both in color and shape. This is 
made more striking by the presence of roughnesses on the sur- 
face which suggest buds, and by the possession of muscles 


which enable them to support themselves rigidly outstretched 
for hours by means of the posterior legs alone, so that the 
axis of the body makes an angle with the branch. 

Other instances of special devices whereby animals protect 
themselves are found in the electric organs of some eels and 
other fishes, in the poisonous fluids with or without special 
stinging organs, as in ccelenterates, bees, some spiders, a few 
fishes (spines) , and some snakes ; also in the repulsive odors of the 
skunk and many caterpillars. 

Caterpillars oftentimes have an acrid or otherwise unpleasant 
taste, but, unless this is associated with a special odor or color 
by which its enemies may recognize the fact, it is not likely to 
prove of any great service to the animal possessing it since a sin- 
gle incision in the soft body made by the bill of a bird is likely to 
.cause death. For similar reasons animals with stings are often 
highly colored. The colors or other marks are, in these cases, in 
the nature of warnings. The "monarch," one of our large con- 
spicuous butterflies is an illustration of the association of color 
and offensive taste; the wasps and the coral-snake, of the asso- 
ciation of calor with the possession of stinging powers. Thus 
owing .to the power of association in the mind of the enemies, 
the advantage comes to lie quite as much in the possession of 
the special color or form as in the presence of the underlying 
protective powers. These facts give rise to the remarkable 
phenomena of mimicry. This term applies to those instances 
where an edible or harmless animal, by reason of its similarity 
to those which are disagreeable, partakes of their immunity 
from attack. Mimicry must not be considered as in any way 
a matter of choice with the animal but simply the result of 
natural selection in preserving and allowing the propagation 
of favorable variations. The viceroy butterfly, though edible, 
seems to be protected by its striking likeness to the monarch. 
The nearest relatives of the viceroy are quite differently marked. 
Mimicry of bees and wasps is found among many flies and some 
moths and beetles. Non-venomous snakes occasionally have 
the marking and the motions of the venomous. 

164. Practical Exercise. Try to discover instances of general protective re- 
semblance among the animals known to you. Analyze each case and see just 


the nature and value of the protection. Treat similarly the subject of special pro- 
tective resemblance. Do you know any really harmless animals which assume 
apparently dangerous attitudes for protection? Accumulate all the available 
references on mimicry. What range of color have you seen illustrated among 
animals? In a single animal? Where, on the earth, are the brightest-hued 
animals found? What are believed to be the causes of colors among animals? 
What are the uses of colors? What is albinism? Where have you seen instances 
of it? See Fig. 62. 

FIG. 62. 

FIG. 62. Albino Opossum (Didelphys virginiana). Photo by Folsom. 

Questions on the figure. What is albinism ? In what structures is it manifest ? 
Among what groups of animals can you find that it occurs? To what is color in 
hair due? What natural conditions tend to produce color in organisms? What is 
the chief value of color as an adaptation, individual and social? 

165. Parasitism. Of a nature which combines the quali- 
ties of commensals and of the preying animals is the association 
known as parasitism. It is an association of individuals of 
different species in which one member (the parasite) gets all 
the benefits, and the other (the host) suffers the loss. It is 
a case where one species preys on another, but in which it is 
to the advantage of the parasite, especially if a permanent one, 
as well as of the host that the life of the latter shall not be 
suddenly destroyed. It will be readily seen that the parasite 
increases the work to be done by the host, thus being a handi- 
cap in the struggle for existence. This might easily bring 
about the destruction of these species which serve as host were 
it not for the fact that nearly or quite all animals support vari- 
ous parasites. Parasites are of two classes, external, as the 
fleas, lice, and the like; or internal, as most of the parasitic 



worms. The fleas are transient parasites, as are many other 
insects which are free in the adult stage but lay their eggs in 
or on the body of the host where they undergo partial develop- 
ment as parasites. In other instances the parasite must spend 
its whole life in the body of one or more hosts. These are 
called permanent parasites. 

FIG. 63. 

FIG. 63. Caterpillar of Piatysamia cecropia parasitized. From Lugger. 

Questions on the figure. Seek in your reference literature all figures and refer- 
ences to caterpillars attacked by parasites. Why would caterpillars be rather 
favorable hosts for parasites? What are a few of the parasitic enemies of cater- 
pillars? What economic importance has this phenomenon? 

In addition to the drain on the resources of the host, the 
presence of the parasite may so irritate the tissues of the host 
as to produce abnormal growth and disease therein. In many 
of the transient parasites the life of the individual host is of 
no consequence after the end of the period of parasitism and 
hence the entire destruction of the host's body may occur just 
as truly as in the ordinary preying species. Very profound 
modifications occur in the structure of the parasite, which are 
the outcome of, and in part an adaptation to, the special mode 
of life. There is usually a degeneration of the organs of di- 
gestion, of motion, and of sensation, since the parasite de- 


pends on the host for the performance of these functions. The 
explanation of this degeneration of useless or unused organs 
is not quite certain. It is known that disuse causes structures 
to deteriorate in the life of the individual, and some naturalists 
claim that part of this IQSS is transmitted to the next genera- 
tion. The claim is denied by many, who are disposed to con- 
sider that it is merely a case of natural selection working for 
simplification of organs and the economizing of materials. 
The reproductive organs on the contrary become much more 
complicated and the reproductive elements are produced in great 

FIG. 64. 

FIG. 64. Lake Lamprey (Petromyzon marinus unicolor) clinging to Sucker. (From Bull. U. S. 
Fish Commission, by Surface.) 

Questions on the figure.^Does it seem that this is an instance of parasitism 
or simple preying? What special organs has the lamprey adapting it to this 
habit? What references can you find to the breeding habits of the lamprey? 

abundance. This is an adaptation to the difficulties involved 
in finding the special host in which development may proceed. 
This is more striking because many parasites require two dif- 
ferent hosts in order to complete the life cycle, and great mor- 
tality accompanies the passage from one host to another. A 
good illustration of such parasites is the tape-worm which 
infests the trout in Yellowstone Lake. The larvae enter the 
tissues of the trout and by their ravages weaken and kill the 
host. The dead fish are eaten by pelicans. The worms de- 
velop to the adult, sexual condition in the digestive canal of 


this second host and the eggs or young embryos escape into 
the water with the excreta and from there are taken up by 
other trout whose destruction is again wrought by the tissue- 
infesting larvae. This passage from one host to another prob- 
ably arose, and is helped, by the carnivorous habit among 

The parasites are almost exclusively invertebrates. The 
worms and arthropods furnish the most numerous representa- 
tives. The gregarines, and some other Protozoa, are internal 
parasites, often being parasitic within the cells . There are only 
a few parasitic vertebrates, and these are transient. They be- 
long to the lower fishes (lamprey, Fig. 64) . 

Parasitism is a very successful adaptation to a much limited 
environment in which the organism has bartered its original 
powers for a life of comparative ease. The only necessity 
still resting upon it is in the matter of reproduction, and the 
success with which this needful function is accomplished shows 
us that the parasite must be considered well adapted to its 
conditions, notwithstanding its degeneracy. Its chief hazards 
are met in the passage from host to host and these are over- 
come by the carnivorous and omnivorous habits of hosts and 
the extraordinary powers of multiplication on the part of the 

i6s a . Practical Exercises. Enumerate all the parasites, transient and per 
manent, known to infest man, and find to what groups of animals they belong. 
Report on the habits of the principal parasites on man: as tape- worm, trichina, 
hook-worm, etc. What other hosts are demanded to complete the life cycle? 
What are the principal sanitary conclusions to be reached ? Examine the mouth- 
parts of the mosquito (see Fig. 65). To what kind of feeding are they adapted? 

1 66. Habits and Instincts in Relation to Adaptation. In 

the study of adaptations there is constant danger lest we 
come to consider that structures alone are adaptive. In reality, 
adaptation in the manner of doing things is quite as important 
as in the structure of the organs by which work is done. When 
even the simplest organisms are acted on by an external stimulus 
they respond to it in some way. This response may be either 
advantageous or disadvantageous to the organism. If unfav- 
orable, the result may be disastrous. If favorable later repe- 


titions of the stimulus are all the more likely to be answered 
by the same kinds of response as in the first instance. This in- 
dividual acquirement of a special mode of responding to stimuli 
is known as habit. Since responses in higher organisms occur 
by means of the nervous system we rightly associate habits 
with the nervous activities. In reality, however, mere pro- 
toplasm may acquire these habitual modes of action, and one 

FIG. 65. 


FIG. 65. The head of female Mosquito (Culex). After Dimmock. a. antennae; c, clypeus; h, 
hypopharynx; m, mandibles; ma., maxillae; m.p., maxillary palpus; I, labium; la., labrum (epi- 

Questions on the figure. In what way and for what purpose are the mouth- 
parts of the mosquito used? What are the probable functions of the antennae? 
Compare the antennas and the mouth-parts of a male and female mosquito. See 
also Fig. 42. Mention some respects in which the mosquito is adapted to its mode 
of life? What extent of horopter do its eyes command? To what degree is the 
mosquito parasitic? 

might say that all such adaptations are dependent on the power 
of protoplasm to respond to external stimuli. By reason of 
this power of adaptive responses, organisms may become 
habituated or acclimatized to changes in their environment, 
their habits or responses changing according to the necessities 
of the case. It is a matter of common observation that animals 
can thus gradually be brought to the endurance of conditions 
which would originally have killed them. Such must have 
been true of the animals which have come to live in the waters 
of hot springs. Such must have been the way in which other 
animals were changed from the marine to the fresh- water habit, 
since all fresh-water animals are believed to have been derived 
from marine forms. 


Similarly in the history of any species those individuals 
which respond in suitable or advantageous ways to the stimuli 
brought to bear on them are selected from generation to gen- 
eration in preference to those not so responding, and in the 
course of time certain modes of action become characteristic 
of the species, even without the necessity of individual ex- 
perience. In other words the protoplasm has become so modi- 
fied in a series of generations that responses of a definite kind 
may be expected of it, which cannot be looked upon as in- 
dividually acquired habits. These are instincts and embrace 
many of the most interesting activities which have been men- 
tioned as characteristic of animals. The instincts of feeding, 
mating, and the like are examples. If instincts are in conflict, 
the stronger prevails. In this possibility of situations arising 
in which the instincts are in conflict, or are unequal to a correct 
solution, lies the advantage of intelligence and choice, as adap- 
tations whereby correct responses may be made to external 
conditions. Of the utmost importance in the development of 
intelligence is the introduction of imitation, of training, of 
experience, of memory, factors more or less represented in 
the activities of all the higher animals. It is necessary to re- 
member that what we call intelligence does not arise suddenly 
in the animal kingdom and is not confined to the highest animals. 
Many of the acts usually spoken of as instinctive are not 
purely so, but are the results, in part, of imitation, parental 
or social training, and individual trial and error, and are there- 
fore to be classed as intelligent. 

167. The Dispersal of Animals and the Formation of 
Special Faunas. In section 140 we see that every point occu- 
pied by the individuals of any species becomes, under natural 
influences, a centre of distribution from which the species will 
spread in all directions, unless kept back by adequate barriers. 
Thus we should expect all animals to be found all over the 
earth if all the conditions were equally suitable and all animals 
were equally adaptable to varying conditions. This, however, 
is not so. Species have unequal powers of adaptation to the 
different conditions and thus it comes to be that certain groups 


of species adapted to some special environment will be found 
together in certain regions, but will be absent from others. 
The total animal life of any region is known as its fauna. 

1 68. The Original Home of Animals, and the Sea-faunas. 
There can be no reasonable doubt that animal life began in the 
sea and close to its surface, and probably not close to the shore. 
From this region the various nooks and crannies of the earth 
have been occupied, until now it seems that there is no place 
which does not have at least a few animals suited to its condi- 
tions. The fauna of the surface of the mid-ocean is known as 
the pelagic fauna. It is made up largely of protozoa; certain 
more or less transparent, free-swimming types of invertebrates, 
as worms, jelly-fishes, tunicates; many minute Crustacea and 
fishes. The abyssal or deep-sea fauna contains representatives 
of all types of animals from protozoa to fishes, notwithstanding 
the darkness and the great pressure of the water. Many of 
the forms are highly modified, differing markedly from the 
corresponding species found in other life-regions. The littoral 
or shore fauna is the most varied, abundant, and interesting of 
the sea-f o aunas. Indeed there is no place on the earth where life 
is more abundant. This is true because of the wonderful food 
supply brought from sea and land and broken up by the 
waves, and the great variety of physical conditions at the 
meeting of land and water. 

169. Library Exercises. What are the special conditions of each of the 
regions indicated in the preceding section, which are likely to be favorable or un- 
favorable to life? Illustrate more fully the typical forms characterizing each 
region? Find instances of the special adaptations which seem peculiarly advan- 
tageous to some of the animals frequenting each region. 

170. Fresh -water Faunas. From the littoral regions of the 
sea, animals doubtless originally migrated into the brackish 
water of the mouths of rivers. Thus certain types came to 
inhabit the fresh waters of the streams, and as the result of 
the adaptations thus made necessary new species arose dis- 
tinctly different from their relatives which remained in the 
sea. The most of the branches or phyla of the animal king- 
dom have their fresh- water representatives, but very few 


species of the sponges, the jelly-fish group, and none of the 
starfish group have left the salt water. Some species of 
animals, as the salmon and eels, pass back and forth from fresh 
to salt water in obedience to their spawning or other instincts, 
but these are not very numerous. 

171. From the fresh-water fauna or from the ocean shore 
fauna have come those species which have acquired the power 
of breathing by means of the air. These embrace some worms 
and mollusks, the insects, and the vertebrates above the fishes. 
This adaptation, which is one of the most important acquired 
in the history of animal life on the earth, may have come about 
by the gradual or periodic drying up of fresh-water basins, 
or by means of temporary excursions to the land, such as we 
see some water forms capable of enduring today. Several 
types of these terrestrial animals have achieved a more or less 
complete mastery over the air (aerial fauna) by means of 
flight. Chief among these are the insects, the first group to 
accomplish the task; a group of reptiles in early geological 
times; the birds; and a few mammals (as the bats). Animals 
after passing from one region to another may in their descend- 
ants reoccupy their old habitat. Thus the whales and seals are 
air breathing mammals and are probably descended from land 
forms, but have become aquatic. The same is true of some 
reptiles. Some birds have lost their powers of flight and have 
become purely terrestrial. 

Other divisions of the continental and oceanic faunas into 
geographical faunas are made, depending on the climatic con- 
ditions and the geological history of the regions. The prin- 
ciples governing this division are too complicated for our 
present purposes. 

172. Distribution of Animals in Time. This distribution 
of animals on the surface of the earth does not come from modern 
conditions merely. Throughout all the millions of years that the 
earth has been reaching its present conditions and has been 
inhabited by living things, forces similar to those we now know 
have been at work. During all this time, with geographic 


and climatic changes, the living things were changing both in 
their nature and their position on the earth. Thus during 
the various geologic periods, the distribution and character of 
the life of one period determines the life of the next. 

All our knowledge of the life of the earlier times is gained 
from fossil remains found in the limestone, sandstone, clay and 
other strata of rock. Of course only the hard parts can be 
preserved, and only a small proportion of these are found in a 
form to give us much information. Notwithstanding, we are 
able to get from the strata a very fair idea of the progress of 
life on the globe. In the earliest fossil-bearing strata we find 
only invertebrate remains. The invertebrates have continued 
through all the successive strata to the present time, but in 
doing so they increase in differentiation and become more and 
more like present invertebrates. Of the vertebrates the 
fishes appeared first, then the amphibians, reptiles, mammals, 
and birds. None of these when they first appeared are like 
their modern types. As we pass upward through the strata, 
old species become extinct and new ones more and more like 
the species of the present arise from them, presumably by 
the changes made necessary in becoming adapted to the chang- 
ing earth conditions. In a general way the fossils of any 
age are intermediate " connecting links" between those of 
the ages preceding and following. In other words plants and 
animals have been making progress toward present forms 
through all these ages. The study of this department of 
adaptation of animals is known as Pal&ozoology. 

173. Summary. 

1. It is necessary to consider the individual not merely as 
a group of cells and tissues but as a unit acting and being acted 
upon by all external forces and by other organisms. 

2. Characteristics derived from the germ cells of parents, 
whether resulting in similar qualities or in new ones, are de- 
scribed as hereditary. The reproductive cells are the carriers 
of ancestral qualities. 

3. Individuals vary as the result (i) of internal conditions 
and changes, the causes of which are obscure, and (2) of differ- 


ences in the environment. The environment may produce 
very important changes during the single life of the individual. 

4. The food supply of animals is limited, since all ultimately 
depend on plants; any species multiplying at its average rate 
of increase could in a short time, if unchecked, stock the earth 
up to its limits of support; that this does not occur is due to a 
struggle for food among the excessive numbers which are 
born, whereby only a small percentage of them reach maturity. 
In the main, those survive which possess some qualities which 
tend to fit them for the environment in which they find them- 
selves. These are thus enabled to transmit their qualities to 
their offspring, the fittest of which are again chosen. The 
result is adaptation, and the process is known as natural 

5. A similar result is effected by man in domestic animals 
by artificially selecting individuals in accordance with the pos- 
session of certain features. The resulting forms are frequently 
very unsuited to the natural environment, and could not survive 
if left to themselves. 

6. As the result of various causes animals become dispersed 
from their point of origin, and in becoming adapted to the 
different regions into which they go, or through variation 
within a given region, give rise to new varieties. When, by 
any means, these groups have become perfectly adapted to 
their new special environment and permanently different from 
their parent stock and from each other, without intermediate 
individuals which manifestly connect the varieties, they are 
recognized as new species. Through the influence of heredity 
and by natural selection these differences may accumulate, 
apparently to any amount. 

7. The nutritive function relates particularly to the con- 
tinued existence of the individual; the reproductive function 
looks to the continuance of the species, and is a tax on the in- 
dividual. Nature has specially favored those organisms in 
which an increasing degree of energy is given to the production 
and care of the young. As it is sometimes expressed, nature 
sacrifices the individual to the welfare of the species. 

8. Animals become adapted to all the influences that tend to 


make or mar their success in life. The more powerful the 
influence the more certain the adaptation, because the destruc- 
tion is the more certain in case of failure. The principal 
classes of adaptations are, those relating to the using of the 
favorable and resisting the unfavorable features of the inani- 
mate environment; those assisting in the obtaining of food 
whether vegetable or animal; those of mating and care of 
young; those of offense and defense, in predaceous animals 
and their prey. The relations and adaptations range all the 
way from indifference to friendship, and from feeding at the 
same table on the one hand, to the utmost antagonism on the 

9. Perhaps the most important and the least understood of 
the series of adaptations which animals acquire are those con- 
nected with the nervous system and its functions: the habits, 
instincts, and intelligence of animals. They are inseparable 
from those already enumerated, and yet in fundamental im- 
portance they form a group of their own. They seem pri- 
marily to depend upon the irritability of protoplasm which 
enables it not merely to respond but to become permanently 
changed by that response a kind of organic memory. From 
this fact acclimatization and adjustment become possible. 

10. In being scattered from their starting place, animals 
with similar powers of response and adaptation come to be 
located in the same kinds of conditions. This results in faunas 
more or less characteristic of all the important kinds of en- 
vironments: as marine, brackish water, fresh water, terrestrial, 
aerial, cavern faunas, etc. 

11. The origin of animal life was in the ocean, and from 
these marine types it is believed that all other forms of animal 
life have come, by gradual adaptation to their present mode 
of life. 

12. The various climatic zones of the earth and the principal 
geographical regions are characterized by distinct forms of 
life. For example, the lake life of Africa differs from that of 
North America, and similarly for all the various types of 
fauna. An analysis of such facts and an explanation of them 
belongs to the geographical distribution of animals. 


13. Fossils are remains of former plant and animal life 
preserved in the rocks. We read much of the ancient life 
history by a study of these fossils. We learn chiefly that life 
started in a more lowly and more simple form than we now find 
it; that it has been getting more complex and more like the 
present with each passing age. What we find then at the present 
moment on the earth is not the result of present forces. It is 
the result of all the past. 

174. Topics for investigation, in field, laboratory and library: 

1 . What constitutes individuality in animals ? 

2. In what respects (enumerate) and to what degree have you ever noticed 
variety in a given species? In the offspring of a pair of parents? 

3. Have you ever observed any changes in structure in animals which could 
reasonably be attributed to change in environment ? Give evidence. 

4. Does use or disuse produce changes in the organs of an individual? Why? 
Give illustrations. 

5. Enumerate some facts of your own observation which illustrate heredity. 

6. Cite observed instances of associations among animals of the same species, 
and determine as well as you can from your observations what ends are gained by 
the association. 

7. Make an effort to classify a series of objects, noting carefully your basis of 
classification; that is, the characters which you select in separating and grouping 
the individuals. The teacher can make this a most instructive exercise. A few 
objects of considerable diversity may be chosen, as sand, pebbles, shells, crystals, 
a plant, an animal, and the student may be required to examine each as fully as 
he can, write out the characters which he discovers as belonging to each, being 
sure that he uses a simple and observed feature in each statement. On the basis of 
these recorded observations let him compare and group the objects. Or take a 
large number of relatively similar individuals and, without stopping to write 
their characters, let the student place or distribute them in groups near or remote 
from each other in proportion to their unlikenesses, allowing intermediate forms 
to stand between. Afterward he may be caused to determine and justify his classi- 
fication and to see whether other classification could be made with a different basis. 
Gasteropod shells, illustrating varieties of the same and different species; beetles; 
butterflies; grass-hoppers; or even books of diverse shape, binding and contents 
may be used. 

8. Can you suggest any cause for the degeneracy of parasites? 

9. Cite instances from you own observation in which animals use the leap or 
spring in capturing prey or escaping enemies. Why is it a peculiarly advantageous 
adaptation ? 

10. Cite instances of the food-storing instinct, with all observed details. What 
is the most remarkable fact about them? How is it useful? 

11. From reading and observation would you say that there is any definite 
relation between the instinct for home-building and parental care? 

12. Study sleep among animals. What is its relation to rest? Is it found in 
the lower animals ? To what is sleep an adaptation ? When does sleep commonly 
occur among animals ? Why? Do plants show any sleeping qualities? 


13. What are the principal geographical faunas recognized by zoologists. 
Enumerate the more important means by which dispersal of animals from one 
region to another occurs. What are the chief barriers to the dispersal of land 
animals? of aquatic animals? 

14. Study the different authors to which you have access, as to the significance 
of the terms species and variety (or sub-species). 

15. What were the older views concerning the "fixity" or invariability of 

1 6. What are the different views of the "Origin of Species," as based on the 
views of the meaning of species ? 

17. What is the essential difference between the theories of "natural selection" 
and "definite variation" as explaining adaptation of organisms to their environ- 

1 8. What additional ideas are introduced by the "mutation" theory of De 
Vries? What reasons does he give for thinking that natural selection is not a very 
important factor in adaptation ? 



Before undertaking the study of the special groups into 
which animals are arranged because of their apparent kinships, 
it will be advantageous for the student to look briefly at the 
whole field of animals, the "animal kingdom." See Fig. 66. 

175. Class Mammals. Beginning with man himself it is easy 
to see that there are numerous animals (as the apes and monkeys; 
the various quadrupeds, as the horse, ox, dog, cat, bears and 
squirrels; the whales and seals; and many others) which differ 
much in general appearance from him but are like him in very 
many remarkable particulars. For example, they all bring 
forth their young alive and in a more mature condition than 
is usual for other types of animals, the young being carried 
in a special organ of the mother's body, often until develop- 
ment is well advanced. After birth the mother produces milk 
in special glands for the nourishment of the young to a still 
more mature stage. This is seen in no other group of animals 
beside the mammals. The skin produces hair or wool as a 
covering for the body. Man differs from the other mammals 
in certain particulars but not nearly so much as he and they 
differ from other animals. 

176. Class Birds. Another well-developed and numerous 
group of animals is the class known as birds. There is scarcely 
another class of animals so easy to distinguish at sight as this. 
They equal or surpass the- mammals in specialization, but are 
very different from them. They are especially to be recognized 
by the body-covering of feathers, the modification of the front 
limbs into wings for purposes of flight, and the fact that the 
jaws are sheathed in horny matter and, at least in present day, 
birds do not possess teeth. 

177. Class Reptiles. This is a class recognized by zoologists 
which is not nearly so easy to define or to identify as either of 



the preceding. This is partly because the animals composing 
it differ more among themselves than in the other classes. It 
includes snakes, lizards, turtles, and crocodiles. The reptiles 

FIG. 66. 





FIG. 66. Diagram showing the general relations of the chief divisions of the animal kingdom. 
The number of species belonging to each is roughly approximate, only. During the last 30 years 
the number of species of animals recognized by zoologists has increased at the average rate of 
12,000 per year. 

have some features which indicate that they may be distantly 
related to both birds and mammals, as well as to the next 
class. This is an additional reason why the group of reptiles 
is a difficult one to define. In general they may be recognized 


by the fact that their bodies are covered by scales or plates 
instead of hair or feathers. They always breathe oxygen from 
the air, as do birds and mammals. They usually have only 
three chambers to the heart whereas in the former groups 
there are four. The blood is not constantly warm as in birds 
and mammals. They lay eggs very much like those of birds. 

178. Class Amphibians. In external appearance the mem- 
bers of this class often look somewhat like reptiles, and they 
have certain possessions in common with them, as the cold blood 
and the three-chambered heart. They are especially noteworthy 
from the fact that they begin life breathing oxygen from the 
water as fishes do, and later in life lose their gills, acquire 
lungs, and get their oxygen from the air, as do the reptiles 
and higher forms. Amphibians include the frogs, toads and 
salamanders. This is not a very important group in nature, 
but is intensely interesting to the student of zoology because 
it seems to be a connecting link between the air-breathing and 
the water-breathing forms. 

179. Class Fishes. Fishes are characterized by the fact 
that they breathe by means of gills throughout life. The body 
is often scaly; the appendages are fin-like; the blood is cold, 
and the heart has two chambers. They are beautifully adapted 
to life in the water and are easily recognized. 

1 80. Vertebrates and Invertebrates. All the animals of 
which we have thus far spoken agree in certain particulars. 
They all possess a dorsal rod of supporting matter (notockord; 
see 349), which is often surrounded by cartilage or bone (the 
vertebral column). The nervous system in all of them is dorsal 
to this rod and to the digestive tract, and is tubular in char- 
acter. The heart is ventral to the digestive tract and the 
blood has red corpuscles. They are called Chordata (having 
notochord) or Vertebrata. This is the highest, best developed 
phylum of the animal kingdom. The five classes which have 
been mentioned are included in it. All other animals, with the 
exception of a few which seem intermediate in some respects, 
are classed as Invertebrates, and agree in general in the follow- 


ing facts: there is no notochord or vertebral column; the 
nervous system is chiefly ventral to the digestive tract; the 
heart, when present is dorsal; and the blood usually has only 
colorless cells. The principal phyla of the Invertebrates 

181. Phylum Arthropoda (jointed feet). This is the most 
numerous phylum of the animal kingdom. It embraces 
crayfish, lobsters, crabs (Crustacea), which for the most part 
have gills and live in water; the Insects, as bees, flies, beetles, 
butterflies, etc., which usually live in the air and get their 
oxygen from it; the spiders, whose habits and appearance are 
somewhat similar to those of the insects. Arthropods are 
especially to be recognized by the fact that their bodies are 
segmented, are bilaterally symmetrical, and have paired jointed 
appendages to many of the segments. In addition to this there 
is a covering of resistant substance (chitin) developed by the 
skin. This serves for the protection of the animal and for the 
attachment of the muscles within. 

182. Phylum Mollusca (soft). This branch of the inverte- 
brates includes the snail, clam and oyster, the squid and devil- 
fish, and their kind. They differ very much among themselves 
but agree in the lack of segmentation of their bodies, in the 
absence of paired appendages, and in those types most 
commonly known to the student, in the presence of a shell 
of one or two valves, which is secreted by a fold of the skin 
called the mantle. While many of the mollusks are lowly in 
organization and in intelligence, one group of them that which 
includes the squid, has the most highly developed brain 
found below the vertebrates. It occupies among the inverte-, 
brates somewhat the place which man has among the mammals. 

183. Phylum Echinodermata (spiny skin). These are easily 
recognized by the possession of five or more arms or rays in 
the adult stage'. Usually a skeleton is developed in the skin. 
This is often covered with spines, and from this fact the phylum 
has its name. They are marine and are poor movers, a few 
being fixed by stalks to objects in the ocean. The starfish, 
sea-urchin and sea-lilies are representatives. 


184. Phylum Annulata (Segmented Worms: with rings). 
This phylum is similar to the arthropods in that the body is 
bilaterally symmetrical, is segmented, and has paired append- 
ages to many of the segments. It differs from them in the 
fact that the appendages, when present, are not jointed but 
are merely setae or hairs in sockets or on fleshy prominences. 
The segments are more nearly homonomous than in typical 
arthropods. The earthworm, many types of aquatic worms, 
and leeches are included here. 

185. Unsegmented Worms (embracing numerous ill-as- 
sorted animals of doubtful relationship). Here may be in- 
cluded a number of small groups many of which have long 
been grouped with the Annulata and called "worms." They 
are not sufficiently alike to be regarded as one distinct phylum; 
Indeed there are probably three or four small phyla included. 
The majority of them are bilaterally symmetrical, unSegmented 
and without appendages. They differ from the mollusks in 
that they do not possess a mantle and do not secrete a shell. 
Many of them are parasitic. Among these animals of doubtful 
relationship may be included the "flat- worms," "thread- worms," 
the nemertea, rotifers, and others. 

1 86. Ccelomata (with ccdom) and Ccelenterata (hollow 
inside). All the animals thus far considered possess during 
some stage of life a more or less developed body cavity or 
ccelom (see 58) distinct from the digestive tract. For this 
reason they are sometimes known collectively as Ccelomata. 
All the remaining many- celled animals have a general cavity 
which serves both as a body cavity and a digestive tract (gastro- 
vascular cavity), or to speak more exactly, there is no true 
body cavity. Of these the phylum Ccelenterata is the chief 
illustration. Here belong the jelly-fish, sea-anemone, corals. 
They are all aquatic and are more or less tubular, sac-shaped 
animals often attached by one end, with the mouth, which also 
functions as the anus, at the other surrounded by clusters of 
tentacles. Many secrete skeletons, and some form immense 
attached colonies. 


187. Phylum Porifera (pore-bearing). This group, to which 
belong the sponges, is sometimes classed with the Ccelenterata. 
While similar to them in habit the sponges are much less highly 
organized and unified. Instead of a single mouth opening into 
the digestive tract, sponges have many openings or pores 
(whence the name Porifera) which are the beginnings of tubes 
entering a central cloaca or sewer. This is in reality not a true 
digestive tract. It communicates with the exterior by one or 
more large passages. They are attached and usually form large 
colonies by budding. 

1 88. Phylum Protozoa (first animals). All the preceding 
phyla of animals consist, in the adult stage, of many cells among 
which there is more or less differentiation. In all of them the 
adult passes through stages in which the cells are arranged in at 
least two layers (ectoderm and entoderm; see 55), from which 
the tissue-masses arise. These animals are known as Metazoa. 
In the remaining phylum the Protozoa the animals are 
single cells, or at most, loose aggregations of similar cells. 
They are the lowest of animals and are for the most part in- 
visible to the naked eye. 

189. An Artificial Key to the Phyla of the Animal Kingdom. 

Many-celled animals METAZOA. 

With true coelom Coelomata. 

Possessing notochord (and often vertebral column), 

Phylum Chordata. 
Possess functional gills. 

Throughout life Class Fishes. 

In embryonic life only (with a few exceptions), 

Class Amphibia. 
Do not possess functional gills. 

Epidermal covering of scales Class Reptiles. 

Epidermal covering of feathers Class Birds. 

Epidermal covering of hair Class Mammals. 

Without notochord* Invertebrata. 

Bilaterally symmetrical (chiefly). 
Body made up of segments. 

Paired appendages jointed Phylum Arthropoda. 

Paired appendages unjointed Phylum Annulata. _ 

Body unsegmented; without paired appendages. 
With mantle often secreting shell, 

Phylum Mollusca. 
No mantle Unsegmented Worms. 


Radially symmetrical in adult Phylum Echinodermata. 

Without true coelom. 

With a single mouth, which also functions as an anus: stinging cells 

Phylum Ccslenterata. 
With numerous incurrent openings or pores, and only one or few 

excurrent. No stinging cells Phylum Porifera. 

Single-celled animals (chiefly) Phylum Protozoa. 


PHYLUM I. PROTOZOA (Primitive Animals) 


Without compound microscopes this branch of animals can- 
not be studied with profit in the laboratory. The Amoeba is 
one of the most interesting of the Protozoa and serves well to 
illustrate the simplest forms of animal life, but large specimens 
in sufficient numbers for profitable study in an elementary class 
are usually so difficult to secure at the right time that it be- 
comes a question whether the teacher should be advised to 
depend on them. My advice is, make every arrangement you 
can to secure them, use them for demonstration or study 
whenever they appear, but depend on Paramecium. Perhaps 
the surest method for securing Amoeba is to chop up the soft 
parts of three or four fresh-water mussels, placing the pieces, 
together with the shells, in a large shallow basin. Allow a 
gentle stream of water to drip into this. This keeps the water 
slightly agitated, causes it to run over, and prevents an undue 
accumulation of bacteria. The addition of a little of the sur- 
face mud secured from the bottom of several streams or ponds 
will make the success of the preparation all the surer. Amcebas 
should appear at the surface of the mud, about the shells, 
or at the margins of the vessel near the surface of the water. 
Test all these places every day, and sooner or later the amoebas 
are practically sure to be found. Paramecia will be likely 
to occur in the same preparation. Any abundant Protozoan 
which may appear may be studied instead of Paramecium or 
in addition to it, by means of the outline below. The mode 
of securing the materials should be explained to the class to 
make clearer the habits of these organisms. 

190. Paramecium. This protozoan may be obtained 
readily by allowing fresh- water algae, with hay or leaves, to 



decay in water. This infusion should be examined every day. 
If the bacteria become too abundant some of the surface water 
may be poured off and fresh water added. The paramecia, 
which are just visible to the naked eye, appear as a whitish 
cloud in the water or may accumulate as a film at the surface. 
Often a sufficient number for study may be secured by scraping 
with a scalpel the matter which accumulates on the sides of the 
vessel just beneath the water surface, even when they are not 
sufficiently numerous to cloud the infusion. The cover-glass 
should be supported by sediment or by bits of cover-glass. 
Make outline sketches of everything which can be shown in 
that way. 

I. With the low power of the microscope study the follow- 
ing points: 

1. Activities. Describe, and figure as well as possible, the 
nature of all the movements of which the animal seems capable, 
using arrows to indicate directions. Can you distinguish an 
anterior from a posterior end? By what characteristics? 

Do you find any reasons for believing that the paramecia 
are sensitive to external influences? What evidences? To 
what sorts of influences do they respond? Do they avoid ob- 
jects? Do they collide with each other in motion? Do they 
tend to collect ? Where ? Are they as active at the end of the 
hour as at the beginning ? 

Make a new preparation in which the paramecia are uni- 
formly distributed in a drop of water. Place a very small 
grain of salt at the edge of the drop. What is the result? 
Watch the individuals under the microscope as they come into 
the salt solution. On a new preparation, try similarly a minute 
amount of acetic acid (Ko to % per cent, solution) applied with 
a capillary tube. Compare results. Try sugar; quinine. 

Do you discover any instances of division or conjugation? 
If so, describe. 

2. General form of the body. How would you describe 
its shape? To what degree is it capable of change? Is the 
body symmetrical ? Give evidences. Make diagrams showing 
your idea of a cross-section through the middle; also of one, 
one-third way from each end. 


II. With the high power, study, 

3. Cilia: where found? Are they uniform in length? How do they act? 
What results do they produce? (Place a small amount of water containing finely 
powdered indigo or carmine at edge of cover-glass. If the movements are too 
rapid a little gelatine added to the water will be of advantage.) 

4. Find the mouth, with the oral groove leading to it. Position and shape? 
How are food particles captured? Can you find them within the body (food 
vacuoles) ? How are the food vacuoles formed ? Do the food vacuoles move within 
the cell? If so, trace their course? What finally becomes of them? Evidences? 

5. Contractile vacuoles (clear spherical objects rhythmically disappearing and 
reappearing). Number? Position? Rate of contraction? Do they contract 
at the same time? What becomes of the clear material during the contraction of 
the vacuole? Are they deep or superficial structures? Your evidences? Does 
change of temperature cause any change in their rate of contraction? 

6. Distinguish between the inner mass of protoplasm (endosarc) and an outer 
layer (ectosarc). What are the characteristics of each as regards motion, clearness, 
firmness, etc. ? Note the changes in these portions on the addition of dilute acetic 
acid or iodine at the edge of the cover-glass. 

7. Discover, if possible, nuclear bodies. These are not usually recognizable 
without careful staining. Place at the edge of the cover-glass, in a fresh preparation 
of Paramecia, a 5-10% aqueous solution of methyl green. Compare the result 
with a permanent mount stained by suitable methods (see Appendix : Suggestions 
for the Laboratory). 

191. Other Protozoa. If the class is supplied with microscopes, the pupils 
should be allowed to examine stagnant water for as many types of protozoa as 
may be found. Allow them to compare these, noting the points of similarity and 
difference in general structure and activities. Especially profitable protozoa for 
laboratory work are the green flagellate infusorian, Euglena, which often tinges the 
water, or forms a green scum over shallow pools of water; the colonial ciliate form, 
Vorticella, found attached to submerged objects in ponds or pools of slowly moving 
streams in which there is considerable decaying organic matter. The colonies are 
easily visible to the naked eye. Stentor is a very large trumpet-shaped infusorian 
which may be alternately attached and free-swimming. It lives upon submerged 
sticks and leaves and may often be found attached to the sides of vessels in which 
such matter has been placed. In all such studies and identification of the protozoa 
the question of evidence of the unicellular character of the organism should be kept 
before the student. 


192. In this first and lowest group of animals, the individu- 
als of which consist of single cells or loosely associated simi- 
lar cells, we find something of the variety of shape which we 
observed in the tissue cells of the higher animals (Chapter V). 
The Protozoa are especially interesting to the biologist because 
they represent the simplest forms of animal life now found 
on the earth and because some of their representatives are very 


like some of the simplest plants. Indeed some of them are 
claimed by both the botanists and the zoologists. It also seems 
probable that the first animal life to appear on the globe had 
the general characteristics of some of the Protozoa. Whether 
some type of protozoan is to be considered as the ancestor of 
the higher many-celled animals or not, it is true that we find 
illustrated here in the simplest possible way the beginning of 
all those functions . which are so completely distributed among 
the special organs of the complex animals. The Paramecium 
does in a simple yet satisfactory way all that any complex living 
animal needs to do in order to live and perpetuate its species. 

FIG. 67. Amoeba, ec., ectosarc; en., endosarc, containing food vacuoles (/); n, nucleus; p, pseudo- 
podium; p.v., pulsating vacuole. 

Questions on the figure. Define the various terms used above in describing 
the parts of the amoeba. What changes may the amoeba undergo in its life history ? 
Compare with figures 2 and 7. 

193. General Characters. 

1. Mostly unicellular throughout life. May have one or 
more nuclei (Figs. 68-71). 

2. The protoplasm usually consists of a clearer outer por- 
tion (ectosarc) and a more granular inside portion (endosarc) 
(Fig. 68, ec, en). 

3. There is usually what is known as a pulsating vacuole, 
in which some of the more fluid cell-contents collect, to be 
forced out of the vacuole again by the contraction of the denser 
protoplasm (Fig. 68, pv). 


4. Reproduction is effected chiefly by dividing into two or 
more parts or cells, which occasionally remain associated. The 
nucleus, when present, divides with the division of the cell 
(Fig. 7). 

FIG. 68. 


FIG. 68. Paramecium in optical section (semi-diagrammatic). A, anterior end; c, cilia; t.c., 
ectosarc; e.n., endosarc; /.., food "vacuole"; g, gullet; N, meganucleus; n, micronucleus; o, oral 
groove, leading to the mouth; p.v., pulsating vacuoles in different stages of contraction; tr., tricho- 
cysts; v, food vacuole in process of formation. 

Questions on the figure. In what sense is the term "vacuole" descriptive of 
the structures to which it is applied in Paramecium? Describe the special adapta- 
tions of the anterior end. Judging from their distribution have the cilia any other 
function than locomotion? In what way are the food vacuoles formed? Why do 
some food vacuoles appear lighter than others? 

194. Habitat. Protozoa in their active stages require 
abundant moisture, hence they are found in water, fresh or 
salt, and as parasites in the bodies of other, animals. The 
Sporozoa are parasitic. Some amoeboid Rhizopods infest the 
digestive tract of man and other animals, producing irritation 
and disease. The Infusoria occur in water in which there is 
decaying organic matter and minute organisms of various 
kinds. Vohox and Euglena, green forms often classed as 
Protozoa, have the power which green plants possess of using 
the inorganic substances found in ordinary water in building 
up their substance. 

195. Organization. We cannot say that Protozoa have 
organs in the sense in which we have defined that term hitherto, 
yet they are certainly organized. The organization shows 
itself in the nucleus, in the distinction of ectosarc and endosarc, 


in the pulsating and food vacuoles, in temporary projections 
of protoplasm called pseudopodia, in more permanent vibratile 
projections of the ectosarc known as cilia or flagella, in the 
mouth found in many forms, in cell- wall and secreted skele- 
ton, in delicate contractile fibres in the ectosarc, and in stalks 
for attachment to objects (see Figs. 68 and 70). By means 
of these differentiations all the functions necessary to life are 
performed. There are many colonial Protozoa. In such (as 
Volvox) there may be some division of labor among the cells, 
as between reproductive cells and body cells (Figs. 72, 73). 

196. Nutrition. The parasites absorb food, already di- 
gested and fitted for absorption, directly from their hosts. 
Most of the free forms take solid particles directly into the 
endosarc through permanent or temporary openings in the 
ectosarc. In some shelled forms, in which there is no mouth, 
the food is digested outside the body proper (Fig. 74) by the 
pseudopodia. These envelop the food and gradually transfer 
it to the main body of protoplasm through openings in the 
shell. In the other instances the digestion takes place in 
the body of the protoplasm. The ferments found in the proto- 
plasm are doubtless responsible for the digestive changes and 
act in much the same way as the special ferments secreted 
from the cells of the digestive glands in the higher animals. 
Circulation is effected 'by the general protoplasmic motion. 
Respiration, whereby the protoplasm gets rid of C0 2 and 
receives O, occurs through the cell surface without special 
structures. All projections of the cell-body assist in this 
exchange by increasing the area of the surface. Excretion 
may take place from the surface of the cell, and it seems probable 
that the contractile vacuole has an excretory function. 

197. Movement. The majority of Protozoa move freely 
in their medium. In Amoeba the motion is of a gliding character 
and is effected by putting forth processes into which the proto- 
plasm streams. The process or pseudopodium thus enlarges at 
the expense of the body of the cell and progress is had in the 
direction of the growing pseudopodium. The direction of 
motion is changed by the breaking out of new processes in a new 


direction. In those Protozoa which have a cell-wall special 
devices become necessary to enable the animals to move. Most 
of the free-swimming forms possess cilia or flagella, which 
act as oars on the water and thus propel them. In Stentor, 
Spirostomum, Vorticella, etc., there are clearly defined strands 
of contractile material developed in the ectosarc by which the 
shape of the animal may be strikingly changed. In the at- 
tached forms these strands extend from the body proper into 
the stalk. Vorticella (Fig. 70) by this device may change its 
position with much suddenness. Attached forms are able to 
break loose from their moorings and become free-swimming 
for a time. Still other species are encased in shells and are 
almost or wholly destitute of the power of independent motion. 
Even the most active types may assume the non-motile or 
resting stage, by which they pass uninjured through such 
unfavorable conditions as drouth, cold, and the like. 

198. Sensation and Behavior. All the Protozoa show more 
or less sensitiveness to external conditions. They may be 
caused to contract and move by mechanical stimuli such as 
contact or jarring, by chemically active substances in the water, 
by light, by changes in temperature, and the like. Vorticella 
and Spirostomum are exceedingly sensitive to contacts; Amoeba 
avoids the light; many forms seem to find their food as the 
result of the chemical differences in the water and may be 
seen to swarm about suitable objects; the contractile vacuoles 
of many forms contract more rapidly in warm than in cold 
water; Paramecia tend to collect in groups at the edge of the 
cover-glass, around air-bubbles, about green filaments, or 
even without any foreign matter whatever. So far as we 
know, these simple responses do not give evidence of special 
organs, but merely represent a diffused protoplasmic irritability 
and power of responding to stimuli (20, 21). 

On the whole, when protozoa are stimulated, their response 
is an advantageous one. That is it is positive, or toward sub- 
stances or forces that are of help to it; and negative to stimuli 
that are hurtful. It is not believed that the protozoan is con- 
scious of these conditions. It probably means that they have 



inherited the tendencies which through untold generations have 
resulted in safety. Those with wrong tendencies have been 
eliminated. In this way, through generations of trial and error 
and by adjustments on the part of the organisms they have 
become adapted to the conditions of life. We have no evidence 

FIG. 69. 

FlG. 69. Paramecium. i, transverse fission; 2-5, stages in conjugation. Lettering as in Pig. 
68. The meganucleus gradually disintegrates during the process and the micronucleus by two 
successive divisions forms four micronuclei. Two of these disintegrate. One of the remaining 
micronuclei (n) in each animal passes into the other Paramecium and unites with the stationary 
micronucleus (n*). thus fertilizing it. Later a new meganucleus is formed in each animal by the 
division of this body. Nucleus n* is often smaller than n and may represent the male element. 

Questions on the figure. Which is permanently represented in the cell during 
conjugation, the micro- or the mega-nucleus? Which seems to correspond most 
nearly to the ordinary nucleus of higher forms? What really transpires in the act 
of conjugating? Compare this with more elaborate figures and descriptions in 
reference texts. 

that they learn by experience. . They transmit their native 
qualities, but not the results of accident to the individual body. 

199. Reproduction. In the Protozoa we discover methods 
of reproduction which are to be looked upon as suggestions of 



FIG. 70. 

FIG. 71. 

FIG. 70. A, Vorticella, a stalked ciliate Infusorian: i, contracted; 2, extended. /, food 
"vacuoles"; g, gullet; m, contractile fibre (muscular); w, nucleus; o, mouth, surrounded by ciliated 
disc; p.v., pulsating vacuole; s, stalk. B, a colonial type similar to Vorticella. 

Questions on the figures. Compare the internal structure of Vorticella with 
that of Paramecium (Fig. 68). What are the principal differences? Likenesses? 
How is a colonial type (as B} formed? How are new colonies started? In what 
way does the animal become extended after contraction? Compare living 

FIG. 71. A, Euglena viridis, a flagellate Infusorian. i, typical swimming condition; 2, some- 
what contracted; 3, spherical resting condition; 4, encysted stage in which fission has taken place. 
c, cyst;/, flagellum; n, nucleus; o, mouth; p.v., pulsating vacuole; sp, pigment spot. 

B, Podophrya, a stalked Infusorian bearing tentacles (<) P> Infusorian captured for food; s, 

Questions on the figures. How does multiplication in Euglena differ from that 
of Paramecium? What are the differences in the method of feeding employed in 
Vorticella and in Podophrya? What is the structure and function of the tentacles 
in the latter? 


methods found in the Metazoa. Reproduction among the 
Protozoa is, primarily, mere fission or division of the cell-sub- 
stance. In some instances this division is little more than an 
irregular breaking up or fragmentation of the protoplasm. In 
others, one or more buds may arise from the parent cell. A 
more typical method is by the equal division of the parent into 
two new individuals. In still other instances, especially among 
the Sporozoa, there is the formation of a cyst, within which 
the protoplasm rearranges itself in numerous small bits. These 
finally break from the cyst as new individuals. In all such 
cases the old nuclear material is distributed among the daughter 

There are some indications that the process of division 
carried on for a long time without cessation results in a gradual 
loss of the vitality of the stock. There are three ways in 
which this untoward result is overcome, so that a kind of re- 
juvenation occurs. In the first place, a thick wall may be 
formed and a period of rest ensue (encystment). It appears 
also that new vitality may be given to a culture of paramecia 
by rhythmic changes in the conditions of life, particularly in 
the foods. Or in the third place, there may be a temporary 
(Paramecium) or permanent (Volvox, Vorticella) union of two or 
more individuals. This is conjugation. The essential thing in 
conjugation seems to be the introduction of new nuclear matter 
into the cell. The conjugation-cells (gametes) may be alike 
(Paramecium), or diverse (Vorticella or Volvox). 

Paramecium may reproduce for many generations by divi- 
sion, and then two individuals may conjugate, exchange certain 
nuclear elements, and separate, beginning once more their 
process of division. There is here no sign of sexual dimorphism 
in the Paramecia themselves. It has been discovered, however, 
that the portion of the nucleus which passes out of each con- 
jugant into the other is smaller than that with which it unites. 
It might therefore be considered a male nucleus. 

In the colonial species, as Vorticella and Volvox, there is 
the union and permanent fusion of the whole protoplasm of 
individuals (cells), distinctly different in form and size, to 
produce the new individual. This is much like the dimorphism 



found in the sexual cells in the Metazoa or many-celled animals, 
and illustrates heterogamy (see 101). Consult Figs. 7, 69, 73. 

200. History. The existence of the Protozoa was prac- 
tically unknown until the compound microscope came into use. 

FIG. 72. 

FIG. 72, Eudorina. A colony of 16 flagellate cells imbedded in a gelatinous matrix. 

FIG. 73- 

FIG. 73. Eudorina. The development of reproductive bodies within the colony from the 
ordinary vegetative cells (). /, a mass of female cells; m, a mass of male or motile cells;/', a single 
female cell surrounded by male cells (mO; w, the boundary of the original colony. 

Questions on figures 72 and 73. What suggests that this is a colony rather 
than an individual? What suggests the reverse? Compare accounts in other 
texts to test your conclusions. What degree of differentiation is shown among the 


A naturalist of Holland, Leeuwenhoek, first discovered the 
Infusoria, and thus opened up one of the most interesting 
departments of zoology. It was not until the middle of the 
nineteenth century that the simple, unicellular structure of 
the Protozoa was really understood. Many of them can endure 
drying, be blown about in the spore stage, and then take up 
active life again on the return of water, so that thereupon, in a 
few hours, Infusoria may literally swarm where none seemed to 

FIG. 74. A compound Foraminiferan Nodosaria. a, aperture of shell; /, food particles 
captured by the strands of protoplasm outside the shell; n, nucleus; sh, shell. 1-4, the successive 
chambers of the shell; i, being the oldest. 

Questions on the figure. Does this seem a colony or a single individual? 
Why ? Why is digestion possible outside the capsule ? Compare this with figures 
of Protozoa in which there is no large aperture to the shell. 

be. This is responsible for the long life of the old belief that 
they arose by "spontaneous generation," that is, without 
parents. It is only in recent years that this belief has been 
finally disproved. It is known that they do not appear in 
water that has been boiled and kept free from exposure to the 
air. Much brilliant work has been done on the group in recent 
years, on structure, on behavior, and on their relation to disease. 

201. Classification of Protozoa. The following are the principal classes of 



Class I. Rhizpoda (root-footed). Type: Amoeba. The Rhizopoda are amoeboid 
in form with pseudopodia, which may be either blunt (Fig. 67) or slender (Fig. 74). 
The protoplasm may be naked (Amceba) or may secrete a shell either calcareous 
(Foraminifera) or siliceous (Radioloria). In the shelled forms the pseudopodia 
pass out through openings in the skeleton (Fig. 75). Reproduction is usually by 
division, or by the formation of many spores. Encystment frequently occurs. 

Class II. Mastigophora (whip bearers). Types: Euglena, Chilomonas, Volvox, 
Trypanosoma. Active protozoa which may be simple or colonial. They bear one 
or more large lashes or flagella. The trypanosomes are blood-parasites. 

Class III. Infusoria (in infusions). Types: Paramecium, Stentor, Vorticella. 
Predominantly active protozoa, usually without shell, but with distinct cortical 
portion from which project permanent vibratile threads of protoplasm (cilia, 
flagella, or tentacles), from the possession of which the sub-classes are named. 

FIG. 75. 

PIG. 75. Actinomma, a radiolarian with a shell and no mouth. A, whole animal with a portion 
of two spheres of shell removed. B, section, showing relation of protoplasm to the skeleton, c., 
central capsule; n, nucleus; p, protoplasm; o, openings through which the pseudopodia extend. 
(From Parker and Haswell.) 

There is usually a permanent mouth. The nucleuses always present and assumes 
a great variety of shapes. The infusoria are typically free-swimming, but many 
are capable of attachment by a contractile stalk, to foreign objects (Vorticella). 
Reproduction is normally by equal division, but budding and spore formation 
occur. Conjugation is common, and may be either temporary or permanent. 

Class IV. Sporozoa (spore- animals). Types: Plasmodium vivax, Gregarina. 
Protozoa predominantly passive in habit, parasitic, with no pseudopodia, and no 
cilia in the adult. Remarkable for encysted resting stages and spore formation. 
Conjugation often precedes the formation of the cyst. 

202. Place in Nature. Protozoa are an important element 
in the food of many aquatic animals. Despite their minute 
size, their immense numbers and universal distribution make 


them important. Together with bacteria they serve to save 
for the organic world much decaying material which no other 
animals could utilize. The bacteria decompose organic matter 
and the protozoa devour bacteria. They in turn become food 
for higher animals. We have seen that there are green forms 
that manufacture their own food. Some live on debris, some 
are predatory, some are parasitic, and some are symbiotic with 
algae. Rhizopod shells dropping to the bottom of the ocean 
form the "ooze," the chalk of later geological epochs. Other 
forms of limestone also are produced by the accumulations of 
these calcareous shells. Similar masses of the siliceous shells 
occur in various parts of the earth. 

Some of the Protozoa, especially the parasitic Sporozoa 
produce diseases in man and other animals. Malaria and 
possibly yellow fever in man are caused by Sporozoa in the 
blood. In both these diseases, species of mosquitoes are 
apparently the cause of the introduction of the spores into the 
human system. Texas fever, one of the most dreaded of the 
diseases of cattle, is believed to be communicated through the 
cattle tick, in which the sporozoan producing the disease 
undergoes a portion of its life history. Trypanosomes, flagel- 
late blood-parasites, are responsible for "sleeping sickness". 
in man in tropical regions. Similar parasites are found in the 
blood of rats and other animals. 

Amoeba-like rhizopods in the intestine of man cause some 
forms of dysentery and other derangements of the tract. Similar 
organisms accompany small-pox, hydrophobia, pyrea, and 
other disease, though it is not known whether they have 
an active influence on the diseases. 

Pieces of such protozoa as Stentor have been shown to be 
able to regenerate a whole animal, provided a portion of both 
nucleus and protoplasm are present, but not otherwise. This 
shows that each is necessary to the activities of the animal. 
Because they are lowly and simple animals, we must not con- 
sider that they are either unimportant or unsuccessful in the 
struggle for existence. Their wonderful reproductive power 
insures that they hold their own whenever the conditions are 
at all favorable for them. They occur in practically all the 


waters of the earth, increasing or decreasing as their food 
varies in abundance. 

203. Supplementary Studies for the Library. 

1. The reactions of Protozoa to light; to chemical substances; to heat; etc. 

2. Their power of resistance to heat; cold; drouth. The practical results 

3. The economic importance of Protozoa. 

4. What is "plankton"? What is the importance of its study? 

5. Conjugation in Protozoa. Compare methods of reproduction and conjuga- 
tion in the various groups. Follow the nuclear changes in conjugation of Para- 

6. Why should Volvox and Euglena be considered animals rather than plants? 

7. Diseases in man or animals believed to be caused by the Sporozoa. The r61e 
of the mosquito in the life history of the sporozoa causing malaria and yellow fever. 
The life cycle of this parasite. The bearing of these facts upon infection and the 
management of these diseases. 

8 Forms of the Protozoa of different classes as shown by the illustrations in the 
larger text-books. 

9. The varying form of the nucleus in different species of Protozoa. 




204. Grantia. This is a marine sponge and in consequence 
the majority of schools will be compelled to depend upon alco- 
holic material. Grantia occurs along our New England coast, 
and is found attached to piles or to stones a few feet below the 
low- tide mark. If the school is near the coast the living sponge 
should be studied in a basin of sea-water. 

1. General Form. (Keep in a watch-glass, covered with 
the preserving fluid.) Make careful outline sketches of every- 
thing discovered. 

Note, the basal or attached portion; the column; the free 
end. How do the ends differ ? Are there any openings ? 

Do you find any connection between individuals (budding) ? 
Are these individuals of equal size ? 

2. Structure. Split the body longitudinally with a sharp 
scalpel, and examine with hand lens or a low power of the 

Study, body wall; cloaca (internal cavity); the relation 
of the cloaca to the osculum (the opening at the unattached 

By what is the osculum surrounded ? Notice in the wall of 
the cloaca the minute openings of the radiating chambers. Do 
they communicate with the exterior? What are the functions 
of the osculum and of the pores? Evidences? 

3. Make thin cross sections with a razor, mount under cover-glass, and examine 
further for points in 2. Do both internal and external pores open into the radial 
chambers? Notice the spicules. Is there any regularity in their arrangement? 
What differences in shape and size have you discovered in the spicules from different 
regions of the body? 

4. Place a bit of the sponge in a small amount of a 5 % solution of caustic potash 
and boil Examine under high power, and draw the differently shaped spicules. 

5. Place a bit of the sponge on slide and allow weak acetic or hydrochloric acid 
to pass under the cover. Note and interpret results. 



205. Comparison Demonstrations. 

1. Fresh-water Sponge. In portions of the country where the streams are clear, 
swift, and with rocky bottoms, a fresh- water sponge may often be found which will 
be valuable to compare with Grantia or to substitute for it. It grows attached to 
submerged objects and is commonly of a dirty greenish color, though this may 
vary. This sponge is firm and gritty to the touch, and may be either compact or 
branched. Use the general outline prepared for Grantia, noting the points of 
contrast. Is there anything like the osculum? the cloaca? Gemmules or re- 
productive bodies may occur imbedded in the flesh, especially at the base. 

2. The Sponge of Commerce. This is merely the skeleton of a sponge from which 
all the cellular part has been removed. Select a small rounded specimen. Do you 
find any signs of the attached end? of an osculum? Split the sponge with scissors, 
beginning with an osculum. Are there any canals as in Grantia? If so, what is 
their arrangement? Examine a small portion of the skeleton under the micro- 
scope. Test as before (for calcic carbonate) with dilute acid. Is the skeleton elas- 
tic? Why? 


206. The Protozoa are unicellular animals, or at most, 
masses of similar cells in a more or less globular form. This 
condition is comparable to the morula stage of the embryos of 
higher animals (see 54). In all the other groups (Metazoa) 
the cells at some stage in development are in at least two 
layers, an inner and an outer or superficial layer, a structural 
condition which we have seen at its simplest in the gastrula 
(see 55). The exact position of the Porifera in the animal 
series has long been a matter of debate, but the great majority 
of zoologists agree that they stand below all the other Meta- 
zoa, presenting transitional features between the Protozoa 
and Metazoa. For this reason they are especially interesting. 
Some authors include them with the next phylum the Ccelen- 
terata. They possess two cell-layers, but the division of labor 
among the cells is not so decided as in the Ccelenterata, and 
the individual cells are very much more independent of each 
other in consequence. 

207. General Characters. 

i. Porifera possess a system of internal chambers through 
which the water flows. The water enters by means of many 
minute pores at the surface, passes along radiating tubes (in- 
current channels) to the central cavity (cloaca) and escapes 
through one or more larger openings (oscula) at the unat- 
tached end. There is no true ccelom (see 58). 



2. Parts are arranged about the central cavity but not usu- 
ally in a symmetrical fashion. 

3. There are two distinct layers, ectoderm and entoderm. 
These are separated by a gelatinous middle region in which are 
included cells of different kinds (mesenchyma or mesoderm) 

FIG. 76. 

FIG. 77- 

FIG. 76. Leucandra, a simple type of sponge. (From Delage and Herouard; "Traitede Zoologie 


Questions on the figure. What is the position of the osculum ? Which is the 
attached end? How many individuals are represented in the cut? 

FIG. 77. Diagrams to illustrate the development of one of the simpler types of sponge: i, the 
egg; 2, section of 16- to 32-celled stage; 3, section of later stage, a ciliated larva (blastula) ; 4, gastrula; 
5. section through older larva which has become attached by the end containing the blastopore. 
New openings break through by the coalescence and perforation of the ectoderm and entoderm, 
and a form results such as is figured in Fig. 78. a, archenteron; &/., blastopore; ec., ectoderm; ., 
entoderm; mes., mesenchyma; s, segmentation cavity. 

Questions on the figures. What terms would be applied to the cleavage and 
gastrulation in this sponge? What is suggested as to the mode of forming meso- 
derm ? The attachment of the sponges by the blastopore end of the larva necessi- 
tates what later development ? See Fig. 78. Examine figures in other texts of the 
development in other species. 

not in a true layer. In the cells of the mesenchyma spicules 
are produced, forming the supporting skeleton (Fig. 79, C). 

4. Non-sexual reproduction is prevalent, but dimorphic 
sexual cells are also formed in the mesenchyma. The sexually 
produced larva is free-swimming; the adult is attached. 


I6 7 

5. Mostly marine; wholly aquatic. 

208. General Form. The simpler sponges are cylindrical 
or vase-shaped sacs with an opening (the osculum) at the un- 
attached end. From the central cavity (cloaca) of the sac 
numerous radial passages pierce the walls (Fig. 78), and 
terminate directly or indirectly in pores at the surface (whence 


FIG. 78. Diagram of simple type of sponge, more mature than in Fig. 77. c, cloaca; ch, 
chambers, lined with flagellate entoderm; e.p., external pores; i.p., internal pores; mcs., mesenchyma; 
o, osculum; r.c., radiating canals. Other letters as in Fig. 77. In .the adult sponge the canals and 
flagellate chambers become much more complex than figured here. 

Questions on the figure. What portions of the animal are lined with ectoderm? 
With entoderm? What two main types of entoderm are figured? What is the 
actual nature of the mesoderm in sponges? Is there a coelom (a cavity bounded by 
mesoderm) ? What mechanical advantage do you see in the fact that the water 
currents enter by way of the radial canals and find their exit through the osculum, 
rather than the opposite direction? Compare with Pig. 79. 

the name Porifera). In the more complicated sponges there 
is such power of budding and lateral growth that there is 
formed a dense tuft of sponge made up of many individuals 
in organic connection with each other. In such sponges the 
simplicity of the internal structure is lost, and the cloaca may 
branch, opening to the exterior by a number of oscula. The 
radial passages which penetrate the wall become much branched 


and enlarged in special regions until the mesenchyma becomes 
honey-combed with the passages and chambers. No animals 
are more profoundly influenced by their environment, in 
respect to the special form which the individual or colony 
assumes, than the sponges. Individuals which develop in 
active currents differ much in bodily shape from members 
of the same species which grow in sheltered places. In all 
instances the form assumed appears to be correlated to the 
external conditions. 

209. The Structure of the Body. In the typical condition 
the sponge consists of an outer epithelium and its derivatives 
(ectoderm), an inner epithelium (entoderm), and an unorganized 
middle region (mesenchyma). From certain unusual occur- 
rences in the early development of sponges we are not sure 
that the ectoderm and entoderm in them are homologous with 
those layers in the animal kingdom generally. 

The outer epithelium is usually of flattened cells. These 
cover the whole outer surface and line the incurrent canals. 
From this layer arise certain specialized cells which come to 
lie deeper and even to invade the mesenchyma. Among these 
are fibrous contractile cells, and cells that secrete the hard parts, 
as spicules of lime and spongin fibres. In the middle region 
are also amoeboid cells, which ingest, store, and convey foods, 
and reproductive cells. The inner epithelium, or entoderm, 
lines the general cavity and the tubes and chambers which 
penetrate the body wall. In the cloaca the entoderm is flat- 
tened; but in the radial canals it is columnar or flask-shaped, 
collared, and flagellate (Fig. 79,!)). These cells by means of 
their flagella create the inward currents of water that bring 
food to the animal. 

In the mature specimen all these tissues are penetrated 
and supported by the spicules or threads of secreted skeleton. 
These may be calcareous, siliceous, or horny. The sponge of 
commerce illustrates the last. The spicules may be isolated 
and independent as in Grantia, or become fused into a continu- 
ous framework. But for this framework the otherwise soft 
animal would collapse into a shapeless mass and thus close the 
openings whereby water brings the oxygen and food. It is the 


form of the skeleton too which gives the characteristic form to 
the individuals and colonies of the different species. 

210. Nutrition. The food of sponges is essentially similar 
to that of the single-celled Protozoa. It is carried in by the 
water currents, which enter the pores, pass along the canals 
lined with the collared flagellate entoderm into the cloaca, and 
from there reach the exterior by way of the osculum. The food 
particles are taken up principally by the entoderm cells lining 
the radial chambers and by the amoeboid cells which belong to 
the mesenchyma. In these cells digestion takes place as in 
Amoeba. The indigestible parts of the food are returned to 
the current and are eliminated through the osculum. There is 
no other circulation. The digested food apparently diffuses 
from cell to cell or is carried by the amoeboid cells. Respira- 
tion occurs through all the cells which are in contact with 
the water. 

211. Sensation and Motion. Sponges are fixed and vege- 
tative in their adult life, and show very little of the more 
active functions. In addition to the ciliated and amoeboid cells 
already described, the pores may be closed in response to 
stimulus. Contractile elements have been described as occur- 
ring in the epithelium of these regions. It is uncertain whether 
there are any nervous elements. The oscula open and close 
under certain conditions, the flagellate cells work in unison, and 
their rate may be caused to vary by change of conditions. 

212. Reproduction by outgrowth or budding is common. 
In this way large colonies arise from a single individual. New 
colonies may arise, especially in the fresh-water sponges, by 
the separation of gemmules or groups of cells produced asexu- 
ally within the mesenchyma. These, after a period of rest, 
escape and produce new individuals. Sexual reproduction also 
occurs in all sponges. The ova and sperm are developed in 
the mesenchymatous layer. The male and female cells origi- 
nate from the same individual (hermaphroditism) . Usually 
however the sexes mature at different times. 



213. Development. Fertilization of the ovum and early 
cleavage take place in the mesenchyma near the incurrent 

FIG. 80. 

FIG. 79. Diagrams showing the arrangement of the radiating canals in two types of sponges: 
A, Ascon type; B, Sycon type; C, a portion (*) of the latter, more highly magnified, showing char- 
acter of the three layers, ec., ectoderm; en 1 entoderm (flattened layer); en 2 , flagellate entoderm; 
e.p., external pores; f.c., flagellate chambers of the radiating canals; i.p., internal pores; mes., 
mesenchyma; r.c., radiating canals; D, two flagellate cells more highly magnified. After Korschelt 
and Heider. 

Questions on the figures. Trace the relation of ectoderm to the entoderm in 
these two types? Compare these with illustrations in reference texts. Is there 
any way of accounting for this disproportionate growth of the entoderm ? What 
are the apparent functions of the flagellate, collared epithelium ? What structures 
are to be found in the mesenchyma in sponges? 

FlG. 80. Axinella polypoides, showing numerous oscula. After Schmidt. 

Questions on the figure. What are the principal external differences between 
Axinella and Leucandra (Fig. 76)? How many individuals are represented here? 
What are the grounds for your answer? Compare this with the skeleton of the 
sponge of commerce. 

canals, by means of which the spermatozoa find entrance. 
Cleavage is total and for the most part equal (see 53), pro- 


ducing an oval blastula which swims freely by means of cilia 
or flagella. While there are some peculiar features about the 
gastrulation, a gastrula or two-layered embryo is ultimately 
formed. Strangely enough it is the flagellate cells of the 
larva that become the inner or entodermal layer of the adult. 
Conversely, the cells which in other Metazoa give rise to the 
entoderm form the outer epithelium of sponges. 

The embryo finally settles to the bottom and becomes 
attached by the end containing the blastopore, which thus 
becomes obliterated (Fig. 77, bl). An excurrent pore breaks 
through at the opposite end, and the numerous incurrent 
pores are formed at the sides. The mesenchyma seems to 
be formed by special blastomeres or by cells which migrate 
from the other layers into the segmentation cavity, thus filling 
it. The entoderm outpockets into the mesenchyma, establish- 
ing connection with the ingrowing ectoderm, thus forming the 
incurrent canals (see Fig. 78). In most species the process is 
much more complex than that described here. 

214. Classification. 

The divisions of the group Porifera are made on the basis of the differences in 
the skeleton. Two principal classes may be recognized, as follows: 

I. Calcarea. Sponges in which the skeleton is composed of calcareous spicules. 
Laboratory type, Grantia. 

II. Non-Calcarea. Sponges with glassy (siliceous) spicules, or with homy 
(spongin) fibres, or with merely a gelatinous mesenchyma. Laboratory types: 
the Venus Flower-basket; the fresh- water sponge; the commercial sponge. 

215. Ecology. Sponges are chiefly marine animals, and 
flourish in all the seas and at any depth. The larger horny 
sponges, of which the bath sponge is the skeleton, are found in 
the warmer seas and in relatively shallow water. The best 
ones are found in the Mediterranean, in the South Indian Ocean, 
and about Florida and the West Indies. By reason of their 
budding and branching, the sponges form immense colonies 
or beds, and many other forms of life associate with them in 
varying degrees in intimacy. Worms, Crustacea, and the 
larvae of many forms play hide-and-seek and burrow in the 
thickets of growth produced by sponges. Several species of 
crabs allow their shells to be covered by sponges. In this way 
they are less conspicuous. In other cases the sponges may pro- 


tect the crab by its ill odor or taste. Sponges have no power of 
attacking any but microscopic organisms. They may, however, 
smother by their growth fixed animals like the oyster. Because 
of their harsh spicules or their unpleasant secretions they are 
rarely used for food by other animals. 

The horny skeleton of certain species, the "sponges" of 
commerce, is the chief contribution of the group to human uses. 
Fossil sponges, apparently of the same general characteristics 
as those now living, are found in very early geological strata. 

216. Supplementary Library Studies. 

1. Economic value of sponges. Sponge fisheries. Sponge "farming." The 
mode of preparing sponges for market. 

2. What arguments may be advanced for considering the sponges as colonial 
Protozoa? What is the conclusive argument for regarding them as Metazoa? 

3. By comparing the figures of sponges found in your reference books, note the 
different degrees of development of the passages lined with entoderm and ectoderm 
in the walls of various species. 

4. In what special ways do sponges become adapted to the conditions in which 
they are situated? Effect of rapid currents on them? Of quiet water? Of 
muddy water? 





217. Hydra. Hydras are small tubular animals found in 
permanent fresh-water pools, attached to submerged leaves, 
twigs, algae, etc. They are somewhat difficult to recognize 
when disturbed because they contract into small rounded 
masses, close against the supporting object. Promising ma- 
terials should be collected from several ponds, and placed in 
shallow vessels (a white-ware dish is good), and in a short 
time the hydras will become extended. The green hydra 
(H. viridis) is perhaps more common and hardier, but is not 
so satisfactory for general laboratory work as the brown (H. 
fusca), because it is less transparent. 

i. Study the living animal in a glass jar (tumbler). 

Is it free or attached? What happens if it is freed from its 
attachment? Is it lighter or heavier than the water? Evi- 
dences. Can it move from one portion of the vessel to another ? 
If so, does it become detached ? Watch same individuals from 
day to day. What is its position in the water? If the vessel 
containing hydras be placed near the window for several days, 
at which side of the vessel do the animals become collected? 
When the animals are stretched out at their greatest length, 
touch lightly the tip of one of the tentacles. Touch the body. 
Repeat the experiment until you are sure of your results. Note 
and explain as well as you can the results. Of what degree of 
contraction is the animal capable ? Do you notice any contrac- 
tions or motions of parts, when the hydra is undisturbed? 
What seems to be the purpose of the motions? Evidences? 
Bring a piece of meat the size of a pin-head, or a Daphnia or 
Cyclops, in contact with the tip of a tentacle and note the results. 
How do the other tentacles behave ? Place a food-particle di- 



rectly at the base of the tentacles. How is it swallowed ? How 
long does it take? What becomes of it? How long does it 
remain in the body ? Classify the results which you have at- 
tained, under the following heads: motion and locomotion, 
nutrition, sensation. Devise still other experiments to test 
special points which you desire to know. Have you dis- 
covered anything that would lead you to think that they 
learn from experience? 

2. General Structure. Transfer a living animal to the slide, 
covering it with a drop or two of water. Observe with a low 
power without cover-glass. Draw carefully in outline everything 

Note body regions. 

Foot (attached end). 


Tentacles, position, number (examine several specimens). 

Hypostome, surrounded by the tentacles. 


To what extent do these regions vary in their dimensions 
during the different stages of contraction of the hydra ? Would 
you say there is any distinct symmetry? Which is the main 
axis ? Is there any indication of an internal (Castro-vascular) 
cavity ? What is its extent ? Are the tentacles solid structures ? 
Evidences ? Are there any buds in your specimen ? Relation to 
the parent ? To what extent do different parts of the body do 
different work ? 

3. Microscopic Structure. Cover with a cover-glass supported by objects 
nearly as thick as the animal. Study with a higher power. Verify the points 
studied above. Follow the gastro-vascular cavity more fully. Is there an aboral 

Body wall. 

Ectoderm, or outer layer of cells. 
Entoderm, or inner layer of cells. 

Determine the extent of each layer. Are they continued into the tentacles? 
What differences do you find in the thickness of the layers and in the shape and 
character of the cells of each layer in the various parts of the body? Is there 
anything between the ectoderm and entoderm? 

In the ectoderm, especially in the knobs on the tentacles, find highly refractive 
oval bodies, the nettle capsules. Irrigate with a drop of dilute acetic acid, and 
watch the tentacle all the while. What changes have occurred in the nettle cells? 


[A whole animal stained and mounted may be studied profitably in comparison 
with the preceding.] 

4. Histology from Sections. If the teacher is not equipped for imbedding and 
sectioning objects, and desires to carry this work further, stained and mounted 
sections of Hydras and most of the other prepared sections suggested in this book 
can be secured for a reasonable sum by applying to any of the large laboratories. 
By comparison of longitudinal and transverse sections verify your observations 
concerning the extent of ectoderm and entoderm. What occurs between the 
layers? Study the shape and arrangement of the cells in both layers. Compare as 
to size. What is the relation of the nettle cells to the other ectodermal cells? 

5. Histology from Maceration Preparations. Place a specimen in a watch 
glass, and draw away some of the water with a pipette. When the Hydra is well 
extended, pour over it an aqueous solution of hot corrosive sublimate. Rinse 
and place in Muller's fluid or 15% alcohol for 24 hours. Take a portion of the 
body and place on a slide in a drop of glycerine and water. Cover, and tap the 
cover-glass very gently with a needle. The cells thus become separated, and 
their shape may more readily be seen. Instructions for staining may be found in 
texts on histology. 

Study the nettle cells, the ectodermal cells, the entoderm, and the gland cells 
of the foot and gullet. 

218. For comparison with Hydra the teacher should secure some alcoholic 
material of some of the marine hydroids, as Pennaria, Obelia or Campanularia. A 
few slides should be secured bearing whole mounts and sections properly stained. 

The following points should be studied briefly: Relation between individuals 
in the colonies, branching. What classes of individuals are discoverable, i. e., 
how do the different branches end? Is there any covering to the softer portions? 
Tentacles; are they present? If so, what is their arrangement? Hypostome? 
Mouth? Is there a gastro-vascular cavity? Ectoderm? Entoderm? Call 
attention to polymorphism among the polyps or zooids. 

219. Metridium (Sea-anemone). If lack of appropriations will not allow the 
purchase of sufficient material for class work, the teacher should have at least a 
few well-hardened and preserved specimens of sea-anemone. From these should 
be made a series of cross sections from various parts of the body, with a thickness 
of one-eighth to one-fourth inch. These sections may be fastened to cards or to 
plates of mica by thread or fine wire and kept in preserving fluid. One specimen 
should be split lengthwise, and one left whole. Four or five specimens could thus 
be used from year to year until more abundant supplies are obtained. 

The following studies should be made. Make drawings to illustrate all points 
made out. 

1. General Form. 

Base, or aboral disc (the end attached during life). 

Oral disc: zone of tentacles; intermediate zone; lip-zone; mouth; siphono- 
glyphs (grooves in the angles of the mouth), number? 

2. Transverse Sections. 
Body wall. 

Esophagus; does it appear in all the sections? Siphonoglyphs? 
Mesenteries. How is the esophagus held in position? What differences 


do you find in the mesenteries? They are described as complete (or 
primary), and incomplete (or secondary, tertiary, etc.). 
Show by a diagram the number and arrangement of them, especially of the 
primary. Are they in pairs? Notice the inter-mesenteric chambers. 
Can you find the muscular thickenings in the cut mesenteries? Sketch 
their position. Compare with conditions figured in various text-books. 

3. Longitudinal Section. 

Complete your study of the structures mentioned above. 

Compare the complete and incomplete mesenteries. 


Mesenteric filaments (on free edge of mesenteries). 

Genital glands (developed in the substance of the mesentery near the edge). 

Ostia, or ring canal; openings through the mesenteries by means of which 

the mesenterial chambers communicate with one another. 
Are the tentacles solid or hollow? 

4. General Considerations. 

Make diagrams in longitudinal and transverse view to show the distribution 
and connection of the cavities of the body. Is the mouth the only opening into 
the cavity? Describe the symmetry of the anemone. Is it radial or bilateral? 
Give reasons for your answer. 

220. Oculina (or other branching coral). Study the branches and note the 
position of the polyps. Is the arrangement orderly? If so, describe. 

Note with a hand lens the arrangement of the septa, which grow between the 
fleshy mesenteries of the coral. Compare their arrangement with that of the 
mesenteries of anemone. 


221. Some authors place the sponges and the ccelenterates 
in the same group on account of the typical barrel shape, the 
absence of a true ccelom or body cavity, the somewhat similar 
character and origin of the middle mass (mesenchyma) , and 
the agreement of the principal axis of the adult with that of the 
gastrula. In the ccelenterates however there are no lateral 
pores. The principal opening serves as a real mouth as well 
as vent for the voiding of undigested matter, whereas in sponges 
it is not a mouth in any sense. In general the individual, even 
in the colonial forms of ccelenterates, is more distinctly an in- 
dividual than in the sponges. The division of labor among the 
parts and the interdependence of parts is greater than among the 

222. General Characters. 

i. A single system of internal chambers (gastro-vascular 



cai'ity) in which digestion and circulation both occur. No 

2. Parts radially arranged about an oral-aboral axis. Ten- 
tacles usually occur at the oral pole (Figs. 82, 85). 

FIG. 81. 

- s 



PIG. Si. A, T >! in ill section through the body of Hydra (ffi^mniinlii) B. cmaB 
portion of the wall more highly magrrifird. b, bod; ect^ ectodam; cmL. niUxIniu; /. foot; fL. 
flagellcm; g.i., gastro- vascular cavity; ., moath; met., mesenchyma (noo-ceflnlarl; m./-, ! nlii 
processes of the ectodennal cells; IB, nettling cells; **, gam**, exploded; mi , mirl^|- f f ^*y^- y 

Questions on the figures. How many cellular layers are to be distinguished 
in Hydra? What differentiations are represented in the **t**1flrm in different 

regions? In the entoderm? What is the relation of the bud to the adult? Why 
is the cavity called a gastro-vascular cavity? How is contraction effected in 

3. A supporting layer or mass (mesenchyma) between 
ectoderm and entoderm, sometimes without cells. More often 
cells of various kinds occur, which have migrated from the 
other layers. 



4. Nettle-cells are found in practically the whole group 
(Fig. 83). 

5. Nerve cells (sensory) and muscle cells both occur. 

6. Reproduction by non-sexual methods is prevalent. This 
often alternates regularly with the sexual. The individuals of 
the two generations may be very different in appearance and 

7. Wholly aquatic; chiefly mnriae. 

223. General Survey. The group of Ccelenterata embraces 
animals very diverse in general appearance, which may never- 
theless be reduced to two types. The first and most primitive 
is the tubular hydroid type. This is sessile and is essentially a 
gastrula, at the unattached end of which occurs the mouth, 
usually surrounded by tentacles. The cavity of the tentacles 
is continuous with the gastro- vascular cavity (Fig. 81). Of this 
type we may distinguish two conditions: (i) in which the in- 
dividuals (polyps) occur singly (Hydra), or if in colonies, the 
various individuals have the same form (as the corals); (2) 
colonial forms in which the individuals making up the colony 
are very different (as the Siphonophora), embracing . open- 
mouthed nutritive individuals, mouthless reproductive polyps, 
protective polyps abundantly supplied with nettle-cells, bladder- 
like supporting polyps, etc. (Figs. 86, 87). The extreme con- 
ditions of (i) and (2) are connected by forms possessing inter- 
mediate degrees of polymorphism. Though the individual 
polyps are attached, the whole colony may float freely. The 
second type is the active jelly-fish, or medusoid (bell) type. 
The medusae, though varying greatly as to details agree in 
having a shape comparable to that of an umbrella or a bell 
(Fig. 82, 6). The convex surface is normally the upper surface. 
At the margin of the umbrella are tentacles often very numer- 
ous, and frequently much elongated. In the middle of the 
concave surface is a projection, at the lower end of which is the 
mouth-opening. The gullet leads from the mouth into a cavity 
in the central portion of the body of the bell (gastro-vascular 
cavity). From the central cavity radiating passages run 
through the substance of the bell to the margin where they 



may communicate with a circular canal which passes around the 
bell near the bases of the tentacles. This whole internal cavity 
is lined with entoderm, and therefore no portion of it represents 
a ccelom, but is merely a much-modified digestive tract (Fig. 
82, 6). 

FIG. 82. 

-VH CC ' T" * .-^ 

PlG. 82. Sections of types of Ccelenterates (diagrammatic): T (longitudinal) and 2 (transverse) 
of a tubular hydroid; 3, Sea Anemone (longitudinal) ; 4, same (transverse, at the level of the upper 
dotted line); 5, same (transverse, at the level of the lower dotted line); 6, longitudinal or vertica 
section of a Medusa; 7, transverse section of same at the level of the dotted line. The continuous 
line is ectoderm, the broken line, entoderm, and the stippled portion, mesenchyma. c.c., circular 
canal; g, gullet; g.p., gastro- vascular cavity; m, mouth; ma., manubrium; mes., mesentery; mes. 1 
directive mesentery; o, ostium; r. c., radial canal; t, tentacle; v, velum. 

Questions on the figures. By a careful comparison of the diagrams what 
points of similarity do you find in these three types ? What are the principal points 
of difference? Examine similar diagrams in other texts. Why is Ccdenterate an 
appropriate name for all. 

The bell is comparable to an inverted polyp in which the 
main axis has become much shortened, accompanied by a 
thickening of the body in the direction of the other axes. 1 
The gastro-vascular cavity is further modified by the increase 
of the mesenchyma of the aboral disc and by a union of the 
oral and aboral walls of the cavity in certain regions. The 

1 See Textbook of Zoology, Parker and Haswell, Vol. I, p. 127, Pig. 89. 


large chambers between the mesenteries in such forms as the 
sea-anemone thus become limited to small radial canals. 

Frequently both the tubular and the bell types are found in 
the life history of the individuals of a single species. The 
tubular colonial polyp produces, by asexual processes such as 
budding or fission, the bell or medusoid forms which are sexual. 
These may remain attached or become free swimming. They 
produce ova or spermatozoa, or both, and from the sexual 
union of these elements the non-sexual tubular polyp is again 
produced. This regular alternation of sexual and sexless in- 
dividuals is known as alternation oj generation. In some forms, 
however, the polyp has no corresponding bell (as in hydra; 
corals; sea-anemone), and for some bells (as in some large pelagic 
medusae) there is no corresponding polyp stage. 

224. The nutritive processes in the Ccelenterata are marked 
by relative simplicity. Food, consisting mainly of small organ- 
isms and organic debris, is taken into the mouth often with 
the assistance of tentacles. The tentacles are frequently 
armed with numerous special cells in which are developed 
capsules containing long stinging threads, with poisonous tips. 
When these threads are discharged they may penetrate and 
paralyze small organisms. They are very irritating even to 
the human skin. In some types of nettling capsules the thread 
forms a cork-screw coil that may take hold of the hairs or other 
projections of the prey. They serve as organs both of defense 
and food capture (Fig. 83). 

Digestion and circulation both take place in a general 
cavity (gastro-vascular) lined ztith entoderm. In other words the 
circulatory function in this group is not differentiated from 
the digestive. In the colonial forms the gastro- vascular cavity 
of the various polyps in the colony may be directly continuous 
(Fig. 87). Thus a kind of cooperative digestion occurs. In 
the medusa, the corals, and forms like anemone, the cavity is 
much more complicated than in the tubular hydroids, on account 
of the mesenteries. The entoderm seems to take up food from 
the gastro-vascular cavity, in part at least, by means of the 
amoeboid action of some of the entodermic cells a? well as by 



absorption. Pseudopodia are formed, and particles are directly 
taken into the body of the cell. Special gland cells also occur in 
the entoderm, by the secretions of which the food undergoes 
changes preparatory to absorption. There is no anal opening. 
Undigested remnants are eliminated at the mouth. Respira- 

FIG. 83. 

P$?tt--- c 


FIG. 83. Nettling cells of Hydra (after Schmeil). A, unexploded; B, exploded, b, barbs; c t 
the nettling cell in which the nettling organ is developed; en., the cnidocil or "trigger;" cp., the 
capsule or nettling organ; /, the nettling filament or lasso; n, neck of the capsule; nu., nucleus of 
the cell. 

Questions on the figure. Compare the parts of the nettling organ before and 
after explosion and note the difference in position. How would the barbs in the 
neck of the capsule behave as it is forced inside out by the compression of the 
capsule? Find from your reference literature the nature of the fluid secreted on 
the inside of the lasso. 


tion the exchange of carbon dioxide for oxygen takes place 
by means of the individual cells of the body layers, though it 
is probable that it takes place more satisfactorily in the thin- 
walled, more actively moving tentacles. Excretion is likewise 
a general body function. 

225. Motion. All the Ccelenterata are supplied with con- 
tractile fibres. Many of these are modified ectodermal or ento- 
dermal cells rather than true mesoderm (Fig. 81, B). The 
fibres run both longitudinally and transversely. In the more 
active types cross-striate fibres may occur. The attached 
(polyp) forms have well-developed longitudinal fibres in the 
body wall and the mesenteries, which enable the soft parts of 
the animal to be drawn close to the supporting object. In 
the medusoid types locomotion is effected by rhythmic contrac- 
tions of the bell as a whole. By this means the water is expelled 
from the cavity of the bell, and the reaction forces the animal 

226. Support. The attached colonial forms (corals, sea- 
fans, etc.) usually possess a skeleton of calcareous or horny 
matter commonly secreted by the ectoderm. Each polyp con- 
tributes a portion to the common skeleton the corallum. The 
corallum differs greatly in form in the different species. The 
particular form depends on the manner and rate of budding or 
non-sexual reproduction of the polyps, and the activity shown 
by the individual in secreting. In some cases single polyps 
produce a skeleton (cup-corals). The coral reefs of tropical 
seas are illustrations of the power of corals to form and excrete 
carbonate of lime. Much of the lime-stone of the earth's crust 
shows that corals assisted in its formation. 

227. Sensation and Behavior. The nerve cells may be 
scattered diffusely over the surface of the body with a mesh- 
work of fibrils to connect them with the muscular and nettle 
cells and with each other, as in Hydra. In some other polyp- 
forms there is more differentiation of cells and fibres, but the 
elements are still scattered. In the more active types there is 
a collection of the cells either as a connected ring, or in groups, 


in the tentacle-bearing rim of the animal. Associated with this 
collection of the nervous material into a kind of nervous centre, 
there are often special areas of sensory epithelium, or sense 
organs, developed from the ectoderm. It is not wholly clear 
what kinds of stimuli they are suited to receive although they 
are designated as "eye spots," or as "auditory" or "olfactory" 
pits. Otocysts (see m) are found in the Ctenophores and in 
some medusae, and apparently function chiefly as organs of 

Experiments show that ccelenterates are sensitive to dif- 
ferences in intensity of light, to mechanical stimuli, to tempera- 
ture, to gravity, and to chemical stimuli. The responses may 
be local, as when a Hydra withdraws its tentacle upon a light 
touch or uses all its tentacles and the region about the mouth 
when stimulated mechanically and chemically by its prey; or 
general, as when it contracts body and tentacles completely 
upon vigorous mechanical stimulation. 

In addition to such responses to external stimuli there are 
in Hydra bendings of the body, wavings of the tentacles, and 
even contraction and extensions of the whole body that suggest 
"seeking" movements. Hungry specimens are more active 
than well-fed ones. All of this shows that even in the simplest 
representative of the group there is effective nervous and 
muscular coordination. 

228. Reproduction and Development. The occurrence of 
both sexual' and asexual methods of reproduction has already 
been mentioned (223). It is by non-sexual budding that 
colonies are normally produced and a given locality well occupied 
by the species. By means of the sexual method dispersion is 
effected, and new regions are occupied. The ova and sper- 
matozoa develop in special gonads (ovaries or testes) derived 
in some species from the ectoderm, in others from the entoderm. 
The sexual cells usually escape into the gastro-vascular cavity 
and reach the outside by way of the mouth. As a rule the sexes 
occur in separate individuals. After fertilization cleavage is 
total but sometimes not equal. A blastula is formed which is 
often converted into a peculiar, free-swimming, ciliated larva 



.(planula), consisting of a two-layered sac with no opening. 
This condition may arise by the closing up of an ordinary two- 
layered gastrula (as in Aurelia). In other cases the entoderm 
may be formed by cells budding into and finally lining or even 
filling the segmentation cavity of an ordinary blastula (Fig. 
84), resulting in a quite similar condition. The planula after 
a tirief free life becomes attached by one pole and becomes 
elongated ; a mouth surrounded by tentacles is formed at the 


FIG. 84. Diagrams illustrating development in some of the hydroid types. A, blastula in 
which the entoderm (<.) is produced by proliferation from ectoderm (ect.), B, ciliated planula 
formed by the continuance of this process. A split in the entoderm furnishes the beginning of the 
gastro- vascular cavity (g) of the adult. C, more mature condition, in which the planula has become 
fixed: /, foot or attached end; o, oral or free end at which the tentacles and mouth will be developed. 

Questions on the figures. How does this blastula differ from the typical 
blastula in the formation of entoderm ? What is a planula ? Is a gastrula formed ? 
After an opening forms at the oral end what likeness is there in the adult to a gas- 
trula? What changes would C need to undergo to become essentially similar to 

other. Thus it assumes the typical polyp form. In nearly 
all species the polyps may produce new individuals by buds 
either from the wall of the polyp or from special organs (stolons, 
or runners). If, when these are mature, they separate from the 
parent no colony is formed. More commonly the daughters 
remain in association with the parent. The medusoid in- 
dividual, often of a very much simpler type than that de- 
scribed above (223), may be produced in a similar way from 
a bud. This sexual individual may degenerate until it is little 
more than a case for the sexual cells. It usually breaks its 


attachment with the parent stock and, in the perfect medusae, 
becomes free-swimming. 

229. Classification. The following classes of Coelenterata may be recognized. 
Class I. Hydrozoa (Hydra-like animals). Hydrozoa are ccelenterates with two 
cell-layers (ectoderm and entoderm), between which there is a supporting layer 
(the mesoglcea) non-cellular in structure. The reproductive cells arise chiefly 
from the ectoderm. The life cycle may consist of polyps alone (Hydra); or of 
medusae alone; or of both in one life history (Campanularia, Pennaria, Obelia}. 
Medusoid forms may be free or attached. The gastro-vascular cavity is not 
divided by mesenteries. Here are included all the rather scarce fresh-water 
ccelenterates, many tubular marine forms somewhat similar to Hydra, and the 
much diversified colonies of the Siphonophora (as the Portuguese Man-of-War, 
found in mid-ocean, especially in the region of the Gulf Stream). See Figs. 86, 87. 

Class II. Scyphozoa (cup animals). Ccelenterates in which the mesenchyma 
contains cellular elements. The reproductive cells arise from the entoderm and 
escape into the digestive cavity. Chiefly medusoid forms, though in some the 
bell-form alternates with a polyp stage. Types: Aurelia and the larger jelly- 
fishes. The majority of the Scyphozoa swim on the surface of the ocean; some are 
found at considerable depths. Many of them are very large and handsome. An 
especially interesting fact in connection with the development of such a type as 
Aurelia is that its polyp (known as the Scyphistoma} is intermediate in its charac- 
teristics between the polyps of the Hydrozoa and those of the Actinozoa. The 
Scyphistoma has four ridges which partly separate the gastro-vascular cavity as do 
the mesenteries in the Actinozoa. 

Class III. Actinozoa (ray animals). Coelenterates with only the polyp form. 
Cells in the mesenchyma. There is a well-developed ectodermic gullet (stomo- 
daeum). The gastro-vascular cavity is more or less completely divided into 
chambers by mesenteries. Sexual cells entodermal. A skeleton of calcareous or 
horny material often present. 

Types: Sea-anemones; sea-fans and corals. The sea-anemones or sea-roses 
are common on rocks and other objects just below low-water mark. Though 
attached, they have some power of gradually changing their position. Species 
of sea-anemones are known in which the individuals are as much as two feet in 
diameter, though polyps of the colonial forms are usually very small. 

Class IV. Ctenophora (" comb-bearers"}. ---The Ctenophora are free-swimming, 
pear-shaped jelly-fishes, never occurring in colonies, and not associated with a 
polyp stage. They bear eight meridional plates supplied with transverse rows of 
cilia, which function as locomotor and possibly as respiratory organs, and suggest 
the name of the group. There is a well-developed stomodaeum. The gastro- 
vascular canal branches from this and is much divided, one division lying under 
each row of combs. There are two small aboral openings to the digestive canal 
known as excretory pores. The mesenchyma is well developed. There are no 
nettling cells as in the other ccelenterates; but glue-cells with similar structure and 
functions are found. (Some students of the Ctenophora place them in a separate 

230. Ecology, etc. The food of Ccelenterates consists 
largely of organic debris broken up by the waves, and of small 



animals and plants captured by the tentacles. The attached 
forms flourish best in the comparatively shallow water near the 
shore. Food is especially abundant in such regions and hence 
the passive animals are more successful here than elsewhere. 
Hydractina (Fig. 85) and even the sea-anemone form interesting 
partnerships with the hermit-crab. The polyps cover up the 
shell occupied by the crab, thus concealing it from its enemies 
and its prey. In return the polyps doubtless profit by a share 

FIG. 85. 

FIG. 85. Hydractina echinata, after Hincks. c. the cctnosarc, forming an incrustation over the 
object on which it lives; , nutritive polyps; r, reproductive polyps, bearing buds in which are ova; 
/, tentacles. 

Questions on the figure. How many types of individuals seem to be repre- 
sented? What evidence of budding do you see in the species? What is the cceno- 
sarc? What is its nature in Hydractina ? What can you find concerning the habits 
of the members of the genus? How does this colony compare with that in Fig. 86? 

of the food broken to pieces by the crab, as well as by the change 
of place as the crab moves about in search of food. Some anem- 
ones have living algae in their entoderm cells which seem to 
help supply the animal with oxygen in return for foods of other 

Nearly all the ccelenterates are marine. A few species of 
Hydra, one or two partly parasitic types, and a few fresh- water 


I8 7 

medusoids are the only known exceptions. Cordylophora, a 
colonial polyp, is found in brackish tidal waters, and has been 
discovered in inland rivers. It seems to be a species in process 
of adjustment to fresh- water conditions. 

FIG. 86. 

FIG. 86. Physalia, the Portuguese Man-of-War. After Agassiz. 

Questions on the figure. For what is this animal remarkable? To what 
group of coelenterates does it belong? Compare Huxley's figure of the same 
animal (see Parker and Haswell's Zoology, Vol. I, p. 152, and other reference 
texts). What various types of polyps are represented in the colony? Compare 
with Fig. 87. 

The jelly-fish are used as food by fishes and whales, though 
they cannot contain much solid matter. The group has little 
economic value. The red coral of the Mediterranean, of very 
slow growth, is used in making jewelry. The reefrforniing corals 



have changed the land contours by additions of marginal reefs. 
Much of the limestone of the crust of the earth is from corals. 

FIG. 87. 



FIG. 87. A very diagrammatic and generalized illustration of a complex coelenterate colony. 
The shaded portion represents the gastro-vascular cavity. The light portion, the body tissues. 
b., a bell-like individual developed into an air-bladder; m. t mouth; n.z., nutritive individual; p.*.. 
protective individual; r.z. 1 , r.z.*, r. z.* f different types of reproductive individuals; s.b., swimming 
bell; /, tentacles, which are sensory and protective structures. After Lang. 

Questions on the figure. What is meant by "generalized" above? How does 
such a polymorphic colony as this differ from a highly organized individual? 
In what respect is it similar to an individual ? What is the function of the gastro- 
vascular system? What is the gain in its wide distribution through the colony? 
How do the Siphonophora differ from the other colonial coelenterates ? 


Many interesting experiments have been performed on mem- 
bers of this group illustrating the power of regenerating lost 
parts. Many of the polyps have been shown to have this power 
and even the medusa may become perfect animals again after 
having lost very considerable portions of their structure. Hydra, 
one of the simplest members of the group, is most famous for 
its power of regaining its original form, no matter to what sort 
of mutilation it has been subjected. As long as there is a piece 
of the trunk of appreciable size containing both ectoderm and 
entoderm it may regenerate the whole animal, stalk, mouth, 
tentacles, and all, under favorable conditions. 

Nothing about the ccelenterates is more interesting to the 
zoologist than the way in which the individuals in the poly- 
morphic colonies (as in the Siphonophora) come to do the work 
done by special organs in the higher Metazoa. 

231. Supplementary Studies, for field and library. 

1. Make a list of all the places where Hydra may be found 
in your locality. 

2 . Can you find an account of any other fresh-water Ccelen- 
terata ? 

3. What facts can you find concerning the power of regen- 
eration in Hydra or other coelente rates ? 

4. Coral reefs: kinds and mode of formation. Conditions 
of life necessary to the reef -forming corals. 

5. Polyp colonies. Show, by reference to all the specimens 
and figures you can find, where the newest bud appears and 
how this helps determine the shape of the- colony. 

6. Polymorphism and division of labor in polyp colonies. 

7. Corals in geological time. 

8. Sense organs among ccelenterates. 

9. Alternation of generation in Obelia. In Amelia. 

10. The symmetry of the ccelenterates. 

11. The structure, position and uses of the nettling cells in 
the phylum. 

12. Study the polyp of Aurelia (Scypkistoma) from descrip- 
tions and cuts, and show in what respects it seems to stand 
intermediate between the Hydrozoa and the Actinozoa. 



232. It seems desirable, for the sake of convenience and 
in order to prevent a confusing array of details, to embrace 
under this head a number of groups of animals which do not 
have very much in common except their place of uncertainty 
in the animal kingdom. They are not to be considered as 
forming a phylum of animals, although in the past they have 
often been included by authors with the Annulata (Chapter 
XV) under the head of Vermes. There is abundant evidence 
indeed to enable one to believe that four or five distinct phyla 
are here included. Some of these groups, however, have 
members which bear more or less striking resemblances to 
animals belonging to the recognized phyla, especially to embry- 
onic stages of them. These facts render them of the greatest 
possible interest to the zoologist, because they furnish grounds 
for the hope that, through the study of this heterogeneous 
assemblage, the origin and kinships of the other phyla may 
be made more clear. The same facts make them unfit objects 
for extended study in elementary classes. 

233. Points of General Resemblance. In external form 
these animals differ -very greatly. They may vary from a 
cylindrical or even a globular form to a thin ribbon-shape. 
They agree for the most part, however, in having a main 
axis which in the free-swimming forms is usually horizontal 
in position, the anterior end of which is structurally distin- 
guishable from the posterior. There is usually a distinct 
bilateral symmetry (see 120) which takes the place of the 
radial symmetry found in the Ccelenterates. In some types of 
the 'Ccelenterates there are certain suggestions of bilateral 
symmetry but never to the complete exclusion of the radial. 
For the first time is found an assemblage of multi-cellular 



animals whose individuals move with one end continually 
foremost and one of the body surfaces continually up and 
the other down. This is a distinct gain in organization and 
accompanies a more active life. The Polyzoa are attached 
in adult life and have lost this symmetry, and many of the 
Rotifers, while having definite anterior and posterior ends, 
have lost their right-left symmetry in part, but the embryonic 
stages of these are in many respects similar to the more typical 
forms. By saying that these animals are unsegmented it 
is meant that in a distinct individual there is not usually a 
linear series of equivalent body-parts or metameres. There 
are however several types which reproduce new individuals 
by transverse division ("fission"). These new individuals 
may remain together, temporarily at least, in a chain, as in 
Microstomum (Fig. 91) or the tape- worm (Fig. 93), form- 
ing a strobila. In this condition there is a repetition of all 
the essential organs in each of the "segments." Some authors 
regard this process of strobilation as the condition from which 
the ordinary segmentation, as seen in the Annulata, has arisen, 
by the adhesion and gradual differentiation and coordination 
of the originally similar individuals. 

The animals of these groups agree in the fact that the third 
or mesodermal layer of tissue becomes more important than 
it is among the Crelenterates. They are therefore triploblastic 
animals. In addition to this the mesoderm often, though 
not universally, splits, forming a ccelom or body cavity (58) 
wholly separate from the digestive tract. The ccelom is lined 
with mesoderm. All the animal phyla above the Ccelenterates 
possess this character in some measure and on this account 
are called Ccelomata. 

These animals further agree with those above them in the 
scale of development in possessing a system of excretory tubules 
which connect the ccelom, or the mesodermal tissue if there 
is no ccelom, with the outside world. This system is believed 
to get rid of nitrogenous wastes. 

234. Laboratory Exercises. An extended laboratory study of these groups is 
not desirable, yet the teacher should secure enough material representing the 
various included phyla to enable the student to justify the separation of these 


uncertain forms from the more exactly denned phyla, and to show him how 
ill-defined is the assemblage which we have thus brought together. The Tape- 
worm of man may sometimes be secured from physicians, and other species of 
worms are found not infrequently as intestinal parasites in cats, dogs, or other animals 
dissected in the laboratory. The general form, the method of attachment to the 
host, the progressive development of the proglottides or "segments," and the 
difference between these segments and those of the earth-worm should be noted. 
Permanent whole mounts of a mature proglottis may be made, showing the 
embryos in the uterus. Demonstrations of the structure of the proglottis may be 
given by properly prepared transverse sections, if the equipment and time allow. 

An hour's work may profitably be devoted to the study of some one or more of 
the common Rotifers, which may be found in water taken from the stagnant pools 
in which there is much decaying matter. They are microscopic animals and are to 
be recognized by the possession of discs at the anterior end, which present the 
appearance of rotating wheels because of a rhythmic action of the cilia. Make 
sketches showing the change of shape which the animal undergoes. How is the 
change effected? How is locomotion accomplished? What evidences have you 
of its ability to receive stimuli and to respond to them? How does it get food? 
Can you trace the digestive tract in the body of the animal? Notice the contract- 
ing object just back of the mouth. What conclusions do you reach as to its func- 
tion? Give your evidences. Verify by consulting some textbook. Can you 
prove from what you see that this is not a single-celled animal like Stentor? The 
student should be cautioned against taking these specimens as closely typical of 
the whole group of Rotifers, since there is very great variety of form among them. 

Planarians often appear in the laboratory in water containing an abundance 
of decomposing organic matter, taken from ponds and foul streams. The most 
important points to be noticed are their general form, the method of locomotion, 
sensitiveness to stimuli, and life habits. Non-sexual reproduction by fission is 
frequent among them. 

The Polyzoa occur as tufts of many minute animals in colonies attached to 
objects in the water. Plumatella is a rather common fresh-water form and makes 
a beautiful demonstration to illustrate the ordinary physiological processes, as 
motion, feeding, the action of the digestive tract in churning the food, sensitive- 
ness to stimulus and the like. Schools near the sea-shore will find an abundance 
of marine material for the comparison of the colonial forms of different species 
of Polyzoa, since they are more common in salt than in fresh water. 

235. Classification and Description. Phylum IV: Platyhelminthes (Flat- 
worms}. In the worms of this phylum the body is flattened or compressed in a 
dorso-ventral direction, and from this fact the name is given. They are soft- 
bodied animals without any true skeleton. There is no body cavity and no true 
blood-vascular system. The space which would be given to such structures is 
filled with a spongy connective tissue. Through this body-mass run the minute 
tubes of the excretory or water-vascular system (Fig. 88, ex.}, often terminating 
internally in special cells (flame cells, Fig. 90). These tubes have .external pores. 
By means of this system of organs waste products, probably of a nitrogenous 
nature, are eliminated from the tissues. The digestive tract may be wholly want- 
ing as in the Cestodes, or a simple or forked sac, or a central sac with lateral 
branches. It is blind, i.e., has only the oral opening. In the more complicated 
types of stomach the much-branched sac serves the function of carrying the digested 



food to all parts of the body. Many of these forms are parasitic and in conse- 
quence the organs referred to are often very much simplified and degenerate. The 
digestive tract, for example, may be entirely lost. Reproduction by transverse 
division is not uncommon. By this method strobilae or chains of more or less 
closely connected individuals occur (Figs. 91, 93). The sexual organs are ex- 
ceedingly complex, particularly in the parasitic members of the group (Fig. 94). 
The development is in some instances direct, in others indirect. The principal 
classes are the Turbellaria, Trematodes and Cestodes. 


FIG. 88. Diagram of a Turbellarian, showing the general- arrangement of the nervous structures 
and one of the modes of occurrence of the excretory tubules, which in this case open separately into 
the pharynx, on the ventral side of the animal, b, brain; , eye-spots; ex, excretory canals con 
sisting of a transverse portion passing from the mouth toward the dorsal side (see also Fig. 89), 
and longitudinal tubes which branch into the capillary vessels terminating in/, the flame cells; I.e., 
lateral nerve cords; m, mouth. 

Questions on the figure. Compare this figure with the next and identify the 
structures shown in both. What other positions of the mouth do you discover in 
the Turbellaria, as figured in reference texts? What other arrangement of the 
excretory canals and pores? 

Class I. Turbellaria (Planarians, etc.}. These are mostly small non-parasitic 
Platyhelminthes with a ciliated ectoderm. They are chiefly aquatic and are car- 
nivorous. The ventral mouth may be anterior, posterior, or median in position. 
It opens into a muscular eversible pharynx, which may be used to assist in locomo- 
tion as well as in capturing food. The digestive tract may be simple or very much 
branched. The brain consists of a pair of ganglia in the anterior region. From 
the brain lateral nerve cords pass backward through the body. The excretory 
organs (Figs. 88, 89) usually consist of two or more longitudinal tubes which open 
on the exterior separately or by a common orifice. The position of the opening 



FIG. 89. 

FIG. 90. 

FIG. 89. Diagram of transverse section of a Turbellarian through the region of the mouth. 
d.m., dermo-muscular wall containing longitudinal fibres; ex, excretory system; /, flame cells; g, 
gut; I.e., lateral nerve cord; m, mouth; m./., muscle fibres; ph., pharynx; t, testis; , uterus; y, 
yolk glands. 

Questions on the figure. Determine with care the relation of this to the 
preceding diagram and identify the common structures. What new structures 
are represented here? What would be their position in the former figure? The 
great range in position of the muscle fibres and the spongy character of the body 
contribute to what powers? 

PIG. 90. Diagram of flame cell, the internal terminus of the excretory tubules, c, cilia lining 
the tubule;/, special cilia constituting the flame; n, nucleus of flame cell; p, cell processes; v, vacuole 
or cavity in cell communicating with the capillary tubules (I). 

Questions on the figure. What is the function of the cell itself ? Of the flame ? 

FIG. 91. Diagrammatic sagittal section of Microstomum, showing a chain of four zooids 
produced by fission, b, brain of the original zooid (the exponents indicating corresponding struc- 
tures of the more recently formed zo6ids); c, ciliated pit; d, dissepiments indicating different stages 
in the separation of the zooids; e, eye-spot; wf, entoderm;g, gut; gl., glandular cells about the mouth; 
m, mouth of the original worm. 

Questions on the figure. What various evidences can be found of the relative 
age of the zooids? Is the mouth formed apparently from entoderm or ectoderm? 
Is the gut a blind sac? What incidents seem necessary when this chain separates 
at the oldest plane of division, and forms two chains, in order that each may be 
like the parent? How is this like segmentation in annulates (see Fig. 101) ? How 


varies very much in the different orders. The tubules are much branched 
interiorly and penetrate the soft tissues of the body as minute capillaries with 
thin walls. They terminate in cells of special structure which are excretory in 
function. A group of cilia (the flame, Fig. 90, /) helps in creating a current in the 
capillary tubes. The lining of the tube may also be supplied with cilia. Repro- 
duction is by division (fission, Fig. 91) or by eggs and sperm. The Turbellaria 
have remarkable powers of regenerating lost portions. Experiments show that 
very small portions of an individual will, under favorable conditions, reproduce all 
the parts of a complete animal. In habit they may be terrestrial, fresh-water or 
marine. They vary in size from microscopic fresh-water forms to a length of six 
inches or more in the case of the marine and land types (Figs. 88-91). 

Class II. Trematoda. The Trematodes are small, usually parasitic, Platyhel- 
minthes. The ectoderm is provided with a protective "cuticle" and is conse- 
quently destitute of cilia. They possess a well-developed and often much- 
branched digestive sac, which has only one opening the mouth. Usually one or 
more sucking discs are present. By means of these the parasite attaches itself to 
the host. The nervous and excretory systems are similar in general to those of the 
Turbellaria, but are somewhat better developed and more complex. In those 
members of the class which are external parasites there is usually no metamorphosis 
in the development. In the internal parasites, as the Liver-fluke of the Sheep, 
there is a most complicated metamorphosis coupled with an alternation of sexual 
and non-sexual generation (see 223). A Liver-fluke (Distomum hepaticum) is 
found in the bile ducts of the liver of the sheep, where it gives rise to a much- 
dreaded disease "liver rot." The eggs which are formed, fertilized and pass 
through the early stages of cleavage here, pass out of the bile ducts to the intestine 
and thence to the exterior. If the larva reaches water it develops into a free- 
swimming larva (Fig. 92, C), which to insure further development must bore into 
the tissues of a particular pond-snail (Limnaa truncatula). It there develops into 
a kind of sac (sporocysf) from the inner cells of which special cells are budded 
(Fig. 92, ). These cells have the power of developing into embryos of a second 
generation by cell division that is to say, non-sexually. Several such non-sexual 
reproductions may occur in the body of the snail (Fig. 92, +). These later genera- 
tions of larvae pass, often by the death of the snail, into the water, whence they may 
enter the alimentary tract of the sheep in drinking. The larvae find their way to 
the liver and develop there again into the adult fluke. It is evident that such a 
form must have immense powers of reproduction, when it is considered that the 
reproduction takes place at several points in the life cycle (Fig. 90, + *). This 
may be seen to be a necessity to compensate for the great loss of life involved in 
changing from host to host. It is said that a single fluke may produce half a million 
eggs. Each of these which succeeds in reaching the host snail may produce 
thousands of the last generation of asexual individuals, and yet the numbers of the 
species probably do not increase. The disease is prevalent only in those countries 
where this species of Limncza occurs. It is much worse in wet years. Millions of 
sheep have died in England alone, in a single year, from the attacks of this parasite. 
Trematode parasites are common among animals and frequent most diverse organs. 
As compared with the Turbellaria, the Trematodes have lost their eye-spots, have 
less well developed sense organs and central nervous systems, and have highly 
elaborate reproductive organs and metamorphosis. These facts are related to the 
parasitic habit. 


FIG. 92. A series of diagrams illustrating the life cycle in the LIVER-FLUKE (Distomum). After 
Thomas, Leuckart, and others. A, egg in its case; B, early embryo, still in case; C, free-swim- 
ming ciliated embryo ; D, same after encysting in tissues of snail (sporocysf) ; E, sporocyst at later 
stage producing by internal, non-sexual processes new sporocysts, and redia. (r) which break from the 
sporocyst and lead an independent life of their own in the tissues of the snail; F, a mature redia 
producing within itself new generations of rediae, and a new type of larva, cercaria, which escape 
by a birth-pore (b.p.) and make their way into the water; G, cercaria; H, same after losing its 
tail and becoming encysted; 7, the young fluke in the liver of the sheep, where it becomes sexually 
mature and produces perhaps 500,000 new eggs, b, brain; b.p., birth-pore; c, cercaria; c.m. t cell 
masses, embryos formed non-sexually within sporocysts and rediae; e, eye-spots; ex., excretory 
tubules and pore (only the posterior portion shows); g, gut; m, mouth; ph, pharynx; r, redia; s, 
suckers; sc, sporocyst; +, stages in which non-sexual reproduction occurs; *, stage at which sexual 
reproduction occurs. 

Questions on the figures. In which stages are eyespots found? Number and 
position of the suckers? In which stages found? What is the result of increasing 
the points at which reproduction occurs in the cycle? Is this a combination of 
metamorphosis and alternation of generation? Your reasons for your answer? 
Compare this with the life history of the tape-worm. Note the encysted stage by 
which it passes from water to its host in each instance. 




-doom CO3 


PIG. 93. Diagram showing some stages in the life history of the Tapeworm (Tcenia). A, 
Cysticercus or Bladderworm stage, before the "head" protrudes from the bladder; B, same, later 
stage; C, Strobila, or chain of proglottides, many being omitted; D, embryo, such as fill the uterus 
of the mature proglottides. It is protected by a shell, b, bladder; ex., excretory canals; g, genital 
pore; A, head or scolex provided with hooks and suckers (5); , uterus in a mature posterior pro- 
glottis; s, zone of budding or segment formation. The numerals show the approximate number of 
the segments, reckoning from the front. Not more than 5 per cent, of real length of the chain 
is represented. 

Questions on the figures. What arguments do you find from the figure for 
considering the strobila an individual? What for considering it a colony? 
Where does non-sexual reproduction occur? Where sexual? Seek figures of 
stages between D and A in the reference books. 



Class III. Cestodes (Tape-worm, etc.). The Cestodes are internal parasites 
having a complicated life history usually involving two hosts. In the tissues of 
the first host occurs the "bladder- worm," Cysticercus, or embryonic stage (Fig. 93, 
A) ; in the intestine of a second host the strobila or adult tape-worm (Fig. 93, Q is 
found. The adult form has no mouth or digestive tract, the animal taking its 
food by absorption of the digested material in which it is bathed. The anterior 
end is supplied with hooks or suckers by means of which it attaches itself to the 
intestinal wall. Just behind this "head" is a region in which transverse division 

fl 3. 

FIG. 94. Diagiam of a sexually mature proglottis of Toenia. A, anterior end; e, embryos; 
ex., excretory canals; g.p., genital pore; ov., ovaries (paired); r.s., receptaculum seminis; 5.?., shell 
gland; t, testes; ut., uterus filled with embryos; v, vagina; v.d. t vas deferens; y.g., yolk gland. 

Questions on the figure. Why is self-fertilization possible in tapeworm? 
What is the function of the various portions of the reproductive apparatus? 
Trace the following steps and indicate where each incident happens: formation of 
eggs and sperm; passage of sperm to vas deferens and into vagina; storing of sperm 
in receptaculum seminis; fertilization in the oviduct; addition of yolk; ovum 
covered with shell secretion; passage into uterus where development proceeds. 

(Fig. 93, z; and 126) is continually taking place. This results in the continuous 
formation of new segments or proglottides, the older ones being pushed further from 
the head by those newly formed. Each proglottis becomes in time a sexually 
mature hermaphrodite individual. All stages of sexual maturity are found in one 
strobila or colony, the posterior individuals being most mature. At the posterior 
end of an old colony the proglottides (Figs. 93, 94) are rilled with the developing 


embryos, and on breaking away from the chain these brood cases pass with the 
fecal matter from the intestine. In this way it becomes possible for the embryos 
to find the way into a new host. On being swallowed by some suitable animal 
they break from their cysts, bore through the wall of the digestive tract into the 
tissues. Here they grow, become encysted and at this stage develop, in antici- 
pation of the needs of the adult worm, the head or scolex which remains attached 
to the bladder-like cyst (Fig. 93, A, B). Development stops at this point unless 
the flesh of this host is eaten by some other animal. When this happens the blad- 
der is thrown off, the head becomes attached to the wall of the intestine of the 
carnivorous host, and the active formation of the chain of proglottides begins 

The more common tape- worms of man are Tania solium and Tcenia saginata. 
The former is more common in Europe and is received into the system by eating 
the raw flesh of the pig, in which the bladder-worm stage occurs. The latter is 
obtained chiefly from beef and is more common in America. Only by adequate 
cooking is the danger of infection removed. The American habit of eating beef 
rare "contributes to the spread of the pest. Other tapeworms infest, as their 
double host, the dog and the rabbit; man and fish; the cat and the mouse; the 
shark and other fishes. 

The excretory system is a pair of continuous lateral tubes with transverse 
connections in the various proglottides (Fig. 94, ex}. The nervous system in the 
adult tapeworm includes a rather complex series of loops containing nerve-cells, 
in the scolex, with right and left lateral lines of nervous tissue running the length 
of the strobila. There are numerous longitudinal, transverse (circular), and dorso- 
ventral muscle fibres passing through the spongy tissue of the worm. There is 
a well-developed external cuticle which helps protect the animal from the action 
of the digestive juices of the host. 

Phylum V: Nemathelminthes (Round- or Thread-worms). Nemathelminthes 
are elongated, cylindrical forms which taper at the ends. The body is covered by 
a dense cuticle. Many are aquatic, but some are parasitic at least during a part 
of their life. An alimentary tract is present and has both a mouth and an anus. 
There is a ccelom which is not divided into chambers and contains a fluid without 
corpuscles. There is no circulatory system other than this. There are no special 
respiratory organs. The central nervous system consists of a ring around the 
esophagus. This contains some nerve cells. From this ring nerves arise at 
various points and pass both forward and backward. The chief posterior nerve is 
ventral, but there may be also dorsal and lateral ones. The sexes are usually 
separate. Development is sometimes direct, sometimes indirect. The best- 
known representatives are the round-worms (Ascaris), different species of which 
are found in the intestine of man, of the pig, and of the horse; vinegar- "eels"; 
trichina; and numerous free-swimming forms. 

Trichinella is one of the most dangerous of the nematode parasites. The 
sexually mature worm occurs in the intestine of the rat, the pig, man, or other 
mammal. The young are retained by the mother in the uterus until well developed. 
When born the young bore through the wall of the intestine of the host and make 
ther way to the muscles, where they become encysted and cause degeneration 
of he muscle fibres and often other acute symptoms of the disease known as 
trichinosis. The larvae remain in their cysts indefinitely or until the death of 
their host. For further development the flesh must be eaten. In the intestine 
of the new host where the cyst is dissolved the adult condition is quickly reached, 


reproduction takes place again, the embryos migrate into the muscles and the new 
cycle is begun. We do not find here the non-sexual reproduction that helped make 
the Liver-fluke so prolific, but the reproductive power of Trichinella is very great 
without this. It is estimated that an ounce of "measly " pork may contain 80,000 
cysts of Trichinella, and that each female produced from these embryos may con- 
tain at one time 1,000 or more embryos. During her life she may produce ten 
times this number. Thus the 40,000 females from such a meal would soon supply 
40,000,000 young worms for the infection of the muscles, with the ability of renew- 
ing the supply at short periods. Perfect cooking is the only sure safeguard against 
the possibility of infection. 

The hookworm, Necator americanus, belongs to this phylum and is a common 
parasite of man in the southeastern states. It has only one host in its life cycle. 
The adult worms attach to the wall of the intestine with a sucker-like mouth. 
The teeth pierce the wall and the esophagus works as a pump to extract the blood. 
From the mouth, secretions are poured that prevent the blood from clotting. 
In this way the wound may continue to bleed after the worm goes to a new spot. 
One worm may thus make many wounds ; and in some cases more than a thousand 
have been found in one person. The female may produce thousands of eggs, but 
these cannot develop in the intestine of man. They pass from the intestine, and 
then hatch and undergo their early development in the moist soil. If they do not, 
at a certain stage, find their way back into man or some similar host they die. 
Possibly they may get back into the intestine on raw vegetables, but a much more 
wonderful way has been demonstrated. It has been found that they can pene- 
trate the skin, and many of the poorer people go barefoot in these regions. The 
larvae bore into the capillaries, are taken to the heart and go with the blood to the 
lungs. Here they bore into the lung cavity, pass up the bronchial tubes, through the 
trachea into the gullet and on into the intestine. It takes about seventy days from 
the time the larvae enter the skin until a new generation of eggs appear from the 
intestine. The effect of infection upon human beings is bloodlessness, weakness, 
deranged digestion and poor nutrition, abnormal appetite, and laziness. 

It has been found that the worms may be driven from the intestine by thymol, 
which stuns the worms and makes them loosen their hold, followed by epsom salts 
which flushes them from the intestine. Thymol, however, should be taken only 
by the prescription of a physician as it acts strongly on the heart. Prevention of 
infection involves stopping the miscellaneous infection of the soil by discharges 
from the intestine and protecting the skin from exposure. Drouth and freezing 
are fatal to the larvae. 

Phylum VI: Trochelminthes (wheel-worms or rotifers'). The Rotifers or wheel- 
animalcules are microscopic animals. They usually tend toward bilateral sym- 
metry. The anterior end possesses a retractile disc supplied with cilia variously 
arranged, the rhythmic motions of which often give the appearance of a rotating 
wheel. From this the name of the group comes. This organ assists in locomotion 
and produces currents in the water by which food is brought within reach of the 
mouth. There is a digestive tract with both mouth and anus. The pharynx into 
which the mouth opens is provided with a chitinous grinding apparatus (mastax). 
Usually a pair of digestive glands open into the stomach. The nervous system is 
usually limited to a single ganglion dorsal to the pharynx. Eye-spots and other 
sense organs, called tactile rods or antennae, are present. There is a true ccelom 
communicating with the exterior by means of excretory tubules. For a diagram- 
matic view of these structures see Fig. 95. 



The sexes are distinct and are frequently very different in appearance. The 
males are often much smaller than the females, are much less numerous, and are 
often degenerate. The summer eggs are of two kinds large and small and 
develop parthenogenetically. The large eggs produce females and the small, 
males. The winter eggs have a thick shell and are believed to require fertiliza- 
tion in order to develop. They rest during the winter and in the spring develop 
into females. Development is direct. The adult condition in the Rotifers sug- 
gests the larval- (trochophore) condition in some Annulata. There are some traces 

FIG. 95. 


FIG. 95. Diagram of a sagittal section of a Rotifer. Jb, brain; bl., excretory bladder; c, cloaca; 
the common opening of digestive and reproductive organs; co, coelom; e, eyespot; ex, excretory 
canal; /, flame cells; f.g., foot gland; ft., foot; g, gut; m, mouth; m.f., longitudinal muscle fibres, 
mx, mastax; o, ovary; ph., pharynx; s.g., salivary gland; t, tentacle; tr, trochus, or cilia-bearing disc. 

Questions on the figure. What sets of organs and functions are indicated in 
the diagram? Does this seem a lower or higher form than the other types studied 
in this chapter? What are your grounds for your answer? What indications of 
segmentation are represented in the figure? Is the mastax in the stomodaeum or 
mesenteron? Where do the various authors classify Rotifers? 



of external segmentation in the tail or foot region in some species and for these 
reasons some authors class the Rotifers near the Annulata. Rotifers are aquatic, 
being more common in fresh water than in the sea. They are abundant in water- 
troughs, gutters, ponds. They are capable of resuming activity after having been 
dried up in the mud for a year or more. This power must be of great value in 
preserving the species as well as in spreading it. 

Phylum VII: Molluscoidea (mollusk-like} . The two groups included here 
are quite diverse in general appearance and habit. They are probably not as 
closely related as this classification would suggest. Their larval stages have more 
points in common than the adult. There is in the adult a variously shaped ten- 
tacle-bearing ridge (lophophore} about the mouth. The central nervous system 
consists of one or two ganglia about the esophagus. The Brachyopoda have often 
been grouped with the mollusks, but authors are agreed that much of the seeming 
resemblance to mollusks is superficial. 

FIG. 96. 

FIG. 96. A fresh-water polyzoan, Plumatella. From Parker and Haswell, after Allman. a, 
anus; fu., funiculus, a band of tissue anchoring the intestine to the body wall; g, ganglion; int., 
intestine; m, mouth; o, esophagus; r, reproductive gland; rt, retractor muscle; si, stomach; stat, 
statoblast; t, tentacles. 

Questions on the figure. Is this an individual or a colony? What is the 
function of the retractor muscles? To what degree are the polyps capable of 
contraction as shown in the figure? The value of this power? What are the 

Class I. Polyzoa (Bryozoa; sea-mats; corallines}. The Polyzoa are colonial 
animals which resemble in general appearance some of the compound hydroids. 
The individual animals however are very different in their structure. They are 
found both in salt and fresh water. In Polyzoa (Fig. 96) the digestive tract is 
sharply bent, the anus opening close to the mouth either within or outside the 
circle of tentacles (lophophore}. A distinct coelom is typically present. There 
are no blood vessels. An exoskeleton is formed by the ectoderm, by means of 
which the individuals of the colony are held together. Each member of the colony 


may retire into its own particular portion of the exoskeleton, when disturbed, by 
the contraction of appropriate muscles. The brain consists of a single ganglion 
lying between the mouth and anus. The two sexes usually occur in the same 
individual. The colonies are formed by budding which takes place in each species 
in a way that is characteristic of that species. Thus it comes about that the colo- 
nies differ as much in general appearance as their individuals do in structure. 

Class II. BracTiiopoda (arm-footed; lamp-shells). Brachiopods are marine 
forms chiefly of geological interest, as there are at present only a few living species. 
They were very prevalent in early geological times. They possess a bivalved 
shell which suggests that of the bivalve Mollusca (as the clam). From this 
external resemblance they have long been classed as mollusks. The valves are 
strictly dorsal and ventral in the Brachiopods, however; whereas in the mollusks 
they are right and left. Their shell is therefore no longer considered as homologous 
with the mollusk shell. The internal structure is still further removed from that 
of the clam. The digestive tract is often bent much as in the Polyzoa, and the 
mouth is surrounded by a tentacle-bearing lophophore (the "arms"). The 
lophophore may have a skeletal support which in different types assumes different 
shapes (loop, helix, or spiral). A peduncle usually extrudes at the hinge, by means 
of which the animal attaches itself to foreign objects. The Brachiopods are not 
colonial. The student is referred to the more extended texts for illustrations of 
this group of animals. 

236. Some Forms of Doubtful Relationship. The old group "Vermes," or 
worms, has in recent years had several fairly definite phyla removed from it. 
Clearest among these is the phylum Annulata or segmented worms (Ch. XV). 
More recently the Phyla Platyhelminthes, Nemathelminthes, Trochelminthes, 
and Molluscoidea have been recognized by many students. In this old waste- 
basket of worm-like animals are still the raw-materials for several more phyla 
when we learn enough about them to distinguish them. None of these groups 
is numerous in species, or of much economic or human interest. They cannot 
receive much attention in an elementary course. Among them are: 

1. Mesozoa, which are simple, parasitic forms less complex even than the diplo- 
Uastic animals. It ha? been thought that they are intermediate between protozoa 
and metazoa. 

2. Nemertinea, similar in some ways to flat worms. They are peculiar in de- 
velopment, and have some systems of organs better developed than other un- 
segmented worms. They live in water, chiefly salt, and in moist earth. Some 
attain a length of ninety feet. They are slender and usually flattened. 

3. Acanthocephala, spine-headed parasites sometimes classed with the nema- 
todes. Pound in most vertebrates. 

4. Chatognatha, which includes Sagitta, the arrow- worm. Free-swimming 
marine animals, sometimes placed near the segmented worms. 

5. Gephyrea, marine worms living in sand or mud, unsegmented with large 
body cavity, and protrusible proboscis. The sipunculids are the best known 

The student is asked to seek figures of some of these rarer types in the larger 
texts, and notice the various ways in which they are classified. 

237. Notes on Ecology and Distribution. The organisim 
included in this chapter represent the most varied modes of self. 


The Turbellarians are free animals and may be terrestrial, 
fresh- water or marine; the Rotifers are as a rule free-swimming 
and occur chiefly in fresh water; the Polyzoa are aquatic, 
attached, colonial forms but lead for the most part an independ- 
ent existence, or may occasionally be commensal with other 
types of animals; the Brachiopods are marine and may be 
attached, but are not colonial; the Trematodes and Cestodes 
represent all kinds and degrees of parasitism. Even if all 
these classes of animals could be considered akin, their habits 
of life and their consequent adaptations are so various as to 
produce the greatest range of general form and special structure. 
If we consider the relatively small number of species of 
animals in these groups, the species of the Platyhelminthes are 
among the most widely distributed of the metazoa. This is 
true both of the free Turbellaria and Nematodes and the parasitic 
Trematodes and Cestodes. There is probably not a large 
species of the higher Metazoa which escapes being the host of 
one or more of these worms at some stage of its life history. 
The fact of parasitism, the ability to carry on the life cycle in a 
series of hosts, and the prevalence of the carnivorous habit 
among its hosts all help the distribution. The organs more 
commonly infested by the parasites are the digestive tube, the 
blood and lymphatic vessels, the ccelomic cavity or other 
organs where the nutritive fluids of the body are abundant. 
They produce all sorts of disorders from mere functional dis- 
turbance (such as digestive disorders and anaemia from the 
presence of the tape-worm) to the destruction of the tissues of 
the organs involved. It is very commonly true that the adult 
or sexually mature individuals are produced in one host, and 
the eggs or larvae produced by them find their way into another 
species of host where a portion of the development toward 
maturity occurs. The transfer of the parasite from the second 
back to the first host-species is necessary to complete the cycle. 
In some instances there is not a change from one animal to 
another, but merely from one organ to another in the same 
animal, as in T&nia murina of the rat. In size the unseg- 
mented worms vary from minute microscopic dimensions to 
thirty feet in length in the tape-worm, Bothriocephalus latus. 


Some suggestion of their importance to man and the higher 
animals may be gathered by reference to the following table 
(p. 206). 

238. Supplementary Studies for the Library. 

1. In what different ways are the forms included in this 
chapter classified in the various textbooks to which you have 
access ? 

2. Consider the economic importance of the parasites in- 
cluded in this chapter. 

3. Make a further study of the life histories of selected repre- 
sentatives of these parasites. 

4. Illustrate by means of the unsegmented worms the de- 
generation and simplification which attends parasitism. 

5. In what various ways do the intestinal parasites in the 
group adhere to the walls of the digestive tract of the host ? 

6. Do you think the domestic animals are more or less likely 
to be attacked and suffer from these internal parasites than the 
wild ? What evidences would you offer for your view ? 

7. Prepare for the class a diagram of the reproductive organs 
in the Tape-worm, indicating the function of each of the por- 

8. What is meant by the ' ' dermo-muscular " sac in worms? 
Its functions ? 

9. Report on the importance of the Brachiopods in early 
geological time, with the main structural features of the class. 




a I . ?. 
s sis 

s s 




239. Asterias (Starfish). Both dry and alcoholic, or other- 
wise preserved, materials should be at hand. 

1. General form. 

Central disc. 

Rays; number, form, size, etc. Compare several in- 

Oral surface (contains mouth); aboral surface. Note 
the general differences between these surfaces both 
in the arms and the disc. 

The axis of an arm is known as a radius; the space be- 
tween is inter radial. 

Is the body bilaterally symmetrical or radially sym- 
metrical ? Give the reasons for your conclusion. 

2. External anatomy. 
Oral surface. 

Mouth: position and surroundings. 

Ambulacral groove: position, relation to the mouth, 

Ambulacral feet: how arranged? Is the foot hollow 

or solid? Pull off one, and examine with lens or 

low power of the microscope. 
Aboral surface. 

Madreporic body: position (radial or interradial?), 

shape, size, structure. 
Bivium; trivium (see text, 243). 
Examine the spines on both surfaces and determine 

the arrangement and shape in different regions. 

How are they fixed to the body ? 
Pedicellariae (at the base of the spines); papulae (soft 

bodies among the spines). Examine with lens. 


Make an outline drawing of each surface, filling in the de- 
tails of the disc and one arm and showing the points above de- 
termined. Sketch one of each of the various classes of spines 
in profile. 

3. Organs of the body cavity. 

Using alcoholic or other moist preparations, cut into one side of an arm of the 
trivium, making an incision from near the tip almost to the disc. Cut across the 
back of the arm near the tip and make a similar incision on the other side. Lift 
the flap thus separated and notice the organs attached to it. The material should 
be dissected under water or 50 per cent, alcohol, or kept moistened therewith. 
Hepatic caeca; extent, number, and attachment. 

Detach the hepatic caeca from the aboral wall by breaking the mesenteries, 
and treat all the arms of the trivium as above. 

Carefully connect the incisions across the interradii and remove the entire 
aboral wall except that around the madreporic body and that between it and the 
centre of the disc, being careful to disturb none of the soft parts. If material is 
scarce the teacher should make a few dissections to be used as demonstrations. 

Body cavity, its extent and contents. 

Stomach: pyloric (aboral) portion; shape, position. Are the hepatic 

caeca connected with it? Verify. (The stomach opens aborally into a 

small, short rectum and anus usually very difficult of demonstration.) 

Rectal diverticula? number and position? 

Cardiac (oral) portion of stomach; pouches, number and form; retractor 

muscles, attached to the floor of the arms. 
Mouth; peristome. 

Remove the hepatic caeca from one arm and find the genital glands which 
lie in the floor of the body cavity. What is their number and arrange- 
ment? At what point do they connect with the body wall? Can you 
prove that they communicate with the exterior? 
Ampullae (on ventral floor) : determine if they connect with ambulacral 


Make three diagrams showing the position of the organs thus far studied: 
(i) the aboral surface with the wall removed, showing the stomach in 
the disc, the hepatic caeca in one arm, the reproductive bodies in a 
second, and the ampullae and retractor muscles in a third; (2) a trans- 
verse section of an arm about midway between its ends; and (3) a sagittal 
section of an arm continued through the disc. 
4. Ambulacral system. 

In a specimen from which the preceding organs have been removed, make a 
transverse section of an arm about an inch from the disc. Find the radial water 
canal, a small tube lying just outside the skeleton in the ambulacral groove. Force 
air into it with a blow-pipe, or inject a colored solution with a hypodermic syringe. 
What other structures are affected? Trace connection between radial canal, 
ampulla, and ambulacral feet. Compare the number of ampullae and the number 
of feet. Follow the radial canal toward the disc. How does it terminate? 

From the madreporic body trace the S-shaped stone canal toward the oral 
surface. How does it terminate? 


Ring canal: its position. Are there any other structures (sacs) in communi- 
cation with the circum-oral ring-canal beside the stone canal and the radial water- 
tubes? form and position? 

5. Nervous system. 

There is a radial nerve (in the skin) superficial to the radial water canal in each 
arm. The radial nerves unite to form a circumoral nerve ring. 

6. Skeletal parts. 

Dried material and portions soaked for a day or so in a 10 per cent, solution of 
potash should be used to supplement the alcoholic specimens. 

Is the skeleton complete, i.e., are the ossicles in contact? 

Are they similarly arranged on the aboral and oral surfaces? Which sur- 
face shows the greater differentiation? Illustrate, and find a reason if you can. 
How are the ossicles related to the spines? to the papulae? Study with some care 
the ossicles forming the ambulacral groove, beginning at the middle line. 

Ambulacral rafters: shape and arrangement. 

Ambulacral pores; are they in, or between, the ossicles? 

Adambulacral ossicles (just lateral to the former); how do these compare in 
number with the ambulacral ossicles? 

"Cross-shaped" ossicles. 

Which of the above bear spines? what kind? 

Place some of the cleaned ossicles in dilute hydrochloric acid. Result? What 
is the significance of this result? 

7. Physiological experiments are possible only near the seashore. The animals 
must be kept in sea water, and studied soon after being collected. When possible, 
locomotion, the action of the ambulacral feet, feeding, and sensitiveness should be 
studied. Do you find any indications, among the specimens provided, of the power 
to renew a lost arm? With care and perseverance, at the proper time of year, the 
sexual elements may be collected and the maturation, fertilization, and cleavage of 
the ovum illustrated in this group. Teachers in inland schools should procure, 
whenever possible, slides demonstrating the early development of the starfish or 

8. Compare briefly the external features of other "stars" with that already 

240. Sea-urchin (Echinus or Arbacia}. 

A few skeletons of sea-urchins and sand-dollars will be of great value in enabling 
the pupil to see how the same general plan of structure may be varied, in different 

1 . Spines (if present) : arrangement and method of attachment. Are they of 
the same appearance and composition as the skeleton? Do you find any signs 
of the former presence of ambulacral feet? If so what, and how arranged? Can 
they all have the same function as in the starfish? Proofs? 

2. Ossicles; Make out the boundaries. Compare with the condition in the 
starfish. What are the special advantages gained by each arrangement? Can 
you find anything corresponding to ambulacral ossicles? (Look for the pores.) 
What corresponds to the ambulacral groove in Asterias? Identify the interambu- 
lacral ossicles. How arranged ? What is radial and what interradial in the urchin ? 
What in the sea-urchin would correspond to the oral and aboral surfaces in the 
starfish? Evidences? Find the madreporic body. Make a plot of all the os- 
sicles in this region, noting the differences. Find the genital pores. 



3. "Aristotle's lantern 1 ' (the mouth apparatus). 

Examine the structure as a whole. How related to the body? Study the 
parts in their relation to each other. Number and method of action ? 


241. The Echinoderms (spiny- skinned) form a very distinct 
group of animals, which in the adult condition at least show a 
decided radial symmetry. They possess a more or less extensive 
calcareous exo-skeleton with outwardly directed spines. The 
starfishes, sea-urchins, brittle stars, sea-lilies, and sea-cu- 
cumbers are representatives. They are marine in habit and 
may be either fixed or slow-moving. They agree with the 
Ccelenterates in having radial symmetry, and in the absence 
of a well-marked brain and other signs of cephalization. There 
is considerable ground for believing that this is an outcome of 
their sluggish habit, since the larval free-swimming condition is 
bilaterally symmetrical, and radial symmetry is clearly adapted 
to a passive life. It is difficult to determine the relationships 
of the Echinoderms; yet it seems probable that their ancestors 
were bilateral forms. Perhaps they should be considered as 
connected with the worms rather than with the Ccelenterates. 

242. General Characters. 

1. Larvae are bilaterally symmetrical; in the adult there is 
a more or less complete radial arrangement of equivalent parts, 
usually on the plan of five. In this radial plan all the princi- 
pal sets of organs share: as the nervous, digestive, reproduc- 
tive, etc. 

2. There is a complete differentiation of digestive tract and 
body cavity. The latter is large. 

3. The blood- vascular system is partially differentiated 
from the body cavity, but communicates with it. 

4. A calcareous exo-skeleton occurs, derived from the meso- 
derm. It may consist of isolated spicules or united plates. 
Associated with these are usually spines, from which the group 
is named. 

5. A water- vascular system, consisting of a series of tubes 
(closed except at one point), muscular sacs (ampullae) and dis- 
tensible feet, serves a locomotor and respiratory function. 



6. Reproduction sexual; development usually indirect, *.>., 
with a metamorphosis. Reproduction by budding does not 

243. General Survey. The majority of echinoderms have 
a central disc in which is located portions of the various sets 
of organs. Ordinarily there radiate from this disc more or less 
clearly defined rays or arms in which lie radial outgrowths from 

FIG. 97. 

FIG. 97. Starfish, from chart of Leuckart and Nitsche. 

Questions on the figure. How would you describe the symmetry of the animal ? 
Identify and name, by comparison with the diagrams and the text, all the struc- 
tures which show in the figure. Compare this with specimens or figures of the 
common American species and note the chief differences. 

certain central organs. The spaces between the rays (inter- 
radii) may be bridged by growth in such a way that the distinc- 
tion between rays and disc is not marked (echinoids) . In crinoids 
the arms may be much branched. The oral-aboral axis is usu- 
ally pronounced, often short, and is vertical in position (aster- 


oids, echinoids, crinoids, etc.), though in the sea-cucumbers 
(holothuroids) it is horizontal and much elongated. Starfish 
are flattened vertically, as are the sand-dollars, but many of the 
urchins (echinoids) are dome-shaped. The antimeres are at 
right angles to this chief axis. In addition to this dominant 
radial symmetry, there is seen even in the adult a suggestion 
of the bilateral condition. The madreporic body generally 
occurs in only one interradius, and a plane passing through 
it and splitting the opposite arm divides the body into two sym- 
metrical halves. No other plane does this. The two arms 
embracing the madreporic body are known as the bivium, the 
remaining three, the trivium. In some of the echinoids the 
bilateral symmetry becomes much more pronounced than in 

244. The integument consists of an outer ectodermal por- 
tion which is often ciliated (cilia wanting in the holothuroids 
and ophiuroids), and a subepithelial, mesodermic layer in which 
is developed the calcareous ossicles. These may occur as spic- 
ules, as rods, or as plates in the various classes. They may be 
bound together by connective and muscle fibres. Frequently 
the ossicles bear spines which may ' or may not be movable. 
The spines are useful in defense and locomotion. Special forms 
of spines known as pedicellaria often occur (asteroids and 
echinoids). They consist of two- or three-pronged pincers 
moved by muscles. They may be mounted on short stalks. It 
is suggested that they help clear the body of foreign objects 
which lodge among the spines. 

245. Digestive System. The mouth and anus usually open 
at opposite poles of the principal axis (asteroids, holothuroids, 
and some echinoids). When the axis is vertical the mouth is 
usually directed downward, in the centre of the oral surface, 
and the anus occupies a more or less central position on the 
upper or aboral surface. In some of the echinoids and crinoids 
the mouth or anus, or both, have vacated their central position 
and may come to occupy opposite margins of the body. The 
digestive tract is a simple tube, in the holothuroids running 
spirally through the body. In the echinoids a similar condition 


is found except that it begins in a complex masticating apparatus 
of five parts (Aristotle's lantern). In the asteroids the mouth 
opens by a short esophagus into an expanded stomach which is 
divided into an oral, or cardiac, and a pyloric portion (Fig. 98). 
From the pyloric part the narrow intestine passes to the anus. 
Outpocketings (caeca) may occur in any of these divisions. 
The most important are the hepatic caeca which are glandular in 

FIG. 98. 

FIG. 98. Vertical (sagittal) section through an arm and an interradius of a Starfish (diagram- 
matic), c, anus; amp., ampulla; c.b.. circular blood vessel; c.w., circular water canal; co., ccelom; 
CO. e. ccelomic epithelium; d.b., dermal branchiae; e, position of the eyespot; ect., ectoderm; ent. t 
entoderm; /, ambulacral foot; g, ambulacral groove; h, hepatic caeca or liver; *', intestine; i.e., 
intestinal caeca; mes, mesoderm; mo., mouth; m.p., madreporic body; n.r., nerve ring; os., ossicles 
in mesoderm; r.n., radial nerve band; r.b., radial blood vessel; r.p., reproductive pore; r.w., radial 
water canal; s.c., stone canal; sp., spines; z, lacunar spaces in the mesoderm. (Adapted from 
various sources.) 

246. The body cavity is well developed in the disc and usu- 
ally in the arms, is lined with a ciliated epithelium, and contains 
a fluid with amoeboid corpuscles. It is completely distinct from 
the digestive cavity. Thin outgrowths of the body- wall (papula 
or branchid) contain extensions of the ccelom. These assist in 

247. Ambulacral or Water-vascular System. This system 
of tubular organs is peculiar to the echinoderms. It originates 
(see also 254), in common with the body cavity, as an outgrowth 
from the archenteron and is to be regarded as a specialized por- 



tion of the body cavity. In some cases these two cavities are 
in communication in the adult. It consists essentially of a 
ring-vessel about the mouth from which pass radial tubes, one in 
each arm. From the radial tubes arise lateral channels which 
communicate directly or through bladder-like ampullae, with 
distensible feet which reach the exterior by pores in the skeleton 
(Figs. 99, 100). The tip of the foot may be provided with a 
sucking-disc, serving as a means of attachment and of locomo- 
tion. Frequently the walls of these feet are thin and apparently 
serve for respiration, and the terminal "foot" at the end of each 

FIG. 99. 

d. b. 


FIG. 99. Transverse section of the arm of a Starfish near the disc. Diagrammatic Lettering 
as in preceding ^figure. a,r. t ambulacral rafter (ossicle); ov., ovary, containing ova. 

| Questions on figures 98 and 99. What are the principal sets of organs repre- 
sented^ the disc of the starfish ? Which of these have radial portions going into the 
arms? Follow carefully the ectodermal, entodermal and mesodermal boundaries. 
Locate and identify the various structures lettered, and determine as far as possible, 
whether the essential part of each is furnished by ectoderm, entoderm or mesoderm. 
Is there a ccelom? Your evidences? What is the relation of the water-vascular 
cavity to the ccelom, in origin? 

radius may be highly modified to form a sense organ (tentacle.) 
The feet, the ampullae, and even the radial vessels may be want- 
ing. The ring-canal, in typical forms, communicates with the 
surrounding sea- water by means of a tube (stone canal) which 
terminates in a sieve-like plate, the madreporic body, through 
which the water enters the water- vascular system. In the 


majority of the Holothuroids the madreporic tubes open into 
the body cavity instead of opening to the exterior. In conse- 
quence the fluid which is found in the water-vascular system is 
the same as that of the body cavity and contains amoeboid cells. 
In the crinoids also the water-vascular system communicates 
directly with the ccelom, but there is no true madreporic canal. 
In its stead is found a system of ciliated water-tubes in connec- 
tion with the ring canal. Identify the elements in the water- 
vascular system from Fig. 100. 

248. Respiration occurs in connection with the water- 
vascular system especially in those forms in which the tentacles 
and ambulacral feet are possessed of thin walls (holothuroids 
and some echinoids). In the asteroids and echinoids there are 
thin outpocketings of the body-wall, papulae or branchiae (Fig. 
99, d.b.), the cavity in which is continuous with the body cavity. 
The body fluids may thus be aerated from the water outside. 
In some forms water is drawn into special branching pockets 
(respiratory tree) in the wall of the rectum, and later is forced 
out again. 

249. Circulation. The circulatory vessels are merely partly 
differentiated portions of the ccelom or body cavity. Our knowl- 
edge is by no means complete but it seems that in none of the 
groups is there a complete separation of the blood spaces from 
the ccelom. There are probably no contractile hearts. The 
walls of the blood spaces may bear cilia which assist in securing 
the motion of the fluid. The blood contains migratory cells, 
usually colorless, and is identical with the fluid in the body 
cavity. The general body contractions are important in caus- 
ing motion of the fluids. It should be remembered that the 
water- vascular system is also partly circulatory in function. 
The blood vessels of the various classes agree in having a central 
circular portion consisting of one or more rings, with radial 
tubes running into the arms, and in some instances vessels which 
accompany the intestine. The vessels of the oral surface are, 
throughout, in close connection with the nervous epithelium 
(Fig. 98, r.b.). 



250. Excretion. It is impossible to name any organs known 
to be solely excretory in function. As in respiration many 
organs may take part in the work. The gaseous and soluble 
excreta are eliminated through the general body surface, the 
papulae, the respiratory tree, or the ambulacral organs. The 
skeletal ossicles in the mesoderm represent, in part at least, 

FIG. 100. 



FIG. 100. Diagram of a portion of the water- vascular (ambulacral) system of the Starfish, a 
ampullse;/, ambulacral feet; m, madreporic body; p, Polian vesicles; r.r., ring canal, with the upper 
portion removed at the right of the figure; r.t., radial water tubes (in r.t.' the upper portion is re- 
moved at the distal end and the proximal portion is represented entire) ; s, stone canal. 

Questions on the figure. Where does the water enter this system of vessels ? 
At what points in the system is it of use ? By comparing all illustrations at your 
disposal, describe the mode of using this system of organs for locomotion. How 
may it be used in respiration? 

the elimination of certain inorganic salts which can not be used 
in the vital activities and are therefore excretions. 

251. The Muscular System. The degree of the develop- 
ment of the muscular system varies. In those forms which 
have a well-developed skeleton the body muscles are not of 


much significance. In the holothurians, on the contrary, the 
body is capable of definite and considerable contractions, by 
reason of both circular and longitudinal fibres. In forms with 
incomplete skeletons, as the starfish, muscular fibres connect 
the ossicles. There is a high degree of flexion of the arms of 
crinoids and aphiuroids. There are also special muscles con- 
trolling the water vascular system, the stomach, the mouth parts, 
the spines and pedicellariae. The fibres are non-striate. 

252. The Nervous System consists of a ring around the 
mouth and a radial nervous band in each arm supplying, by a 
plexus of fibres and cells, all the radial organs. This system is 
superficial (" ventral") to the radial water-tube (Fig. 99, r. n.) 
and in the starfish preserves its connection with the ectoderm 
from which it is in all forms derived. Other deeper lying, and 
even aboral, nervous elements are described for some of the 
members of the group. These elements, when present, have as 
their function the innervation of the muscles of the interior and 
of the aboral wall of the body. 

Sensory organs are not highly developed. The animals 
show evidences of possessing a chemical sense (analogous to 
taste and smell) by which the presence of food is detected. 
This is apparently localized in the tentacles in such forms as 
have them. A tactile sense is also present, and is most highly 
developed in the tentacles, ambulacral feet, and other movable 
outgrowths. At the tip of the antimeres of the asteroids, or of 
the radial nerve (echinoid) are structures bearing pigmented 
spots, which appear to be sensitive to light (eye-spots). These 
cannot give more than a very general impression of light, by 
means of the chemical changes induced in the pigment cells by 
the action of the light. Starfish are more quiet during the day. 
When the starfish is placed on its "back" it makes definite 
coordinated movements to right itself. Experiments show that 
the starfish is modified slowly by its experiences. That is to 
say it learns to do certain things in certain ways. It retains 
("remembers") for at least a few days. 

253. Reproduction is wholly sexual. The sexes are distinct, 
but the males and females are not often distinguishable by 


external characters. The sexual organs, ovaries or testes, are 
lobed bodies occurring usually in pairs in an interradial position. 
These open by pores also interradial, and usually dorsal (Fig. 
99, r. p.). There are typically five pairs of genital glands, but 
in the holothurians the number is reduced to one. Fertilization 
takes place outside the body, and usually the development is 
wholly independent of the parent. In some instances however 
the parent has special pouches in which development proceeds. 

254. Development. The fertilized ovum undergoes total 
and practically equal segmentation, producing a ciliated blastula. 
Gastrulation occurs by invagination resulting in ectoderm and 
entoderm. The mesoderm is formed in two ways: (i) by mi- 
grating cells budded from the entoderm into the segmentation 
cavity (mesenchyma; Fig. 14, c}\ and (2) by the outgrowth of 
ccelomic vesicles or pouches from the wall of the archenteron 
or entoderm (true mesoderm). These latter outpockets of the 
wall of the gut are those which give rise to the ccelom and to 
the water vascular system (see 247). 

In the later larval development the cilia of the gastrula be- 
come limited to two zones, a preoral and a preanal, and the 
shape of the larva is much modified. Numerous paired, lateral 
outgrowths serve to accentuate the fundamental bilateral sym- 
metry. In most members of the group a marked metamorphosis 
occurs in the passage from the larval to the adult condition. 
During this change, the water vascular system and the mid-gut 
of the larva are retained with the necessary modifications. 
About these as a centre, what we might almost call a new animal, 
the radiate starfish, begins to grow at the expense of the larval 
organs which are absorbed by the amoeboid cells, and thus new 
organs appropriate to the adult are formed. During this process 
the bilateral symmetry of the embryo gives place to the radial 
symmetry of the adult. While there is no reproduction by 
budding there is a striking power of renewal of arms or other 
portions which may be lost by injury, or in some instances by 
self -mutilation. Arms are readily reproduced if the disc is 
uninjured (stars, brittle-stars, and crinoids); portions of the 
internal organs, as the digestive tract, are said to be regenerated 


by some of the holothurians. Occasionally, at least, an arm 
and a small portion of disc seems to have the power of reproduc- 
ing a new disc and other arms. This power of throwing off arms 
and replacing them is doubtless a means of protection. 

255. Ecology. The echinoderms are marine. The larvae 
are free-swimming, pelagic, but after the assumption of the 
adult form they usually become much less active. The crinoids 
are typically stalked and often attached. The asteroids and 
echinoids inhabit the bottom of the ocean where they creep more 
or less slowly. They may be found at almost any depth, from 
the shallow pools at low tide to the deepest bottoms. Many 
of them burrow in the mud and sand, and others (some sea- 
urchins) have the power of scouring out burrows in the rocks 
by the action of their spines. Echinoderms, being slow movers, 
are compelled to subsist upon such food as the currents or the 
chance movements of other animals may bring, or upon the 
debris which falls to the bottom of the sea, or upon such organ- 
isms as are attached and cannot escape. The starfishes for 
example are a constant menace to the oyster beds. The fact 
that some starfish are in a measure gregarious makes this all 
the more serious. It is difficult to see how the starfish can get 
the oyster from the protection of its shell, but it manages to get 
the shell open and clasping its arms about its prey it turns the 
cardiac portion of its stomach inside-out over the soft part of 
the oyster and thus leisurely digests it outside its body, so to 
speak, leaving the empty shell behind. Except for this the 
group is of little economic importance. The Chinese esteem 
some species of Holothuria (the trepang, for example) as food. 
The group appeared early in geological time and has had very 
characteristic representatives in all ages up to the present. 
The changes which have taken place in the echinoderms from 
one geological age to another are among the most interesting and 
instructive furnished by the invertebrates. 

256. Classification. Class I., Blastoidea; Class II., Cystoidea. 

(These are both extinct, fossil classes. They comprise stalked and attached 
forms, and perhaps represent the nearest approach of our known species to the 
primitive echinoderms.) 

Class III. Crinoidea (feather-stars and sea-lilies}. These forms are less com- 


mon than in earlier geological times, when they must have been very abundant 
and very beautiful. They contribute much to the formation of the limestone of 
the Palaeozoic. They are usually provided with jointed stalks, by which they may 
be attached to the bottom. At the summit of the stalk is a central disc with five 
arms often much branched and bearing lateral pinnules. The anus is on the oral 
or upper surface, the stalk arising from aboral surface. They are inhabitants of 
moderate to deep seas. 

Class IV. Asteroidea (starfishes) (Fig. 97). The asteroids, of which there 
are several hundred species, are free echinoderms with a central disc and usually 
five arms. The latter are large and contain liberal ccelomic spaces in which are 
lodged outgrowths of the digestive system and other organs. There is a distinct 
oral and aboral surface. The anus and madreporic body are on the latter. Dis- 
tinct ambulacral grooves lie on the oral surface of the arms. Adult starfish may 
vary in size from a few inches to two feet or more in diameter. 

Class V. Ophiuroidea (serpent-stars). These are fragile, free echinoderms 
in which the arms are small and much more distinct from the disc than in the 
asteroids. The organs of the disc are not all continued into the arms. There is 
no anus, no ambulacral grooves, and the madreporic body is on the oral surface. 
Their slender arms are useful in clinging to supports or to prey, and are used in 
locomotion. They are readily broken and regenerated. 

Class VI. Echinoidea (sea-urchins, sand-dollars). These are free echinoderms 
without free arms. The arms are connected by the development of interradial 
plates. The calcareous rods are united into plates which produce a complete exter- 
nal skeleton varying from flat dome-shape (as in sand-dollars) to a globular form 
(Echinus or Arbacia). The mouth is usually in the centre of the oral surface and 
the anus near the centre of the aboral, yet one or both may come to have an excen- 
tric position. In this way the bilateral symmetry is accentuated at the expense of 
the underlying radial symmetry. The madreporic body is aboral and there are 
no ambulacral grooves. The spines of the urchins are usually well developed 
and may be used to scour out rounded pockets in rock in which the animals are 
sometimes found. 

Class VII. Holothuroidea (sea-cucumbers}. These are soit, free echinoderms, 
elongated, cylindrical or flat, with mouth and anus at opposite poles of the hori- 
zontal long axis. The skeleton is not well-developed, usually being represented 
merely by scattered spicules. The water- vascular system in most forms com- 
municates with the body cavity instead of the exterior. Well-developed tentacles 
occur about the mouth. Most holothurians burrow in the sand or mud, but others 
cling to rocks near the surface of the water, and still others occur at great depths 
in the ocean. Their reactions are rather more complex than in the other members 
of the phyla. They are more muscular and more responsive to light and contacts. 
In addition to hiding by burrowing, they often contract the muscular wall violently 
and eject large portions of their viscera. These may be replaced by regeneration. 

257. Suggestive Studies for the Library or Laboratory. 

1. Read and report on the metamorphosis of the various 
members of the group. 

2. Study from dry and moist material and report on the 
structure and mode of action of "Aristotle's lantern" in Echinus. 


3. Construct a table of parallel columns one for each of 
the five living classes and contrast them as to: (i) general 
form of body including symmetry, (2) manner of motion, (3) 
position of mouth and anus, (4) position of madreporic body, (5) 
character of digestive tract, (6) differences in the spines and other 
skeletal structures, (7) the position and character of the ambu- 
lacral feet, (8) habitat and food, (9) parts repeated in the anti- 

4. Report on the habits, appearance, and abundance of 
crinoids in geological time. 

5 . The origin and development of the water- vascular system. 

6. Compare the figures of the various classes as illustrated 
in your reference texts and mark the degree of variation. 



258. The Earthworm (Allolobophora or Lumbricus). The 
principal work should be done with living worms. For what- 
ever anatomical work is undertaken, specimens may be killed 
by exposure to fumes of chloroform while wrapped in cloth 
moistened with water; they should then be pinned out straight, 
and hardened in an abundance of alcohol. If needed in the 
winter they may often be found under manure heaps, or about 
green-houses. They may be kept alive in flower pots contain- 
ing moist earth. 

1. Promorphology; General Form. Is there an anterior and 
a posterior end ? How distinguished ? Is there any distinction 
of dorsal and ventral surfaces ? If so, what ? Is there bilateral 
symmetry ? What external evidences of segmentation do you 
find? How are the similar units (metameres or segments) ar- 
ranged? Compare with the condition in the starfish. Com- 
pare the metameres of different parts of the body, noting differ- 
ences. Is the body divisible into regions (i.e., groups of similar 
metameres) ? Locate (by numbering the segments) all such 
regions. How many segments in the animal ? To what extent 
does this vary in different specimens? Show by a series of 
diagrams the shape of the animal, and the shape and size of 
cross sections in various regions. 

2 . Activities. Describe, after careful observation, the method 
of locomotion in the earthworm. Place the worm on a 
rough board ; on a plate of glass. What is the difference ? And 
why ? Compare the various parts of the body as to size, during 
movement. Cause of the difference ? Can each end move fore- 
most? What seems to determine which end shall protrude as 
the result of the muscular contractions ? 




Does the animal respond equally to contact (with pencil or 
toothpick) at anterior, posterior, and middle parts of the body ? 
Devise a method of determining whether it is sensitive to light. 
Record results. 

Place moist soil and dry soil side by side on a board; place 
the worm in various positions to test his preference. Record 
results. Place a piece of filter paper which has been dipped in 
acetic acid in the path of a worm. How does it react? Try 
similarly a sugar solution; a salt solution; a decoction of decay- 
ing leaves. Will an earthworm pass into water ? Do your ex- 
periments bear in any way on the habits of the earthworm in 
nature? Can you secure any evidence as to the food of the 
earthworm ? How ? Record your results. 

3. Special External Structures. Locate the mouth, the pre- 
oral lobe, clitellum (a series of swollen segments), anus. Com- 
pare preoral lobe with other segments. With a lens and by 
drawing the worm backward between the fingers discover the 
setae or bristles. Are they found on all segments? Number 
and position of the groups of setae in each segment ? What is 
the function of the setae ? Proofs ? 

4. Internal Anatomy. Pin out a large specimen, which has been hardened in 
alcohol, on dissecting board or pan, and carefully remove the dorsal wall from the 
anterior half of the body by making lateral incisions with sharp-pointed scissors, 
or make a single incision along the back a little to one side of the middle line. 
After noting the cross membranes (dissepiments}, their relation to the rings on the 
outside, and their attachments, cut them so the body wall may be folded back and 
pinned. The dissection should proceed under fluid, 50 per cent, alcohol, for 
example. Make all the outline drawings necessary to show all your discoveries. 
Notice the coelom. It is completely divided by the dissepiments? Are the cham- 
bers of equal size? 

(a) Digestive organs: Beginning at the anterior end, note the following re- 

Pharynx, a pear-shaped enlargement: how held in place? In what seg- 
ments is it situated ? 

Esophagus, a narrow tube; crop; gizzard; intestine. 

Determine the segments in which each region occurs. Does the digestive 
tract show any signs of segmentation, i.e., in correspondence with the 
external rings? 

(6) Circulatory system: A living or newly killed specimen is somewhat better 
for this. Discover, if possible: 

Dorsal vessel (just dorsal to the digestive tract). 
Ventral vessel (just ventral to the digestive tract). 


Hearts, transverse vessels connecting the longitudinal vessels, in segments 
VII to XL 

There are other vessels more difficult to find. Examine a drop of the con- 
tents of the blood vessels with the microscope. 

(c) Reproductive System: These organs are rather too complicated for satis- 
factory results in an elementary class. Instead of a detailed examination note the 
reproductive segments (in the region of the esophagus) with the whitish bodies 
showing at the sides of the alimentary canal, and ventral to it. They are attached 
to the septa. (Compare figures in various text-books.) Make a composite dia- 
gram of your own. 

(d) Nervous System: In a well-hardened preparation, identify: 

Brain, a whitish lobed ganglion just dorsal to, and in front of the pharynx; 
collar, around the mouth, connecting the brain with ventral ganglia, the 
first of a longitudinal chain of ganglia which give off nerves in each seg- 
ment. How are the ganglia of the ventral chain related to the dissepi- 
ments ? 

(e) Excretory Organs: Just lateral to the nerve-chain the student may be 
able to find coiled thread-like structures (nephridial tubes) in nearly all the body 
segments (see text, 270). How many in each segment? 

5. Microscopic Demonstration. The teacher should make or secure good, per- 
manent mounts of transverse sections of the earthworm, by means of which the 
students should identify the structures studied in the gross dissection, and make 
out more exactly the nature of the following parts (see Fig. 103) : 

Cuticle, or outer layer. 

Body wall, and the relation of the circular and longitudinal muscles. 

The ventral nerve-chain in position. 

The dorsal and ventral blood vessels. 

The wall of the digestive tract; gland cells, typhlosole, etc. 

259. Dero (or other minute aquatic Annelid). Any one of 
these fresh -water worms may be used very profitably to sup- 
pliment the students' work on the earthworm. Mount the 
living worm, being careful to support the cover-glass. Study 
with low power. Compare at all points with the earthworm. 
Dero may usually be had at any season of the year by taking 
mud and organic matter from the bottoms of foul brooks or 
ponds and placing it in vessels in the laboratory. The worms 
will usually come to the sides of the vessels where they may be 
seen. Owing to its transparent qualities, such a form will be 
especially valuable in giving the student a better idea of the 
performance of function in the group. What evidences of mus- 
cular action are manifest ? How is locomotion effected ? Posi- 
tion and mode of action of setae? Study the capture of food; 
how is its progress through the digestive tract, and its elimina- 
tion therefrom effected? Do you discover any circulation of 


the blood? Direction of flow? Evidences? How accom- 
plished? Test for ability to receive and respond to stimuli of 
different sorts. Where are new segments formed? Discover, 
if possible, instances of fission, by which new individuals are 

260. The Leech. The leech may be studied and compared with the earthworm 
as to its external features, its habits, mode of locomotion, and the like. If large 
specimens can be had some members of the class might substitute it for the earth- 
worm and the results of the studies brought into comparison. 

261. Nereis. If specimens of Nereis can be obtained this worm should be 
compared with the earthworm. (Even two or three good specimens may be made 
useful from year to year as comparative demonstration both of external and inter- 
nal structure.) 

Note especially: 

(a) The specialization of the anterior end; proboscis, mouth, jaws, palps, cirri, 
eyes, antennae. 

(&) The fleshy supports of the bristles, parapodia. 


262. The Annulata are separated from the unsegmented 
worms by the possession of a series of segments or metameres 
which show on the exterior as rings, and contain similar or 
homologous organs or similar portions of a continuous organ. 
There is also a more uniform development of the ccelom than 
in the lower worms. They differ from the ccelenterata in hav- 
ing bilateral rather than radial symmetry in the adult condition. 
The development is often direct, but in many, especially the 
marine forms, there is a metamorphosis. The larva of these 
has a peculiar balloon-shaped form, known as the trochophore 
(Fig. 1 06, ), similar in some respects to the Rotifers. 

263. General Characters. 

1. Body elongated, bilaterally symmetrical and segmented. 

2. External paired appendages (setas, bristles, etc.) not 

3. There is usually a well -developed body cavity. 

4. The excretory organs are typically paired nephridial 
tubules, one pair in each segment, connecting the body cavity 
with the outside. Certain highly modified pairs of these serve 

as outlets for the reproductive bodies. 



5- The nervous system consists of (i) a supra-esophageal 
ganglion (brain), and (2) a circum-esophageal collar or con- 
nective uniting it with (3) a ventral chain of ganglia with a 
ganglion in each segment. 

6. Locomotion is primarily effected by means of the contrac- 
p IG 10I tionsof the body wall, acting on body fluids 

in the cavity within. 

7. Development may be either direct 
or indirect. When indirect, the larva 
passes through a trochophore stage. 

264. General Survey. The Annul ata 
though conforming to the type outlined 
above are very diverse in appearance, 
habits and internal structure, While the 
Chaetopoda, the class to which the forms 
studied in the laboratory belong, are 
taken as the type, the leeches, which have 
no bristles but possess suckers, are un- 
doubtedly related, as is shown by their 
development. The Rotifers and other 
forms are sometimes included among the 
relatives of the Annulata. Metamerism 

FIG. loi. Dero, a. fresh-water oligochaetous annelid, in optical 
(frontal) section. Enlarged 30 times, a, appendages; br., brain; 
d, dissepiments; *, intestine; m, mouth; nph, nephridium; oe\ 
esophagus; p, pavilion, lined with ciliated entoderm; ph., pharynx. 
pr., processes from the anal segment; z, zone immediately in 
front of the anal segment where new segments are continually 
being formed; z', the zone of fission or budding. This takes 
place in the middle of a segment. The anterior half-segment 
of z' will produce a region like z for the anterior half of the 
worm. The posterior half-segment will produce a head and four 
segments like those which contain the pharynx (1-4) of the 
parent worm. 

Questions on the figure. What regions of the 
digestive tract are sufficiently differentiated to de- 
serve notice? What is the number of the segment 
in which fission is taking place? What structures 
must the anterior half of this segment make? The 
segment behind the dividing segment becomes num- 
ber 5 of the new posterior worm. What structures 
then must be developed from the posterior half of the 
dividing segment? 



in animals is a most interesting phenomenon to zoologists. This 
group is the first in which we have found true metamerism. The 
body of the animals is more or less constricted on the outside into 
rings as the name (Annulata) implies. The internal organs also 
show metamerism, but in various ways. These organs may 
pass directly through, with slight segmental modification, as the 
digestive tube and ventral nerve cord; they may be repeated 
independently in each segment, as the setae or nephridial tubules ; 
or they may be represented in only one or a limited number of 

FIG. 102. 

C. V. 

d. rrv 

n. c. 

n. f. 


FIG. 102. Longitudinal section of anterior end of Dero. A, sagittal section; B, frontal section 
to show anterior portion of nervous system. &, brain; co., nervous collar about the mouth; c.v., 
contractile blood vessels ("hearts"); d, dissepiment; d.m., dermo-muscular wall; d.v., dorsal blood 
vessel; m, mouth; n.c., nerve cells; n./., nerve fibres; np., nephridia; p, prostomium; ph., pharynx; 
5, setae; sn., segmental nerves; v.g. ventral chain of ganglia; v.v., ventral blood vessel. Notice that 
only a portion of the blood vascular system is shown, and this appears unsectioned in the figure; 
also that nervous structures and nepridia are only partially represented. 

Questions on the figure. Compare this with the cross section of Dero and 
identify the parts. How do the four anterior segments differ from the others 
figured? Does the ventral nerve cord continue the whole length of such an animal 
as this? Which organs may be described as segmental and which as continuous 
through the segments? 

segments, as the brain or the reproductive bodies. The seg- 
ments are not therefore exactly equivalent, yet the agreement 
between successive segments is sufficient to merit the term 
homonomous (see 125). The number of segments varies from 
a few to hundreds. The body is from four or five to many times 



as long as broad, and is usually cylindrical or flattened dorso- 
vent rally. 

265. The dermo -muscular sac is composed of the integu- 
ment or skin and the muscular layers of the body wall. Being 
filled with the body fluids it is a very important instrument of 
locomotion. This is accomplished by the alternate contractions 
of the circular and longitudinal fibres with which the wall is 
supplied. Externally there is a cuticula, usually very thin, 
overlying and secreted by the layer of epidermal cells. Some 
of the cells of the epidermal layer are glandular and others are 
sensory. The setae or bristles are secretions of the epidermal 

FIG. 103. 

c I 

FIG. 103. Transverse section of Dero. X 300. c., ccelom; c.L, cells of the so-called "lateral 
line;" d.m., dermo- muscular wall including muscles and skin; d.v., dorsal bloodvessel; *, ecto- 
derm; ent, entoderm; g, gut; g. /., giant nerve fibres; gl, glandular cells assisting in digestion; m.c., 
circular muscle fibres; m., longitudinal muscle fibres; n, nephridium; n.v., ventral nerve chain, 
made up of nerve cells and nerve fibres; 5, setae; v.v., ventral blood vessel. 

Questions on the figure. Compare this with Fig. 102 and identify all the struc- 
tures which appear in both. What elements enter into the dermo-muscular wall? 
Identify nerve cells, fibres and the "giant fibres" in the ventral nerve cord. 

cells and lie in sacs in the skin. These structures vary in num- 
ber and position but are usually paired, two or four groups to 
each segment. They are absent in the leeches. Next to the 
skin is a layer of circular muscle fibres, and within these are the 
longitudinal bands of muscle fibres. In the leeches there are 

ANNUL AT A 2 29 

also dorso- ventral fibres. Special groups of fibres occur in 
connection with the setae, the mouth parts, suckers, etc. The 
fibres in worms are spindle-shaped and unstriate. The dermo- 
muscular wall bounds a true body cavity in the chaetopods; but 
in leeches the ccelom is almost filled with connective tissue. 
This suggests the condition in many of the unsegmented worms. 
See Figs. 101, 103. 

266. Worms as a rule have no external skeleton other than 
the cuticle, but in some instances a tubular protective structure 
is formed by secretion or by cementing together small particles 
of foreign matter. Because of the absence of hard skeletal 
parts little is known concerning the worms of past geological 

267. Digestive System and Feeding. The stomodaeum, the 
mesenteron, and proctodaeum (see 93) are all to be distin- 
guished in the enteric canal. The mouth is not quite terminal, 


FIG. 104. Transverse section of the intestine of the Earthworm, ty, typhlosole, an infolded 
longitudinal ridge in the gut in which the gland cells feZ.) are especially aggregated. Other letters 
as in Fig. 103. 

Questions on the figure. Of what conceivable gain is the typhlosole? What 
is it analogous to in the higher types of animals? 

but slightly ventral. The prostomium (or preoral lobe), a mus- 
cular extension of the oral segment, overarches it. There is 
typically an enlarged muscular pharynx which is often eversible, 
followed by a narrow tubular esophagus. Often there is no 
further differentiation, the remainder of the tube being fairly 
uniform and called the intestine. Frequently, however, special 


enlargements occur, chief among which is the stomach or gizzard, 
a grinding organ. In the leeches the alimentary system is 
much modified in accordance with the blood-sucking habit of 
the animal. The crop is capable of great enlargement and may 
contain enough blood to nourish the animal for a long time. 
The mouth is sometimes armed with special cuticular outgrowths 
which serve as teeth. Glands either unicellular or compound 
occur in" various regions of the digestive tract. These secrete 
ferments similar to those of higher animals. In the earthworm 
and related forms there is a dorsal longitudinal fold of the in- 
testinal wall into the lumen of the tube, thus increasing the ex- 
posed surface. This is called the typhlosole (Fig. 104) and is 
supplied with cells which have been described as digestive. 
The entodermal epithelium may secrete a cuticle or may be 
ciliated. This layer is surrounded by connective tissue and 
muscular fibres. 

Worms make use of every possible kind of food. Earth- 
worms eat pieces of vegetable or animal matter, and soil con- 
taining these. They are lovers of darkness, and capture their 
food at night. When the light is dim they may come to the sur- 
face of the ground and, with the tail segments inserted in the 
opening of the burrows, explore the surroundings as far as they 
can reach. After night showers they may come entirely out. 
It is at such times as these that they become the prey of the 
"early bird." Many of the marine forms are actively car- 
nivorous. Most leeches are blood-suckers, though some are 
carnivorous or parasitic. Many small worms flourish in foul 
water where they use much decaying organic matter and micro- 
scopic organisms. 

268. Respiration. The exchange of gases is effected for the 
most part through the general body wall, into which the blood 
capillaries or the lacunae of the coelom may penetrate. In some 
forms there are special thin places and out-pocketings of the body- 
wall (branchiae) by which the exchange is facilitated. These 
are characteristic of the Polychaeta especially (Figs. 107, 108). 

269. Circulation. In some of the simplest worms there are 
no special blood vessels. The ccelomic spaces contain a fluid, 


which possesses corpuscles and is moved by the general body 
contractions. In the typical condition there are two or more 
longitudinal vessels, dorsal and ventral (or lateral) in position. 
These are often connected by transverse loops in a few or many 
segments of the body especially at the anterior and posterior 
ends. The circum-intestinal loops are often contractile, and 
the longitudinal vessels may show a wave of contraction passing 
from one end to the other. Capillaries vary much in perfection 
of development. The blood contains only white corpuscles. 
There is haemoglobin in the plasma. 

270. Excretion takes place by means of the segmental organs 
or nephridia, of which there is usually one pair in each segment, 
with the exception of some of the anterior segments. The 
nephridium is a tubular structure consisting essentially of the 
following portions (Fig. 35): (i) a ciliated funnel, communi- 
cating with the ccelom; (2) a tortuous glandular region; and (3) 
an outlet through the' body wall, often supplied with muscle 
fibres. The nitrogenous waste products find their way into the 
fluid of the ccelom and thence into the nephridium, or directly 
into the nephridium from blood capillaries which may occur in 
its walls, and thus are finally eliminated upon the exterior of 
the body. 

271. Nervous System and Behavior. The "central" nerv- 
ous system may be said to consist of three portions: (i) a mid- 
ventral line of nerve fibres, and nerve cells which are diffusely 
scattered or collected in ganglia, (2) a brain which is anterior 
and dorsal to the pharynx, (3) a connective, or collar about the 
pharynx connecting (i) and (2) (Fig. 102). The brain and 
ventral cord may show distinct right and left lobes or may be 
completely fused into a median mass. From the brain, nerves 
pass to the 'head-parts. From each of the segmental portions 
of the ventral chain nerves pass to the walls, viscera, etc. The 
ventral cord frequently lies in a blood sinus which secures its 
abundant nourishment (leeches). 

The sense organs occur very unequally in the group. The 
Polychaeta and the leeches are best supplied. The skin is 
generally sensitive to mechanical contact, to chemical stimuli 


and to moisture. This sensitiveness is perhaps specially local- 
ized in the tentacles, cirri, and more movable parts. Statocysts, 
fluid-filled cavities bounded by sensory epithelium, occasionally 
occur (see m). Some solid particles (statoliths) float in the 
fluid. These have been described as organs of hearing but the 
sensation resulting is probably quite different from what we 
know as hearing. They are apparently organs of equilibration, 
enabling the animal to appreciate its position in relation to the 
pull of gravity and to appreciate the action of water waves. 
Eyes may consist merely of a group of pigmented cells with 
nervous connections, or may be very complicated, consisting 
of a capsule with refractive media and retina. Images of ob- 
jects are not formed, in all probability, but the direction and 
intensity of light can be appreciated. In the leech there are 
sense organs in each segment somewhat similar in structure to 
the eyes. Their function is unknown. Even in the earthworm 
and other forms in which there are no eyes, the skin is sensitive 
to light. Most worms prefer dark places. 

272. Reproductive Organs. The Oligochaeta and the leeches 
are hermaphrodite. In the Polychaeta the sexes are separate. 
The sexual products are developed from the ccelomic epithelium, 
sometimes on the dissepiments, sometimes on the body wall, or 
in other special regions. The elements may be produced in 
many segments (Polychaeta), or in a few anterior ones (Oligo- 
chaeta). The region is usually distinguishable only about the 
breeding time. In the hermaphrodite forms the ova and sper- 
matozoa often mature at different times and are produced in 
different segments. This of course insures cross-fertilization. 
In the Polychaeta the conditions are relatively simple. The 
elements are freed in the body cavity and when mature find 
their way into the water where fertilization takes place. The 
organs are much more complicated in the hermaphrodite worms. 
The spermatozoa are produced in the testes, are passed into the 
seminal vesicles where they are matured, and at the time of 
copulation escape to the exterior by the vasa deferentia, to be 
deposited in the sperm sacs or receptacula seminis of another 
worm. From this place, any time after copulation, the sperm 



is brought into contact with the ova of this second worm, as 
they pass from the ovary, where they are produced, to the egg-sac 
or to the exterior. It is believed that in some instances at least 
the genital ducts are modified nephridia. 

It will be a profitable practical exercise for the student to 
make for himself a diagram of the sex organs of the earthworm 
or some other hermaphrodite form by reference to several stand- 
ard texts. 

273. Reproduction and Development. Sexual reproduction 
is universal. As we have seen, copulation may occur or the 
elements may come together in the water. In the Oligochaeta 

x mi. 3 

FIG. 105. Two stages in the development of Nereis. A, 8-celled stage; B~, lo-celled stage, 
both viewed from the active or ectodermal pole, mi. 1 , mi,*, and mi.*, the first, second and third 
sets of micromeres separated from ma., the macromeres; s 1 , first somatoblast, one of the second 
group of four cells to be budded from the macromeres; s 2 , second somatoblast, one of the third group, 
which gives rise to the mesoderm. The micromeres are ectodermal and the macromeres produce 
the entoderm. (After Westinghausen.) 

and leeches the fertilized ova, or the ova together with masses 
of spermatozoa, are enclosed in a cocoon of secreted material 
and within this case the young worm is developed. In the 
Polychaeta the larva undergoes its development in a free state. 
Segmentation in Annulata is complete and usually unequal, 
giving rise at the eight- celled stage to four micromeres and four 
macromeres (Fig. 105). The micromeres produce the ectoderm; 
directly or indirectly the macromeres give rise to the entoderm. 
Early in the cleavage "primitive mesoblasts" cells which pro- 
duce the mesodermal structures are separated from the macro- 
meres. A gastrula is formed either by invagination or by over- 



growth. In the earthworm (Oligochaeta) the blastopore of the 
gastrula forms the mouth of the adult worm. In Nereis (Poly- 
chaeta) the blastopore closes by growth, and the stomodaeum 
and proctodaeum arise by ectodermic in vagi nations which finally 
become continuous with the entoderm of the archenteron (Fig. 
106, D, of Polygordius). A ciliated, free-swimming larval stage 



FIG. 106. Diagrams of stages in the metamorphosis of Polygordius, a primitive annelid. Ecto- 
derm throughout is represented as nucleated without cell boundaries; the entoderm has the cell- 
boundaries shown, and the mesoderm is diagonally shaded. A, gastrula; B, same with blastopore 
closed; C and D represent formation of stomodaeum and proctodaeum from ectoderm; E, Tr echo- 
sphere stage showing formation of segments in the posterior portion; F, adult (sagittal): G, adult 
(transverse), a, archenteron; bp., blastopore; br, brain; c, coelom; d, dorsal; di, dissepiments; m, 
inesenteron; pr., proctodaeum; s.c., segmentation cavity; st, stomodaeum; v.n., ventral nerve chain; 
t, zone of formation of nerve segments. (After Fraipont.) 

Questions on the figures. Trace the behavior of ectoderm and entoderm in 
these figures and determine what structures each seems to give rise to. What is a 
trochosphere? Distinguish between somatic (body) and splanchnic mesoderm. 
(See 58.) 

ensues, known as a trochosphere (Fig. 1 06, E) . The trochosphere 
may be looked upon as representing the anterior or head end 
of the adult. The later metamorphosis to the adult condition 


involves the reduction in size of the enormous anterior region, 
and the growth of segments at the posterior end, and is char- 
acteristic of Polychaeta. The development of leeches is direct 
as in the Oligochaeta, or in some instances it might be more 
accurate to say that the process of metamorphosis is very much 
abbreviated, being completed by the time of hatching. 

274. In addition to sexual reproduction many worms, par- 
ticularly the aquatic forms, have the power of multiplying by 
fission. In some instances this may consist of a mere breaking 
in two, as was seen to be possible in the starfish, each part 
regenerating segments corresponding to those lost. In other 
cases (Nais, Dero, etc.) zones of rapidly forming segments are 
produced somewhere in the mid-region of the body, and from this 
zone a new head is developed for the posterior zooid and a new 
tail for the anterior zooid, which usually become structurally 
complete before the separation takes place (Fig. 101, z'). 

In some of the Polychaeta (as Autolytus) a distinct alternation 
of generation is found in which sexual and non-sexual individuals 
are of very different appearance. 

When artificially mutilated the earthworm, and some other 
types as well, may regenerate the lost portions. Groups of 
segments of one worm may be grafted upon another, complete 
healing taking place in such a way as to produce an apparently 
normal worm. Pieces may be grafted on the side of another 
worm in such a way as to produce a forked or otherwise 
abnormal result. 

275. Ecology. The leeches are aquatic in habit and 
many of them live on the blood of higher animals, a kind 
of temporary parasitism; the Polychaeta are marine, and the 
Oligochaeta are chiefly fresh water or terrestrial in habit. A 
few of the latter groups are parasitic. Of the aquatic worms 
some are actively free-swimming, others crawl in and out 
among the living and dead matter of the bottom, others bur- 
row in the sand, or secrete a tubular skeleton into which they 
may retire. Their chief economic importance is that they serve 
as food for fish and other food-animals. The earthworm, in 
forming its underground burrows, eats its way into the earth, 



swallowing the soil for the organic matter which it contains 
and passing it through its digestive tract. These castings 
may often be seen at the mouth of the burrows. Worms thus 
break up the soil, making it more porous and accessible to air, 
moisture, bacteria, and the rootlets of plants. Darwin esti- 
mates that three inches of the subsoil is thus brought to the 
surface in fifteen years through this agency. Doubtless earth- 
worms bring to the surface materials that renew the soil fertility, 
replacing substances taken from the surface soil by plants. 

276. Classification. 

Class I. ChcBtopoda (bristle-footed}. Annulata with metameres usually well 
marked both externally and internally; with setae developed from the epidermis. 
The co3lom is usually voluminous and is divided into chambers by transverse dis- 
sepiments. Closed blood-vascular system. Ventral nerve-chain ordinarily > with 
a distinct ganglion to each segment. 

FIG. 107. 


PIG. 107. Amphitrite ornata, from Yen-ill's "Invertebrate Animals of Vineyard Sound. ' 

Subclass I. Polychata (with numerous bristles}. Marine Chaetopoda with 
numerous setae typically borne on elevations of the body wall (parapodia). Head 
usually well differentiated, bearing eyes, antennae, cirri, etc. Branchiae or gills 
often present. Sexes separate; the reproductive organs simple, and repeated in 
many segments. A metamorphosis occurs; the larva is known as a trochosphere. 

Nereis, the "sand worm" of fishermen, is a type of this group. Autolytus is a 
small worm especially interesting because of its power of reproducing by fission. 
The bud which is freed from the hinder end of the worm differs from the parent 



stock in that it is sexual. Amphitrite is a beautiful worm which represents the 
attached or tube-forming types. As the result of their habits such forms tend to 
lose their segmentation and the appendages of the posterior part of the body. The 
gills and tentacles accumulate about the head. These and other types grow abun- 
dantly in the sand and mud of harbors, amid the vegetation of the bottom, and over 
exposed objects of all sorts from low-water mark to unknown depths. Their 
value in utilizing debris and the more minute organisms as food and thus becoming 
a link in the saving of these to serve as food for the higher organisms cannot be 
overestimated. (Figs. 107 and 108.) 

Subclass II. Oligochata (with few bristles'). These are Chaetopoda with no 
parapodia and comparatively few setae which usually occur in two or four clusters 
in each segment. They are mostly fresh water or terrestrial in habit. Fleshy 

FIG. 108. 

FIG. 108. Cirratulus grandis, from Verrill. 

Questions on Figs. 107 and 108. Are these Chaetopods? What are your 
evidences? What is the nature and function of the numerous outgrowths (bran- 
chial cirri)? In what respects are they differently arranged in the two types? 
Are these Oligochaeta or Polychasta? Your reasons? 

outgrowths, such as gills, are almost universally absent. The sexes are united in 
one individual and the accessory reproductive organs are very complicated. Ova- 
ries and testes limited to a small number of anterior segments; development 
direct. The head not so highly specialized as in the Polychaeta. 

The earthworms, of which there are numerous species, are the best known types 
of this subclass. The genera and species are distinguished chiefly by the position 
of the sexual organs. The aquatic Oligochaeta, which are much smaller, are found 
in practically all ponds and ditches where organic matter is decaying. The more 


common genera are Dero (Fig. 101), a beautiful, almost transparent worm which 
often forms a temporary tube for itself of particles cemented by its own secretion, 
and Tubifex, a longer worm which burrows in the mud at the bottom of streams; 
a portion of the body protrudes from the mud and waves gently back and forth 
in the water. They may occur so thickly that thousands may be seen in the space 
of a few feet. When their home is jarred they speedily withdraw from sight. A 
colony of Tubifex nearly always has associated with it one or more genera of 
smaller worms, as Dero or Nais, a species similar to Dero but with eyespots. Dero 
has an interesting respiratory apparatus at the posterior part of the body (Fig. 
101, p.), one of the few instances where Oligochaeta possess such organs. 

Class II. Discophora (bearing suckers] . Annulata in which there are secondary 
external rings which tend to obscure the metameres, inasmuch as the external 
and internal segmentations do not coincide. There are no bristles. The body 
cavity is much reduced by the growth of muscles and connective tissue. The 
remaining spaces contain blood and are in communication with the vascular 
system. Two sucking discs are present and are powerful organs of attachment. 
The anterior sucker embraces the mouth; the posterior is near the anus. Sexes 
are united in one individual; testes numerous, ovaries a single pair. Development 
direct. Marine, fresh water, terrestrial, or parasitic in habit. 

277. There are several other groups of "worms" of considerable interest to the 
zoologist, sometimes associated with the Annulata, which it seems necessary to pass 
by with mere mention. -(See also 236). 

Class: Archi-annelida ; a few primitive forms, as Polygordius (Fig. 106). 
Class: Sipunculoidea (Gephyrea). With traces of segmentation in the embryo, 
but not in the adult. 

Class: Chaetognatha (arrow worms). 

Some authors would place here also the Rotifers (see 235). 

278. Suggestive Studies for Library and Laboratory. 

1 . Look up the characteristics of the Archi-annelida, the Gephyrea, or Sagitta 
and report on their likenesses to the types studied. 

2. On what grounds might the rotifers be associated with the Annulata? 

3. Compare the "segments" in cestodes and Annulata. 

4. In the Chaetopoda which sets of organs pass through all the segments, which 
are repeated in essentially all, and which are limited to a few? 

5. Examine and report on the habits of the earthworm. (Study in its natural 
haunts or in box of moist earth in laboratory.) What are its haunts? Method 
and rate of burrowing? Does it avoid water? What is its food? How taken? 
Does the animal prefer light or darkness? 

6. If near the sea-shore select other forms and report in a similar way. 

7. Investigate parasitism among the Annulata. 

8. What is the economic value of the earthworm? Of other worms? 

9. Make a study from the text-books of the reproductive organs in any of the 
hermaphrodite Oligochaeta. 

10. In how many species of aquatic Oligochaeta do you find reproduction by 
fission ? In what particulars does the process seem to differ in the different species ? 

11. Outline the life-history of Autolytus, including the origin of the sexes. 



279. The Clam (My a) or Mussel (Anodonta, Unid). Either 
the marine or the fresh-water type will serve. The latter are 
to be found in almost all our streams and small lakes. They 
may be obtained with a long-handled rake from the shore or 
from a boat. They often occur partly buried in the sand or 
mud. If kept in water they may be transported to the labora- 
tory and placed in a tub of water with a few inches of sand at 
the bottom, where something of the physiology may be studied 
with profit. If they cannot be collected when needed for study, 
care should be taken to supply plenty of the preservative fluid 
in which they are kept. 

i . The Living Animal. What facts were observed, in col- 
lecting the material, concerning their haunts, their abundance 
in different localities, their range in size, etc. ? Are there any 
efforts at active feeding, as far as you have seen? Do you 
think all your specimens belong to the same species ? 

In nature or from the specimens in the tub, make out the 
following points: 

Has the animal power of voluntary motion ? If so, what of 
its rate, manner, and the position of the animal during motion ? 
Note the trail. Determine anterior and posterior ends, right 
and left sides, dorsal and ventral surfaces. To what extent 
can the soft parts protrude from the shell? Note briefly, for 
later reference, the position of all visible structures. How 
widely does the shell open during life ? With a pipette place a 
drop of some colored but harmless fluid (carmine in water) 
near the fringes of the posterior end, and note the results. 
Vary by introducing salt, sugar, and acid in the solution. 
Devise experiments to test whether the animal shows sensitive- 



ness to stimuli of various sorts: jars, contacts, currents in the 
water, light, warmth and cold. 

2. General Form. Renew your observations concerning the 
symmetry of the clam by careful examination of the animal. 
Determine and show in a sketch all the points distinguishing 
the anterior and posterior ends. Are the right and left halves 
symmetrical ? Use a pair of empty shells for completer study. 

The shell: what is the relation between the valves? How 
are they held together? Are they normally open or closed? 
Give your evidences? To what extent may the shell open 
without violence? How does the shell vary in thickness at 
various parts ? Contrast the interior and exterior as to finish 
and markings. Make note of everything found, with outline 
drawings, showing position. Locate the following regions and 
structures: hinge, umbo or beak (a prominence near the hinge), 
hinge ligament, hinge teeth, pallial line (a slight depression 
marking the attachment of the mantle muscle), muscle im- 
pressions, lines of growth. Review after studying soft parts. 

What layers are discoverable in a broken shell ? How do 
the inner, outer, and middle layers differ in thickness and 
appearance ? 

Do you find any differences worthy of note in different 
individuals ? 

3. Soft Parts. Remove one valve (say the left) by cutting 
the two muscles which hold the valves together. Leave all 
the soft parts in the right valve as little disturbed as possible. 
Make a sketch showing the relation of the body to the shell. 
If there is any difficulty in cutting the muscles, the clam may be 
made to open by immersing it in water heated to about 140 F. 
Identify : 

Left mantle flap. How related to the right ? to the shell ? 

Siphons : modifications of the posterior margins of the mantle. 
(These will be conspicuous or rudimentary in accordance with 
the species studied.) Number? 

Adductor muscles of the valves ; number and position. 

Mantle cavity. Separate the right and left mantle lobes 
along the ventral margin, except in the region of the siphon, 
and fold back the left. Where is it attached to the body? 


The ventral or incurrent siphon opens into the branchial chamber, 
the dorsal or eoccurrent into a smaller dorsal chamber, the cloacal. 
Verify and sketch. 

Gill plates or sheets; number and attachment. Are they 
symmetrical on the two sides? The eggs and developing em- 
bryos may be found in the outer gill cavity at favorable times. 
(A special study and report may be profitably made by some 
student on the structure of the gills as shown by a hand lens 
and the low power of the microscope. A bit of the living gill 
from a fresh specimen should be examined.) 

Abdomen, the soft, fleshy mass between the pairs of gills, 
which terminates in a more solid part, 

Foot : position and form ? 

Mouth and labial palps; at the anterior end and just below 
the adductor muscle. How many palps ? 

(It is to be remembered that all the structures examined thus 
far are external organs. The body wall has not been penetrated 
at all. If it is the plan to study the anatomy more closely, 
the following are the chief sets of organs deserving attention.) 

4. Other systems of organs. 

Circulatory system. Open the pericardial cavity, just beneath the hinge and 
a little posterior thereto, find the 

Heart: auricles and ventricle. In a fresh preparation the contractions of 

the heart may be observed. 
Vessels: one passes in each direction, but they are not easily seen without 

The intestine passes through the ventricle without open communication 

with it. 
Excretory organ. 

Organ of Bojanus, or kidney, lies just beneath the floor of the pericardial 

cavity, one part on either side. 

Each portion is a dark-colored sac, with an abundant btood supply. 
Nervous system. (Traced best in hardened preparations.) 

Visceral ganglia. Look between the gills in the posterior portion of the 
body, beneath the posterior adductor muscle, and in the floor of the 
cloacal cavity. Number, and closeness of connection? By careful dis- 
section determine what nerves leave them. Trace a pair of these forward 
to the 

Cerebral ganglia, on either side the mouth. Note the connections between 
the cerebral ganglia. Trace from these ganglia the connectives which 
pass ventrally to the 

Pedal ganglia in the muscular foot, close to its union with the abdomen. 
Make a clear diagram showing the relations of these three pairs of ganglia. 


Digestive system. 

Begin with the intestine at the heart. Trace posteriorly to the anus. 
What is its relation to the posterior adductor muscle? Pass a bristle 
into the intestine anteriorly and use it to guide the dissection. Trace 
the intestine through the abdominal mass, and plot its course. Identify 
the stomach, the esophagus, and the mouth. The liver is a brownish 
or greenish mass surrounding the stomach. 
Much of the visceral mass through which the intestine coils is made up of 

the large reproductive glands which open into the mantle cavity. 
5. Cross Sections. A series of cross sections may be made by the teacher, 
numbered, and used with profit as demonstrations. For such sections the soft 
parts of the animal should be hardened for 24 hours in I per cent, chromic acid; 
then one day each in 70 per cent, and 90 per cent, alcohol. Keep in 95 per cent, 
alcohol for a few weeks. Cut one-fourth to one-third inch thick and number so 
as to be able to locate position of section. Float in dish of alcohol and identify 
the parts found. Make sketches of sections passing (i) through the stomach, (2) 
through the heart, and (3) through the middle of the posterior adductor muscle. 
In the absence of these the student should be encouraged to make a diagram of an 
imaginary cross section through the middle of the body. Include the shell. 

286. The Oyster. One or two students should be asked to prepare a report on 
the structure of the oyster and present to the class an account of the chief points 
of contrast between the oyster and the clam. The adult oyster is fixed by one 
of its valves. Is it the same one in all specimens? 

281. The Pond Snail (Limn&d). 

1. The Living Animal. Observe, both in its natural home 
and in glass vessel containing water, in the laboratory. 

To what does the animal adhere in the water ? Must it have 
solid support? Can it swim? What is its method of locomo- 
tion ? What does it eat, and how ? Can you determine whether 
it uses the air in breathing or gets its oxygen from the water ? 
Proof? How is the gliding motion effected? Watch, with a 
lens, one crawling along the side of the glass vessel. Record 
signs of sensitiveness to stimuli, by experiments of your own 

2. General Form. Is there any sign of bilateral symmetry? 
In what parts ? How are anterior and posterior distinguished ? 
Relation of the shell to the animal ? Identify : 

Head: tentacles, number and position; eyes, number and 

Foot, the muscular expansion: shape, changes in form and 



Respiratory orifice, position. Under what circumstances 

3. Shell (secure empty ones). Make sketches of the shell 
and identify the structures referred to in the following terms: 
apex, aperture, lip, spire, whorl, suture, columella. (See 
Fig. no). 

How would you describe the direction of the spiral? How 
many whorls? Have the young and old the same number? 
Can you detect lines of growth ? 

4. Soft Parts. These may be removed by dropping the animal suddenly into 
hot water, and then gradually twisting the soft portion from the shell. It will 
scarcely repay the trouble to do more than re-identify the following parts: mouth, 
respiratory orifice, mantle and mantle chamber, and collar (a portion of the 
mantle). The spiral is occupied by the digestive tract, its glands, the reproduc- 
tive bodies, etc. 

5. Development. Examine the stems of plants and the sides of the vessel 
in which snails have been kept for some days for gelatinous capsules of eggs. They 
are almost transparent and the eggs may be easily located. What seems to be 
the value of the gelatine? Number and arrangement of the eggs? What is the 
shape of the eggs? Get the earliest stages possible, and watch day by day at 
short intervals, or compare capsules of different ages. If care is taken, some idea 
of the early segmentation stages may be obtained. Lookfortheblastula: are the 
cells of the same size? Do you find a gastrula? What are the first signs you find 
of differentiation of parts? Look for different stages of the later development. 
It will not be profitable to try to follow the changes in detail. 

282. A very valuable laboratory exercise may be had by comparing large 
numbers of shells of a single species, found under varying conditions. Compare 
as to shape, markings, etc., and see whether there are individuals connecting your 
extreme groups. The land snail (Helix, Fig. 121) is favorable for such study. 
Helix may be substituted for Limnaa. 

283. The Squid. The teacher should at least have a few specimens of the 
Squid, from which the pupils may be required to get some idea of the general form. 
Drawings should be made, showing all external features. 

Note particularly: 

Head: tentacles, number, comparative length; suckers on the inner surface, 

arrangement of suckers. 
Eyes: number, size, position. 

Olfactory organs opening beneath folds of skin behind the eyes. 

Body: general shape. It is surrounded by the 
Mantle; note the fin expansions at the posterior end. Where are the 

attachments of the mantle to the body? 
Siphon; how related to the mantle cavity? 


What are your conclusions as to the symmetry and the normal position of the 
squid ? Do you find anything from your external examination which would lead 
you to class it with the clam and the snail? 


284. The group Mollusca (mollis, soft) embraces from 
30,000 to 60,000 living species among which there are very great 
differences, as illustrated by forms as unlike as slugs, snails, 
oysters, clams, devil-fishes, and squids. With the exception of 
a few, they are sluggish animals and largely aquatic or fre- 
quenters of moist places. Some are well protected by external 
armor and others are perfectly naked. The typical adult 
mollusk is clearly marked off from both the radiate animals 
such as echinoderms and the segmented animals such as the 
arthropods and the Annulata, but some of the simpler types of 
mollusks, and the larvae of certain of them which undergo a 
metamorphosis, strongly suggest that they may be related to 
some of the worms. 

285. General Characters. 

1. Body soft, unsegmented, bilaterally symmetrical and 
without segmented appendages. 

2. The organ of locomotion is a muscular thickening of the 
body, called the foot, which is variously modified. 

3. A thickened dorsal fold of the body wall, called the 
mantle, is usually present. This encloses a space, external to 
the body, known as the respiratory chamber. 

4. The mantle secretes in many cases a calcareous shell, at 
first single and symmetrical, but usually becoming either spiral 
or separated into a right and left valve. 

5. The central nervous system usually consists of three sets 
of ganglia: (i) the cerebral or "brain," above the mouth, (2) 
the pedal, in the foot and connected with the cerebral by nerves, 
and (3) the visceral, also connected with the brain by a pair of 
nerves (Fig. 38). 

6. Except in the headless forms (Acalephs) a tooth-bearing 
ribbon, the odontophore, is found in the mouth. 

286. General Survey. The more commonly known forms 
are easily recognizable by the hard calcareous shell which 


protects the soft unsegmented body within. The shell may be 
in two sub-equal valves, right and left, or may be in one piece, 
in which case it is usually coiled or spiral (Fig. no). The 
bivalved types are able to open and close the shell after the 
manner of a box, and the soft parts are further capable of 
protrusion from the partly opened shell. This latter power 
is much more pronounced in the univalved types (e.g., snail). 
The fundamental bilateral symmetry is obscured in the more 
sluggish forms, but is very decided in such active animals as 
the squid and some of the bivalves. 

FlG. 109. Shell of a Bivalve Mollusk, inner surface, ad.a., depression showing the attach- 
ment of the anterior adductor muscle; ad. p., posterior adductor muscle; h, hinge with teeth; /, 
attachments of the ligaments; p, pallial line, marking the attachment of the mantle muscles; 5, 
the pallial sinus, marking the attachment of the retractor muscles of the siphon; u, umbo or beak. 

Questions on the figure. Which is the dorsal and which the ventral portion 
of the shell? Is this the right or left valve? What is the effect of the contraction 
of the adductor muscles? What is the value of the teeth on the hinge? To what 
point in the shell of the snail does the umbo correspond? 

One of the most interesting points of difference among the 
members of the group is the degree of development of the 
"head." In the bivalves (lamellibranchs) there is a very 
slight cephalization, or collection of special organs about the 
anterior end. For this reason they are often called Acalephs. 
In the gasteropods (snails, etc.) and cephalopods (squid), on 


the other hand, the head is well developed both as to special 
mouth parts and nervous organs. 

The forms with shells are somewhat more limited in size 
than the cephalopods, which furnish the largest representatives 
of the phylum, measuring in extreme cases 20 to 40 feet in the 
reach of the arms. 

The calcareous shell insures abundant fossil remains, repre- 
sentatives being found in various geologic formations from the 
beginning of the Palaeozoic era to the present. 

FIG. no. 



FIG. no. Helix. A, an empty shell in section from apex to base. B t the relation of the 
animal to the shell when extended, a, apex of shell; an., anus; ap. t aperture of shell; c, columella 
or axis of shell; e, eyestalk; /, foot; I, lip of shell; m, edge of mantle, which secretes the shell; r.o., 
respiratory aperture; 5, suture, between the whorls; t, tentacles. 

Questions on the figures. What suggestions of bilateral symmetry are 
shown by the snail? Where does growth occur in the shell? What are the func- 
tions of the tentacles? What is the function of the edge of the mantle called the 
"collar" (m)? 

287. Integument (skin). This consists of a layer of epi- 
dermal cells, covering a deeper dermal layer derived from the 
mesoderm. The former is made up chiefly of the supporting 
cells and the simple glandular cells which secrete mucus, or 
lime, or pigment. In many forms a large portion of the epi- 
thelium in the mantle cavity (as the inner surface of the mantle 
and the covering of the gills in Lamellibranchs) is ciliate. The 


dermis is a complex of connective tissue, muscle fibres, pigment 
cells, etc. The mantle is a fold of the skin which is very char- 
acteristic of Mollusca. It grows out from the dorsal wall of 
the body and encloses a space known as the mantle cavity. 
It is usually important in respiration, and contains the shell- 

288. Shells are formed in all the classes of Mollusca, al- 
though naked forms occur in several of them. The shell is a 
true secretion or excretion, deposited by the epithelial layer of 
the mantle. It consists of three layers: (a) a thin external 
layer of organic material known as conchiolin, (b) the prismatic 
layer, embracing the greater thickness of the shell and made up 
of prisms of carbonate of lime cemented by conchiolin, and (c) 
the nacreous or pearly layer over the inner surface. The edge 
of the mantle secretes the first and second layers, and they 
usually show lines of growth parallel with the edge of the mantle; 
the pearly layer is deposited by the whole surface of the mantle. 
The point of attachment of the muscles presents a depression in 
this layer because the deposit has been interrupted (see pallial 
line and muscle scars, Fig. 109; and in shell of clam). 

In some Cephalopods there is an internal skeleton in part 
secreted by the mantle (cuttle bone), and in part formed of 
cartilage (the brain case). 

289. The muscular system is made up of unstriped muscle 
fibres, which usually occur in more or less prominent bands or 
muscles. These may be classified as follows: (i) shell or skeletal 
muscles, which embrace (a) adductors, those which draw the 
valves together (lamellibranchs), (b) retractors, which withdraw 
the whole or special portions of the animal into the shell (lamel- 
libranchs and gasteropods) , (c) protractors, or extensors, which 
enable the animal partly to extend itself; (2) pallial (mantle) 
muscles, best developed in cephalopods; (3) the foot, which is a 
mass of muscle and is one of the most characteristic of the 
molluscan organs; and (4) minor muscles controlling the radula 
or tongue, the other mouth parts, and the like. 

Locomotion in the group is accomplished chiefly by the foot, 
in its various modifications, or by rhythmic opening and shut- 


ting of the valves. The squid has a fin-like extension of the 
integument which is an efficient organ of forward motion. The 
siphon of the same animal is regarded as a modification of a part 
of the foot. The tentacles about the mouth are also looked 
upon as arising from the anterior part of the foot, hence the 
name Cephalopoda, which means "head-footed." 

2 go. Digestive Organs. Mouth and anus both occur, and 
are usually widely separated. In the coiled forms (as the snail), 
however, by the looping of the digestive tract they are brought 
close together. In all except the group of headless mollusks 
(lamellibranchs) the mouth is supplied with a radula, or tooth- 
bearing tongue. This lies in the floor of the mouth and, as it is 
worn away in front, is renewed from behind in the radula sac 
(Fig. in). It rasps small particles from solids and conveys 

FIG. in. Diagram of mouth of snail, showing the lingual ribbon (radula). br, brain; c, buccal 
cavity; co., ccelom; g, gullet; j, jaw, against which the radula works; m, mouth; r. t radula; r.s., 
radula sac, in which the radula is renewed as it is worn away in front. 

Questions on the figure. What parts go to make up the "odontophore?" 
How do the parts act in biting? 

them backward into the esophagus. In the gasteropods there 
is a plate in the upper jaw against which this organ works. In 
the cephalopods beak-like jaws occur suited to their carnivorous 
habit. The mouth is followed by a gullet, which may com- 
municate at once with the stomach (lamellibranchs), or may 
expand into a crop (gasteropods and cephalopods) . The stomach 
is well marked and opens into the intestine which is usually long 
enough to make one or more coils in the body mass. It may 
open externally (gasteropods) or in the mantle chamber (cepha- 
lopods and lamellibranchs). Salivary glands pour their secre- 



tion into the mouth cavity or into the gullet, and the so-called 
liver connects with the stomach or intestine. 

291. Respiration. The oxygen may be derived from the 
water (lamellibranchs, cephalopods, and some gasteropods) 
or from the air (pulmonate gasteropods). In the latter a 
pulmonary chamber is formed by the mantle. Blood is richly 
supplied to the walls of this sac and is there aerated after the 
manner of lungs. In the water-breathing forms the gills are 

FIG. 112. 


FIG. 112. Diagram showing the heart and general course of the circulation in the Lamelli- 
branchs. Only a short section is shown, a, auricle (right), with slit to ventricle; b, the body 
(region of spaces, lacunae, capillaries); g, the region of the gills, with capillaries; k, kidneys, with 
their capillaries; m, the mantle and capillaries; v, the ventricle from which arteries pass forward and 
backward; v.c., "vena cava," in which the blood collects on returning from the tissues of the body. 

Questions on the figure. Follow by the arrows and letters the general course of 
the blood flow. How many sets of capillaries are passed by the blood which goes 
to the mantle? By that which goes to the system, before returning to the heart? 
What changes take place in the blood in the capillaries of the various regions? 

variously constructed. Lamellibranchs possess a pair of "gill- 
plates" hanging in the mantle cavity on either side the body. 
These are made up of an immense number of ciliated tubular 
filaments which intercommunicate in a complicated lattice- 
work. To the naked eye they appear as thin sheets with stria- 

2 5 


tions passing from the dorsal to the ventral margin (see dissec- 
tion of clam). The walls of the gills contain blood vessels, and 
the water, assisted by the action of the cilia, circulates over and 
through the gills. In the cephalopods and aquatic gasteropods 
the gills occur as tufts of filaments, which may or may not be 
covered by the mantle. In addition to these special organs the 
mantle and the soft body surface assist in respiration. (For 
figures of the gill structure in the clam see Parker and Has well's 
Text-book of Zoology, Vol. I, Fig. 529.) 

292. Circulation. There is usually a well-developed circu- 
lation of the blood, but a portion of it occurs through irregular 
spaces devoid of proper walls. The organs consist of a contrac- 
tile heart usually with undivided ventricle and a single auricle 



PIG. 113. Diagram showing the general course of the circulation in mollusks. Compare with 
Fig. 112, which shows the organs more nearly in their relative position. 

Question on the figure. Why does the blood which passes to the mantle not 
need to pass the gills before returning to the heart? 

(gasteropods), or one pair of auricles (lamellibranchs, squid), 
or two pairs (Nautilus). Definite arteries pass both forward 
and backward from the ventricle. The blood passes from the 
ventricle to the tissues of the body, whence it gathers into 
venous spaces and passes into the kidneys and the gills by way of 
a principal vein. From the gills it finds its way to the auricles. 
In lamellibranchs the blood which goes from the ventricle to 
the mantle is aerated and returns directly to the auricle. In 
some Cephalopods there are branchial hearts near the gills to 


assist the return of the blood to the heart. The accompanying 
diagrams (Figs. 112, 113) will help the student follow the main 
facts of the circulation. In lamellibranchs the ventricle often 
surrounds the intestine. The corpuscles are colorless and 
amoeboid. The plasma, however, quite commonly contains a 
bluish pigment (hcemocyanin) which assists respiration in much 
the same way as the haemoglobin of the vertebrates. 

293. Excretory Organs. In mollusks the excretory organs 
consist, when reduced to the simplest terms, of one or more 
nephridia which communicate interiorly with the pericardium 
or principal ccelomic space, and with the exterior by way of a 
tubular ureter. The kidney portion of the tube is much 
modified, has glandular walls and is well supplied with blood 
vessels. It lies in the immediate region of the pericardial 
chamber in most cases. 

294. Nervous System. The nervous system of mollusks 
is usually made up of at least three pairs of ganglia: (a) the 
"brain" or cerebral ganglia dorsal to the mouth and varying 
in size according to the degree of development of the head; 
(6) connected with the brain by a pair of connectives are the 
pedal ganglia lying ventral to the mouth and innervating the 
foot; (c) the pi euro- visceral ganglia variously situated in the 
different groups and connected with the brain or both with the 
brain and the pedal ganglia. From it nerves pass to the mantle, 
and to the posterior organs. In gasteropods and cephalopods 
all these ganglia are much closer together and are collected 
about the mouth. Still other ganglia are often associated 
with them. The student should notice how this collection of 
nervous matter accompanies the development of "head" 
organs in the better developed members of the phylum. 

295. The Organs of Special Sense. As usual, scattered 
sensory cells are situated in the exposed epithelial surfaces. 
These .give rise to a diffuse sensitiveness to tactile and chemi- 
cal stimuli. The edges of the mantle and the tentacles are 
especially sensitive. Patches of sensory cells osphradia 
are often found near the bases of the gills, which probably 

2 5 2 


have a value in testing the character of the water flowing over 
them. Still other patches occur about the lips. Otocysts (see 
m), or statocysts, occur in all the groups. Eyes are usually 
found and are of various degrees of complexity. They are 
simplest in the lamellibranchs (Fig. 43), and when found at all 
in this group may occur in great numbers along the mantle. 

FIG. 114. 

FIG. 114. Diagram of a dissection of the reproductive organs of a snail, a.g., albumen gland; 
c.d., common or hermaphrodite duct; e.g., hermaphrodite gland; d.s., dart sac;/, flagellum; g, genital 
aperture; m.g., mucous glands; o, oviduct; p, penis; r.s., receptaculum seminis; v.d., vas deferens. 
The slit from the genital aperture into the oviduct and penis shows the openings of the dart sac, 
mucous glands, and the receptaculum seminis. (After Pelseneer.) 

Questions on the figure. By a careful study of the figure and the text, de- 
termine the functions of the various parts of the system. Does self-fertilization 
occur in a form like this? Evidences. 

In the gasteropods the eyes are borne on the ends of tentacles 
and are frequently destroyed by accidents. The animals have 
the power of regenerating the tentacle, eye and all. This 
manifestly is a very useful adaptation. The eyes of cephalopods 
are the most perfect single eyes found among the invertebrates. 


Though originating in a different way, these are strikingly like 
the vertebrate eye. 

The forms with shells are not so responsive to stimuli as 
free types such as cephalopods, but even the snails, and in less 
degree the clams, are sensitive to light, contact, gravity, 
chemical states of the air and water that would in us arouse the 
sense of taste or smell. The most characteristic reaction of the 
lower forms is to withdraw into the shell or to close it. Gravity 
and light and odor also direct their motions. 

296. Library Reference. Make a report on the position and general structure 
of the eyes in gasteropods, cephalopods and lamellibranchs. 

297. Reproduction and the Genital Organs. Reproduction 
is always sexual. In some of the lamellibranchs (e.g., oyster) 
and many of the simpler gasteropods, including the land snails, 
the individuals are hermaphrodite. The sexes are separate in 
the cephalopods and in most of the lamellibranchs and gastero- 
pods. The organs are more complicated among the her- 
maphrodite gasteropods than elsewhere in the group (see diagram, 
reproductive organs of snail, Fig. 114). The sexual glands are 
usually situated in the visceral mass among the coils of the 
intestine. The ducts ordinarily open into the mantle cavity, 
where fertilization may occur. The eggs after fertilization are 
often, either singly or in masses, surrounded by a gelatinous 
secretion (as in the snail) which serves as a protection from 
drouth and as a means of attachment. In lamellibranchs the 
young are not infrequently retained in the mantle or respiratory 
chamber until partly developed. 

298. Development. Segmentation is total (lamellibranchs 
and gasteropods) or partial anddiscoidal (dibranch cephalopods). 
It is usually unequal in the lamellibranchs and gasteropods, 
but in some of the latter it is equal during the first two divisions, 
producing four equal blastomeres. Each of these divides into 
a large and a small cell macromere and micromere. Still other 
micromeres are formed at the expense of the macromeres, 
and these by continued division form a cap of ectodermal cells 
(Fig. 115). From the macromeres arise ultimately the ento- 
derm and mesoderm. The gastrula may be formed either by 



in vagi nation of the large cells or by the overgrowth of the 
micromeres, depending on the size of the segmentation cavity 
and of the entodermal cells. In the cephalopods, owing to the 
large supply of food substance in the ovum, cleavage is confined 
to a small disc at the active pole. From this point where the 
embryo is destined to be developed, a sheet of cells gradually 
extends itself by growth around the yolk. Thus a yolk-sac is 
formed by means of which the food is used in the further de- 
velopment of the embryo. By the time the embryo is hatched 
the yolk is exhausted. Although the yolk does not segment we 
see that it serves its purpose in the development of the embryo. 

FIG. 115. 



FlG. 115. Diagram of early segmentation stages in a Gasteropod. A, 2-celled stage; B, 4 
celled; C, 8-celled; D t later stage, in section, ect., ectoderm cells (micromeres); ent., entoderm cells, 
macromeres; mes., mesoblasts, early put aside, before gastrulation to form the mesoderm; mi., 
micromeres; ma., macromeres. 

Questions on the figures. What causes are assigned for the differece in the 
size of the cells in the 8-celled stage? In what other ways is mesoderm formed 
in the metazoa? Which cells seem to divide more rapidly, the micromeres or the 
macromeres? Compare with Annelid, Fig. 105. 

The later development is typically indirect, i.e., with a metamor- 
phosis, though many (as the cephalopods) develop directly into 
the adult form. A larval stage (trocko sphere) occurs, suggesting 
the larvae of the Polychaeta. This is followed by another stage 
(veliger) which is more characteristic of the Mollusks. 

299. Library Exercises. Students may well supplement the text by making 
short reports on the following topics: the early segmentation of lamellibranchs 
and gasteropods; of the cephalopods; the veliger of mollusks; the formation of the 


organs in cephalopods; development in the clam or mussel. Illustrations should 
be found in the advanced text-books and presented to the class. 

300. Ecology. The bivalves are sedentary or sluggish in 
their manner of life; the motion of most of the gasteropods 
is slow and difficult. In conformity with their limited powers 
of locomotion, they are scavengers, feeding on the debris and 
the small animals and plants brought to them by the water 
currents (oysters, mussels, etc.), or are largely herbivorous 
(many snails). A very few are parasitic. The cephalopods 
are much more active and are carnivorous. For the most part 
the sluggish forms are well protected by the shells, neverthe- 
less they furnish food for many diverse sorts of animals. Some 
of their enemies are internal parasites, others bore through the 
shells and thus gain access to vital parts. 

The animal within may thwart this attack of its enemies by 
the continued secretion of mother-of-pearl on the inner sur- 
face at the threatened point. Some animals crush the shells, 
or swallow the mollusks, shell and all. Starfishes, as we have 
seen, are especially troublesome to the oyster beds. 

Many of the bivalves are capable of still further protection 
because of their elongated siphons which enable them to 
burrow deeply in the mud or sand, the food being carried in 
through the siphons by the water currents (Fig. 116). Several 
species of marine bivalves have the power of boring into wood 
or even stone. This serves as a protection to them, but often 
results in the complete destruction of piles and other structures 
placed in the ocean by man. 

Many of the mollusks seem more or less gregarious, as is 
illustrated by beds of clams and oysters, the schools of squid, 

Notwithstanding the low organization and sluggishness of 
a large portion of the branch Mollusca, we are compelled to 
consider that it has been a very successful group in that it 
has held its place with practically equal numbers through the 
geological ages, and has succeeded in adapting itself to the 
changes of those ages. Of no less interest is the additional 
fact that there is scarcely a nook into which they have not 
penetrated, except where continuous drouth prevails. On the 



other hand, it is among the more active types the cephalo- 
pods that the ancient geological forms have least success- 
fully adapted themselves to modern conditions. The cephalo- 
pods appear much less numerous and varied now than in earlier 
geological time, although the types which have persisted are 
the most perfect and active ones that have appeared. 

301, Relation to Human Interests. This is the first phylum 
that we have studied that furnishes any food, worthy of mention, 

FIG. 1 1 6. 

PIG. 116. 

Mya arenaria, a burrowing clam. The siphon is represented as fully extended, 
quickly retracted when the animal is disturbed. (After Kingsley.) 

This is 

Questions on the figure. What is the function of the much elongated siphons? 
Which is the anterior end of the animal? Which the dorsal side? What would 
seem to be the chief function of the foot in this case? 

to man. It is estimated that $65,000,000 annually are derived 
from our oyster fisheries. There are several species of clams 


and mussels that are used for food. Less frequently snails and 
squid are eaten. 

Pearl buttons, knife handles, and other objects are made 
from the "mother-of-pearl" shells of mollusks. Pearls, one of 
the most prized ornaments, are produced by a number of species, 
notably fresh-water clams and the marine pearl oyster. 

Squid produce cuttle bone and sepia, and are used for codfish 

The oyster industry and the clam-shell industry have become 
so extensive that they have ceased to be merely "fisheries." 
Promiscuous fishing, if profitable, always results in threatening 
the whole industry. The next step is to limit fishing or devise 
means of encouraging the growth of the organisms, or both. 
This has been done for the oyster and is beginning for the clam. 
The steps involved are to favor the young by artificial means, to 
give them suitable places to settle, to furnish them with food, 
and to protect them from enemies and unfavorable environment. 
This is a form of "farming," rather than fishing. Coupled with 
legal limitations upon fishing such measures make for steadiness 
of supply, and should be encouraged in every way possible. 

302. Classification. The following are the principal classes: 
Class I. Pelecypoda (hatchet-footed) or Lamellibranchiata; (Mussels, Oysters, 
etc.). Lamellibranchs are mollusks 'in which the fundamental bilateral sym- 

FIG. 117. 

FIG. 117. Ensis amcricanus, the razor clam. From Verrill, after Gould. 

Questions on the figure. Where is the hinge, the umbo, etc. ? Trace the lines 
of growth and compare with other figures of bivalves. 

metry is shown in the right and left valves of the shell secreted by a bilobed 
mantle, and in some of the internal organs. There may be one or two adductor 
muscles. The head is undeveloped. The ventral body region is differentiated 
into a muscular foot, shaped like a plow-share. The gills are in sheets (see 
291) usually two on either side, and are suspended in the mantle cavity. Paired 
labial palps occur about the otherwise unspecialized mouth. The three pairs of 
ganglia the cerebro-pleural, the pedal, and the visceral, are usually well sepa- 



rated. The heart consists of two auricles and one ventricle surrounded by a 
pericardial space, which is a portion of the body cavity and communicates with 
the exterior by a pair of nephridial tubes. The reproductive organs are simple; 
the sexes are ordinarily separate. Development by a metamorphosis. 

[The primary subdivisions of the group may be made on the basis either of the 
gill structure, the adductor muscles, or the presence or absence of the siphon.] 

Order I. Isomya: Two adductor muscles which are essentially equal.. 

(a) Siphon well developed, retractile; pallial line (Fig. 109) with a sinus. 
Here occurs Mya arenaria, the common clam of the Atlantic coast. Great heaps 
of shells of this clam show that it was much used by the Indian tribes as food. 
In New England the clam fisheries are of very considerable importance. Mya 
burrows in the mud, using its long siphon to keep it in connection with the water 
from which it gets its food. Of somewhat similar habits is the razor-shell clam. 

FIG. i i 8. 

FIG. 118. Mytilus edulis, a Mussel. From Binney's Gould. 

Questions on the figure. Identify the umbo. What are your evidences that 
it is the umbo? Compare the lines on the shell with those in Fig. 117. What 
is the significance of the specific name (edulis} ? What are the habits of the species ? 

(Fig. 117). The "borer" (Pholas) and the "ship-worm" (Teredo} belong to this 
group and possess the power of boring into wood or stone and are thus of much 
damage to submerged structures in waters where they abound. 

(b) Siphon usually present but not highly developed; no pallial sinus. In this 
group are embraced the more abundant fresh-water mussels (Unio, Anodonta, 
Cyclas), and the cockles (Cardium} of the ocean. The Unionidae are very widely 
distributed and very common in our own fresh waters. They are not much used 
for food at present, though the Indians used them, probably in times of scarcity of 
other food. Their shells are widely employed in the making of buttons, knife 
handles and the like, and pearls of value are not of infrequent occurrence. These 
are merely the mother-of-pearl, which ordinarily lines the shell, secreted about 



some infected or irritated point on the epidermis of the mantle. Great quantities 
of these pearls are sometimes found in the graves of the mound builders. 

In some of the fresh-water clams there is an interesting temporary parasitism 
during the early development. When the young, called glochidia, escape from the 
gills of the mother, they must become attached to the skin or gills of a fish. 
They may become deeply imbedded and be nourished in the tissues of the host for 
several weeks. During this time they pass through their metamorphosis and 
develop the adult organs. This habit brings about a better distribution of the 
clams than would otherwise happen, since it has been shown that adult clams do 
not voluntarily migrate widely. 

FIG. 119. 

PIG. 119. Pecten irradians, a Scallop. From Binney's Gould. 

Questions on the figure. Is this an external or internal view of the shell? 
Where is the umbo? What is peculiar about the hinge in this case? What is the 
significance of the lines nearly concentric with the margin? Of the radial lines? 

Order 2. Heteromya: Two adductor muscles, the anterior much reduced; 
siphon usually wanting. Here are included the horse-mussel (Modiola) and 
Mytilus, edible mussels which occur in clusters just below low tide mark; also the 
pearl-oyster, from which the best pearls are taken. The last mentioned form is 
not found on our own coasts. 

Order 3. Monomya: One adductor muscle (posterior); no siphon. The genus 
Ostrea (oyster) and the genus Pecten (scallop) are the most interesting and impor- 
tant representatives of this order. The species of Ostrea differ much in size in 
different regions. The largest living species is a Japanese form which is known to 
reach a length of two to three feet. The oyster is hermaphrodite. The young 
after a short free life, become attached by one of the valves. The oyster constitutes 
a larger element in the food supply of man than any other invertebrate. The 
scallops are not attached, and swim by a rapid opening and closing of their valves. 



Class II. Gasteropoda (belly-footed; Snails, Slugs, Whelks, and Periwinkles). 
Gasteropods are mollusks with unsymmetrical, univalved, usually spiral shells 
(occasionally lacking the shell altogether). The head and foot ordinarily preserve 
the bilateral symmetry, but the other organs lose their symmetry both from the 
spiral form of the shell and from a twisting which many of the forms undergo by 
which the nervous system and certain other visceral organs lose their original right 
and left relations. The head region is well developed, having tentacles, eyes, and 
a mouth with a tooth-bearing radula. Gills in the mantle cavity two, one, or 
none; in the air-breathing forms there may be merely a pulmonary sac. The sexes 
are separate (Streptoneura} or united in one individual (land snails). Development 
is mostly indirect. 

PIG. 120. 

FIG. 121. 

PIG. 120. Acmcea lestudinalis (Limpet), from Binney's Gould. Upper figure lateral view; lower 

figure, dorsal view. 

Questions on the figure. How do the Limpets differ from the majority of the 
snails? What is the appropriateness of the specific name (testudinalis)? 

FIG. I2i. Helix albolabris, a pulmonate Gasteropod. From Binney's Gould. 

Questions on the figure. What is the significance of Helix? Of albolabris? 
Identify the parts of the shell. Is it a right or left spiral? What do you mean by 
your answer? 

Subclass I. Streptoneura. Gasteropods in which the nerve loop made by the 
visceral commissures, is twisted in development into the form of the figure 8 ; the 
other visceral organs are twisted so that right and left are interchanged. Only one 
pair of tentacles on the head. Sexes separate. Gills usually in front of the 

One of the common representatives of this group is Littorina, the common 
periwinkle of the seashore. Many_pther types of almost infinite variety of form, 
size, and color inhabit the ocean, their shells often being washed ashore by the 
waves; such are the cowries, the whelks, the cone-shells, etc. Here belong the 
uncoiled Limpet and the slightly coiled Crepidula or boat-shell. 

Subclass II. Euthyneura (Land Snails and many naked Mollusks). Gastero- 



pods in which the nerve loop is not twisted. The head usually bears two pairs of 
tentacles. The sexes are united in the same individual. The most important 
of these are the Pulmonata or air breathing Gasteropods, some of which are 
terrestrial and others aquatic. Of the terrestrial snails the genus Helix (Fig. 121) 
is the most widely distributed and interesting. Its variability is such that between 

FIG. 122. 

FIG. 122. Limax flavus, a Slug. From Binney's Gould. 

Questions on the figure. How do the slugs differ from the other Gasteropods? 
In what external respects do they appear similar to them? Compare all the 
figures of slugs you may be able to find. 

FIG. 123. 


FIG. 123. Pearly Nautilus. From Nicholson, e, eye; h, hood, a muscular portion of the foot 
which protects the softer parts; s, siphon; se, septa, separating the successive chambers of the shell; 
sp, siphuncle; t, tentacles. 

Questions on the figure. How does this shell compare with those of the 
Gasteropods? What is considered to be the homology of the tentacles or arms in 
Cephalopods? What is the siphuncle? What is the character of the eye in 

three and four thousand species have been described. Limax (Fig. 122) is a pul- 
monate form in which the shell is practically wanting. It is especially destructive 
to certain types of plants as it is a voracious vegetable feeder. The aquatic pul- 
monates are represented by the "pond-snail" (Limncea), and by Planorbis, a snail 
whose coils are in one plane, presenting a helix rather than a spiral. 


Class III. Cephalopoda (head-footed; Squid, Devil-fish}. The cephalopods 
are bilaterally symmetrical mollusks with a well-developed head in which the front 
part of the foot surrounds the well-armed mouth as a series of lobes or tentacles. 
The head protrudes permanently from the mantle cavity, leaving the mantle 
surrounding the posterior part of the body. The posterior lobe of the foot forms 
a siphon, communicating with the mantle cavity. Into this cavity the nephridia, 
the anus, and the reproductive glands open, and in it the gills lie. The shell may 
be present and external (Nautilus}, internal and slightly developed (Squid), or 
wanting (Octopus). An internal cartilaginous skeleton protects the brain. The 
ccelom is well developed. The ganglia of the nervous system are massed in the 
head region. The sexes are separate and the development direct. The Cephalo- 
poda are to be looked upon as the most highly developed of the Mollusca. They 
are little in evidence now, however, as compared with earlier times. 

FIG. 124. 

FIG. 124. The Devil-fish (Octopus). From Cooke, after Merculiano. A, at rest; B. swimming. 
a, arms, with suckers on the inner aspect; , eye; s, siphon or funnel. 

Questions on the figure. Which is the anterior end of the animal? What 
is the position of the mouth? What is the function of the siphon? Of what 
structure is it a part? 

Subclass I. Tetrabranchiata. Cephalopoda in which the front segment of the 
foot is divided into lobes bearing numerous tentacles, without suckers. Shells 
external and chambered (and in Nautilus, the only living genus, coiled). Two 
pairs of auricles; two pairs of gills; two pairs of nephridia. 

This group is important for its extinct rather than for its living representatives. 
The pearly or chambered nautilus (Fig. 123) found in the Pacific and Indian 
Oceans, is the only important living species. The Nautilus appears to be the only 
remaining representative of the once numerous coiled forms and more remotely 
still of the Orthoceratites, the rulers of the Palaeozoic seas (see Geology). 

Subclass II. Dibranchiata. Cephalopods in which a circlet of 8 to 10 arms 
surround the mouth. These bear sucking discs. Shell internal and rudimentary 



or absent. One pair of gills, one pair of nephridia, and one pair of auricles. An 
ink gland is present. 

Order i, Decapoda, embraces the cuttle-fish and squid. 

Order 2, Octopoda, embraces the devil-fishes (Fig. 124) and the paper nautilus 
(Fig. 125). 

FIG. 125. 

FIG. 125. The Paper Nautilus (Argonauta argo). From Cooke, after Lacaze-Duthiers. , eye; 
ra, mouth;/, siphon; sh, shell; /, tentacles. 

Questions on the figure. In what way does the siphon serve in locomotion? 
In which direction will the animal move by means of the siphon? How does the 
shell of Argonauta differ from that of Nautilus? 

303. Supplementary Studies for Library, Laboratory, and 

1. Compare the clam, snail, and squid with regard to the 
following particulars, putting the results in a tabular form : 

(a) Degree of development of the head. 

(b) Shell, development and method of using, in each. 

(c) Mantle; extent, form and modifications: mantle cavity. 

(d) Foot; parts, differentiation, and uses. 

(e) Respiration ; how accomplished ? 

(/) Sense organs ; position, character, and degree of develop- 

(g) Locomotion ; how effected ? 
(ti) Protection; special devices. 

2. Can you find any indication among the mollusks of a 
relation between the degree of development of the sense organs 


and the activity shown by the animals ? Between the external 
protective structures and activity ? 

3. When did the various classes of mollusks make their 
appearance in the history of the earth ? (See geology.) What 
can you say of their importance in the formation of the sedi- 
mentary rock ? 

4. In what ways may the fresh- water forms have arisen from 
the original salt-water mollusks ? 

5. What members of the group of mollusks are economically 
important ? Indicate in what way and to what extent ? 

6. A report on all the mollusks to be found in your com- 
munity; their distribution, habits, etc. 

7. Formation of pearls, and pearl fisheries. 

8. The industries connected with the use of the shells of the 

9. The life history of the fresh- water clam. 

10. The life history of the oyster. 



304. This group is one especially favorable for the pupils 
to study in the field, in the haunts of the animals themselves. 
For this reason, wherever it is at all possible, the members of 
the class should be required to collect a portion of the material 
needed in the laboratory and to submit a report on such items 
of physiology and ecology as may be expedient in each case. 
The teacher will find suggestions in the supplementary exercises. 

305. The Fresh -water Crayfish (Cambarus). This form 
should be studied when living specimens may be had. They 
may be kept for considerable time in a tub containing an inch 
of water. This should be changed every day or two. Feed 
on small pieces of meat or earthworms. 

I. Physiology. 

1. Locomotion: walking; how effected? swimming: how 
effected? Under what circumstances does the animal swim? 
Do all the walking legs act together in walking? How many 
are at rest at once ? In what order do they act ? 

2. Movements of the parts of the body; segments and ap- 
pendages. Describe the manner and purpose of these motions 
as far as you can determine. In what different ways do the 
various groups of appendages seem to act? Watch them, one 
pair at a time. 

3 . Feeding : kind of food used and manner of securing it. 

4. Respiration: by means of air or water? How can you 
be sure? Does the animal do anything to renew the water, 
by producing currents? Place a minute amount of carmine 
or indigo solution at the side of the animal at the union of 
the abdomen and thorax; at the front of the thorax. What is 
the difference ? What does it signify ? 

5. Evidences of sensitiveness: Devise experiments of your 
own to prove whether the crayfish is stimulated by light; 
contacts; the presence of food in any other way than by sight; 



sound. Are all parts of the body equally sensitive to touch? 
To chemical stimuli? Make use of a 5 per cent, solution of 
acetic acid; strong salt solution; strong beef extract. What 
inferences may be drawn from your experiments? Place a 
crayfish on his back. Describe his actions. What is the nature 
of the stimulus that arouses this reaction? Evidences? 

II. Symmetry. (This group is especially favorable for this 

Notice what is implied in bilateral or tri-axial symmetry. 

Antero-posterior axis : are the poles alike or different ? 

Make a memorandum of all the chief differences. 

Dorso- ventral axis (as above). 

Right-left axis. Record the points of agreement. 
Contrast the axes in length. Can you think of any causes 

for the differences and likenesses discovered above? 

Any advantages arising therefrom ? 

III. General Form. Distinguish two regions; Cephalo- 
thorax and abdomen. 

Cephalo-thorax ; carapace. 
Head; rostrum, eyes, mouth. 
Cervical groove. 

Abdomen; how many segments do you find? What 
seems to determine a segment ? 

Applying these criteria can you find any indications of 
segmentation in the cephalo- thorax ? (Make a tem- 
porary estimate of the number of segments in the 

Make two sketches showing a dorsal and a ventral view 
of the crayfish, preserving proportions. 

Examine one of the abdominal segments (the third or 
fourth from the front). How is it joined to those next 
it ? Follow the line of union. Note tergum, or dorsal 
piece; sternum, or ventral piece; pleura, the lateral pro- 
jections from the tergum. 

Make a sketch of an imaginary cross section showing the 
relation of these parts to each other, together with the 
attachment of the appendages. 


IV. Appendages. Group them into regions and notice 
the general differences and the differences in the uses 
to which they are put. If time will allow, study the 
appendages in detail as follows; 

1. Begin with the third or fourth abdominal appendage (swimmerets) making 
the drawings necessary to show the parts: 

Protopodite, or basal portion. 

Exopodite, or external branch. 

Endopodite or internal (median) branch. 

Compare all the abdominal segments with that studied. Do different indi- 
viduals agree in the appearance of the first and second abdominal segments? 
Compare the last segment (telson) with those studied. How many segments in 
the abdomen? Of what parts is the tail fin made up? 

2. Cephalo-thoracic Appendages. Remove with scissors the over-arching 
portion of the carapace and expose the base of the appendages. Find the third 
maxilliped (the first appendage in front of 'the large claw). Remove by inserting a 
scalpel and bringing away all that belongs to it. 

Identify : 

Protopodite, of two segments (coxopodite, next the body, and basipodite). 

Endopodite and exopodite. How many pieces in each? 

Epipodite, lying in the gill-chamber. Are there any special outgrowths on 

Study and compare with this the large claw, and the other walking append- 
ages. Which part is wanting in these, exopodite or endopodite? Reasons 
for your view? How do these five appendages differ from each other. 

Examine and compare the appendages in front of the third maxilliped in 

Second maxilliped. 
First maxilliped. 
Second maxilla. 
First maxilla. 




Head parts. 

What are the evidences that the antennae and antennules are homologous 
with those already described ? 

Revise your estimate of the number of segments. 

Compare the appendages again by groups, and notice the chief points of 
difference, and the ends served by these differences. Make a careful 
sketch of each type of appendage, labeling all parts. (The names of the 
segments of the larger appendages may be found in fuller texts, if desired.) 

By studying the living specimen, determine just the work done by each 
of the types of appendages. 

Note the position of the eyes. Examine with a low power. 

In the basal joint of each antenna is the opening of the "green gland." 

In the basal joint of the antennules are the otocysts. (Statocysts.) 


V. Gills. Examine the gill-chamber, and the position of 
the gills therein. Which appendages bear gills? How many 
tufts to each appendage? How do they differ as to the place 
of their attachment ? How many in all ? Make a table showing 
these factors. 

VI. Internal Organs. Remove with much care the carapace from the thorax 
and the terga from the abdominal segments, by the use of scissors and forceps. 
Sketch the organs in their natural position. What organs are visible? 

Examine in some detail the following sets of organs, 
(a) The circulatory organs. 

Heart: just beneath the carapace, in a membranous chamber (pericardial 

Apertures, by which the blood enters the heart from the sinus; dorsal, 

ventral, lateral. How many do you find? 
Arteries; anterior, posterior. 

(The teacher should have, if possible, a permanent preparation of the lobster 
in which the arterial circulation has been injected with a colored mass.) 

VII. Reproductive Organs. These will be found immediately beneath the 
pericardial sac as whitish (male), or yellowish to brown (female) lobed structures. 
Depending on the sex there will be found 

Ovaries or testes. Form, position, and number of lobes? 
Oviducts or vasa deferentia. Course, length and outlets? 
Can you determine the sex of your specimen? Note especially the external 
differences between males and females. 

VIII. Digestive Organs. 

Liver, a pair of yellow, brown or reddish masses anterior to the reproductive 

Stomach; sketch in position. Dissect later, if time allows, and note the 
anterior and posterior chambers, and the grinding apparatus. 

How is the mouth situated relatively to the stomach ? 

Follow the intestine backward from the stomach to the 
Anus: position of? 

Make a sketch of the entire tract from a side view, showing in what part of the 
carapace each portion is. 

IX. Muscular System. How is the abdomen flexed and how extended? How 
-do the muscle fibres run ? To what attached ? Are they plain or striate ? How are 

the appendages worked? Split open the segments of the chela. 

X. Nervous System. (If the time is short a demonstration may be made by 
the teacher, preferably with a lobster.) 

Remove the intestine, and cut carefully through the muscles in the median 
line until the white ventral nerve-chain is uncovered. Follow it forward to the 
head, cutting away the covering plates in the thorax. 

How many swellings (ganglia) in the abdominal region? Relation to the 
segments? Where do nerves arise? 

Thoracic ganglia: number and relation to appendages? 

Sub-esophageal ganglion; circum-esophageal connective. 

Supra-esophageal ganglion (brain). 


Do any nerves arise from the brain? Where distributed? Draw from above. 

Make a diagram of the relation of the digestive tract and nervous system from 
the side view. 

XI. Excretory Organs. The green glands occur at the base of the head, in front 
of the mouth. The outlets are at the base of the antennae. 

Make a diagrammatic view of an imaginary cross section of the thorax in the 
region of the heart, and one of the abdomen, showing the position of the internal 
organs. Also a diagram of a sagittal section showing relations of all the parts 

306. Sow-bug (Oniscus, a terrestrial form; or Asellus, a fresh-water Isopod). 
General Form. Use hand lens and identify: 

Head: size, form, number of segments. 
Eyes: number and position. 
Antennules and antennae. 
Mouth-parts: number and structure. 
Thorax: number of segments. What variation therein? 
Abdomen. How many segments ? Proofs? 

Remove carefully, mount in water on a slide, and examine with low power, 

a thoracic appendage. Sketch. 
Examine similarly the other thoracic legs and the mouth parts, and make 

drawings of them arranged in the order of their occurrence. 
Examine similarly the abdominal appendages. What is their number? 

Compare the appendages from the different regions, as to structure, form 

and probable function. Are there any gills? Where situated? 
What! s the number of segments in the body, if there is a pair of appendages 

to each segment? 

Comparisons. Compare the sow-bug with the crayfish as to the degree of 
union of head and thorax; the number of segments represented in each of the 
three regions; the degree of differentiation among the appendages; the mode of 
respiration; the presence of both exopodite and endopodite; as to food, and habits. 
Physiology and Ecology. A study and report of the animal's habitat, food 
habits, methods of motion, sensitiveness to light and to other classes of stimuli, 
should be made. How does Oniscus behave when touched? Do you find any 
trace of eggs or young? What facts are to be noted concerning them? 

307. Cyclops. These minute fresh-water Crustacea may be found in almost 
any pool where aquatic plants are found. They flourish well in aquaria. Select 
several of the larger specimens with egg masses, one on each side the abdomen. 
Examine in a watch glass with a little water to which a drop of chloroform has 
been added. Use low power of microscope. 

General Form. (Study both dorsal and ventral surface.) 
Cephalo-thorax : 

Anterior portion covered with the carapace. How many segments 
represented? How can we know that this is not merely the head, or 
the whole cephalo- thorax? 
Posterior portion (four free thoracic segments). How is it known that 

these are not abdominal segments? 
Abdomen: form; number and character of the segments. 


Appendages. Antennae, oral, thoracic, abdominal. Number and general 
character of each. Where and how are the egg-cases attached? 

Sense Organs. 

Eye-spot (appearing as one, from which the name Cyclops). 
Do you find any organs which suggest a tactile function? 

Report on all available points of physiology: as food habits; methods of locomo- 
tion; reaction to light and other stimuli. 

308. Comparisons. Collect all the minute fresh-water Crustacea possible and 
compare them with Cyclops. Learn to identify them by their manner of moving 
in the vessels of water. Daphnia is especially favorable for microscopic study on 
account of its semi-transparency. 

309. Spider (any common species large enough for study). 
General Form. Study the relations of head, thorax, and 

abdomen. Are there any antennae ? Oral appendages ? Num- 
ber and character of the thoracic appendages ? Does the abdo- 
men show any signs of segmentation ? Has it any appendages ? 
Make sketches showing a ventral and a lateral view. 
Special Organs. 

Examine the head with a hand lens and locate the eyes. 
Note more particularly the types of appendage found, 
and the degree of differentiation. Find the openings 
to the air-sacs on the ventral surface of the abdomen. 
Locate the spinning glands. Number ? 
Activities and Habits. How do the legs act in walking? 
At what joints are they flexed at various parts of the step? 
Do all the legs on one side act in unison? Observe the spin- 
ning action. Does the spider ever produce the threads except 
when weaving a web? Describe. Determine if possible the 
kind of web formed by the species studied. Or find as many 
types of nest or web as possible and learn to recognize the 
spiders producing them. How does the spider travel on its 
web? Where do spiders place their webs? Place a living fly 
or other insect in a newly constructed web and record the 
actions of the spider. Can you devise means to prove whether 
the spider possesses the sense of smell ? 

310. The Grasshopper. Several species of the locusts may 
be found in almost every locality. They are especially abundant 
in the early autumn. For laboratory study select the largest 
species found in sufficient abundance. In connection with the 


securing of material the students should -make observations on 
the following points : 

1. Habits. Where and under what circumstances found? 
At what time of the year does this species occur in greatest 
abundance ? Under what circumstances are they most active ? 

2. Methods of Locomotion. How many methods seem 
available? Degree of efficiency of each? Under what circum- 
stances is each used? What distance can be attained at one 
effort? Continue the study later in more limited quarters, as 
in the room and under a bell-glass. Compare the work of the 
various legs. Are the wings used at all in jumping ? 

3. Protective Features. Coloring; to what extent do you 
find this of protective value ? Reasons. Does the animal show 
a distinct instinct for hiding ? Compare all available species in 
these regards. 

4. Do they produce definite sounds? Under what circum- 
stances ? Do you find any hint as to the method of their pro- 
duction ? 

5. Do you detect any movements which suggest respiration? 
Rate ? (Find spiracles in the thorax and abdomen.) 

6. Supply hungry animals with fresh leaves and study the 
feeding process. Dip the leaves in various solutions and notice 
whether it makes any difference to the grasshopper. 

If alcoholic material is used for the following morphological 
studies it should not be allowed to become dry. If dipped in 
a mixture of glycerine and 50 per cent, alcohol, specimens will 
not dry so rapidly. 

The sexes differ, particularly in the abdominal region. Pro- 
cure specimens thus differing by examining a number of individuals, 
and keep both kinds for comparison. Sketch dorsal, lateral and 
ventral views of each (especially in the regions of difference). 
External Features. Study the following points : 
i. The regions of the body. 
Head; thorax; abdomen. 

What are the signs of segmentation in these three regions ? 
Where is it most clearly indicated? Where are the 
segments most similar? 


2. Abdomen. 

Number of segments (differs in male and female). 
Dorsal and ventral plates. Are they equally developed in 

all segments. 
Appendages : which segments possess them ? 

Ovipositors (paired outgrowths found only in the female). 

Anal cerci (examine the male). Are they found in the 

female? To what segments do these appendages 

belong ? 

Spiracles (small openings at the side of the segments); 

number and distribution ? 

Tympanic membrane, at the sides of the first abdominal 

3. Thorax: studying from the front, backward, find: 
Prothorax; mesothorax; metathorax. Note the form, 

size, and structure of each part. 
Appendages of each segment. 

Legs: number; relative size; parts (beginning at the 
body), coxa, trochanter, femur, tibia, tarsus. Com- 
pare the legs. 

Wings (can these be regarded as homologous with the 
jointed appendages?): number; position, at rest 
and in motion; characteristics; position of veins. 
Compare the two pairs in all essential particulars. 
Are there any spiracles in the thorax ? Position ? 

4. Head (is there any "neck" ?). The head is covered with 

chitinous plates ; identify : 

Epicranium, the dorsal plate. 

Clypeus, the anterior plate. 

Genae, the lateral plates. 

Labrum or upper lip, anterior to the clypeus. 
Examine the compound eyes, their form and relation to the 

plates. Slice off a portion of the surface and study the 

surface with a low-power objective. 

Ocelli or simple eyes. How many and in what position? 
Mouth aperture; position. 
Appendages of the head : 

Antennae, near the eyes; number. 


Mouth-parts. These are complicated and demand careful study, if satis- 
factorily made out. Remove the labrum and proceed from before, 

Mandibles; a pair of horny tooth-bearing jaws. Draw in position. 
Maxilla; a pair of compound jointed organs made up of three portions, 

the lacinia (nearest the median line), the galea, and the maxillary 

palpus (external). 
Labium or lower lip; this also bears a palpus. The labium may be 

studied and removed before the study of the maxillae. 

How many segments seem to be represented in the head? Evidences. 
Internal structure. 

Select large female specimens preferably. Clip the wings close to the 

body, and pin the specimen to a board, dorsal surface upward. 
With a pair of fine, sharp pointed scissors make a longitudinal incision 

into the integument of the abdomen near each side. Gradually and 

carefully remove the skin between the cuts from behind forward. 

Look for the heart, a long, thin-walled, mid-dorsal vessel, which if 

not removed with the skin may be seen just beneath it. Unroof both 

the abdomen and thorax. Note the exposed muscles of the thorax, 

also the whitish fat bodies next the body wall. 

1. Trachea. If the specimen is freshly killed the tracheae will be filled with 
air and will show as white, glistening tubes. Seek their connection with 
the spiracles, and note their ramification and unions in the body. Isolate 
some of the smaller branches and study under the microscope. Prove 
that they are tubes. How kept open? 

2. Reproductive Organs. (These are much more difficult in the male.) 
Ovaries: In how many masses? Notice the subdivisions of the ovaries. 

These contain the eggs and communicate by means of an oviduct 
with the outside. In what segment? Examine an ovum with the 
microscope. Mash, and notice the yolk. 

3. How do the muscles of the thoracic region differ from those in the 
abdominal? Are the fibres plain or striate? 

4. Digestive Tube. 

Dissect forward into the head, and press the other organs aside so that 
the course of the tract may be revealed. It consists of the following 
parts, which should be identified: 

Esophagus; size and course. 

Crop (an enlargement of the esophagus); shape, position. 
Stomach; character and extent. (At the anterior end is a ring of 
tubular appendages which are glandular in function, the gastric 
caeca; at the posterior end it is limited by a circle of fine tubes 
Malpighian tubules which are excretory.) 
Intestine; length, course and size. 
Anal opening; position. 

Make drawing of digestive tract from side view, showing in outline the 
body regions and the relation of the portions of the tract to these. 

5. Nervous System. (Remove the alimentary tube and examine the floor 

of the abdominal cavity.) 


Ventral nerve cord. Is it single or double? 
Ganglia; number, and relation to the segments. 
Nerves; origin and distribution. 
Trace forward into the thorax and head. 
Ganglia; number and position. How connected? Is there any 

portion dorsal to the digestive tract (brain) ? 
Compare the nervous system of the grasshopper part by part with that 

of the crayfish. 

Make diagrammatic representations of imaginary cross sections through 
thorax and abdomen showing the relation of the different structures; 
likewise of a sagittal section. 

The cricket or cockroach may be substituted for or compared with the grass- 

311. Supplementary Laboratory and Field Work. It is 

perhaps inexpedient for students in an elementary course to 
make dissections of other representatives of the Arthropoda, 
but the common air-breathing forms are so numerous, so varied, 
and have such interesting habits and histories, that they may 
profitably be used as a basis for individual field and laboratory 
work and to serve in the comparison of homologous organs in 
related groups. The following outlines are suggestive rather 
than exhaustive. 

I. Make a table in which can be displayed the points of con- 
trast between the crayfish, the grasshopper, the "June bug" 
(or other beetle), the squash-bug or the cicada ("locust"), 
the butterfly, the wasp, the fly, the spider, and the centipede, in 
the following particulars: 

1. The regions of the body; head, thorax, and abdomen; 
their degree of development and separateness. 

2. The number of segments in the body, and the clearness 
with which they are manifest. 

3. The degree of diversity shown by the segments in the 
various parts of the body. 

4. The points of structure which the various segments 
possess in common. 

II. Make a similar table, for the same animals, of the 
appendages : 

i. Head appendages: antennae; mouth parts, number and 


2. Thoracic appendages: 

Legs: number, position, kinds, joints, special adaptations 

to special work. 
Wings: number, size, position, structure, principal veins. 

Compare the first and second pairs as to size, structure 

and function. 

3. Abdominal appendages: number, structure, function. 

III. Make a table comparing these and other available forms 
as to their eyes, simple and compound. 

IV. Find, if possible, another form embodying the same 
general features found in each of the above-mentioned animals. 

V. Compare these (or other forms which may be selected) 
from the point of view of their habits. Introduce all discovered 
correlation between structure and function. 

1. Haunts and place of living. If peculiarly local, can you 
find any reasons ? 

2. Locomotion: methods, and the efficiency of. 

3 . Feeding : material used, and the method of obtaining it. 

4. Respiration: organs and their location; any special points 
as to their use. 

5. Special senses: physiological evidences; morphological 

6. The laying of eggs and provision for the young. 

(The library may be used profitably to supplement field 
work in this section.) 

VI. Study by observation the homes, temporary or perma- 
nent, their mode of construction and uses, in the following: 
Spiders (as many species as possible), the paper- wasp, the 
mud-wasp, the honey-bee, the bumble-bee, ants, flies, etc. 

VII. Development or life history. Studies may be made in 
natural conditions in many cases by periodic observations. 
When this is not possible, animals may often be reared in con- 
finement by supplying the appropriate conditions. This is a 
very attractive line of investigation and one in which real 
contributions to knowledge may be made. The following are 
some of the matters to be kept in mind. 

i . Is there a metamorphosis or is development direct ? (See 
text, 330.) 


2. Eggs: where deposited ? In what numbers ? Relation to 
future food supply ? 

3. Larval condition ("grub," "maggot," "caterpillar"). 
Form, segmentation, general external characters, special 
organs; habits, food, coloration, enemies. 

4. Pupa (a resting and transforming stage) ; how pro- 
tected? What is the origin and character of the pro- 
tecting structure? What changes are undergone at this 
stage ? 

5. Adult. How do the larva and pupa compare with it in 
segments, appendages, etc. 

The following forms may be studied and compared as to life 
history : 

Squash-bug; all stages are to be found on squash, gourd, 
cucumber and similar vines. 

Potato beetle; equally abundant on the Irish potato plant 
in some years. 

Bees and wasps; to be found in their nests. 

"Blue-bottle" fly. This form may be studied in confine- 
ment. (Expose raw meat for the eggs to be laid. 
Place on a chip in a dish of moist earth or sand. Invert 
a tumbler or bell- jar over it and watch the growth and 
changes, as decomposition proceeds.) 

Mosquito. The Iarva3 may be found in stagnant pools, 
and watched in confinement. 

Cabbage butterfly. This form may be studied in the 
garden, or in the laboratory by placing the cabbage 
leaves with the larvae under a bell- jar and keeping the 
conditions favorable. 

Some large caterpillar should be studied with some de- 
gree of care in order to ascertain the general arrange- 
ment of organs. 

Spider. If a mass of spiders' eggs can be found, the stu- 
dent by watching may be able to determine whether 
the development is direct or indirect. 

Silk-worm. The various stages may be studied in con- 
VIII. Group the Arthropoda known to you, in three classes: 


(i) those hurtful to man's interests, (2) those beneficial thereto, 
and (3) the harmless. State the grounds of your classifica- 
tion of each form. In what stage of its metamorphosis is 
each species hurtful or helpful. Extend your own knowledge 
by inquiry, by observation, and by reading. 


312. The group of Arthropoda (jointed-legs) embraces more 
than one-half the species in the animal kingdom, and is cor- 
respondingly rich in individuals. This is the same as saying 
that they are remarkably variable and adaptable to various 
conditions of life. The segmented, bilaterally symmetrical body 
and the arrangement of the nervous system are the most 
important points of similarity with the Annulata. The general 
resemblance is more striking in some of the lower forms (Peri- 
patus), and in the larval stages of those which undergo a meta- 
morphosis. The subdivisions of the phylum (if it can be con- 
sidered a single phylum) are quite diverse and their relation- 
ships uncertain. There are many parasitic and otherwise 
degenerate forms which make the problem of classification 
more difficult. 

313. General Characters. 

1. Elongated, bilaterally symmetrical body. 

2. Segmented; somites heteronomous, and typically grouped 
into three regions: (i) head, (2) thorax, (3) abdomen. 

3. An outer skeleton, of a secreted chitinous substance. 

4. Each somite has typically a pair of jointed appendages 
(whence the name arthropod). 

5. Central nervous system similar to that of Annulata: (i) 
brain, (2) a nerve ring around the esophagus connecting the 
brain with (3) a ventral, ladder-like chain of ganglia. 

~6. Heart, dorsal to the digestive trace. 

7. Ccelom Tepresented largely by secondary blood spaces 
connecting with the circulatory system. 

314. General Survey. The symmetry of the Arthropods 
is very pronounced, except in the case of fixed, parasitic, or 
otherwise degenerate forms (as barnacles, Sdcculina, etc.). 


The group presents great diversities expressive of a high de- 
gree of adaptation to almost every conceivable mode of life. 
They may be parasitic internal or external, symbiotic, social, 
or independent; they may be aquatic, terrestrial, burrowing or 
aerial; they use all sorts of food; they bore, crawl, swim, jump, 
fly, or may be fixed. In geographical distribution they are 
practically cosmopolitan. The group is one of the most suc- 
cessful in the animal series. None of the living species, however, 
attains a very great size. The king-crab and the lobster 
are among the largest. Many are microscopic. 

315. The Segments. There is a great deal of diversity 
among the segments of the body as to size, shape, the form 
and use of their appendages, as well as in their contained 
structures. In the more primitive forms (Peripatus and the 
centipedes) and in the larval condition, the somites are well 
marked externally, but in the majority of forms there is more 
or less fusion of contiguous somites in certain body regions. 
A variable number of segments at the anterior end, which 
bear the mouth parts and sense organs, form the head. Be- 
hind these a group of three (insects), or more (crayfish), may 
fuse to form the thorax. These two regions, head and 
thorax, are often fused into one piece the cephalotkoraoo. 
The abdominal segments are usually unfused. 

316. The Appendages also differ much in form in the 
various representatives and on different segments of the same 
individual. This diversity of structure is closely connected 
with the variety of work to be done, and is an excellent illus- 
tration of the differentiation which accompanies "division of 
labor.*' They are unquestionably serially homologous organs 
as is shown by their similarity of origin and by the funda- 
mental likeness of structure, clearly to be seen in the primi- 
tive forms. They may be said to consist typically of a basal 
portion with one or more segments, supporting two jointed 
branches, a median and an external. Appendages may be 
entirely wanting (as in the abdominal segments of insects) ; 
and yet these may appear in a rudimentary form in the early 
stages of the embryo, only to disappear later. Where the 
metamerism is obscured by fusion, the number of appendages 


may be the only indication we have of the number of segments; 
but as we have seen, the appendages themselves are some- 
times aborted in regions where they are no longer needed. 
So it is not always possible to determine how ma^Sy segments 
are really represented in an animal. 

General groups of appendages are as follows: (i) preoral, 
mostly sensory, as antennae; (2) oral, biting and sucking 
structures, mandible and maxillae; (3) thoracic, chiefly walk- 
ing appendages; (4) abdominal, variously modified (as swim- 
merets, gills, etc.), or wanting. The wings are not to be re- 
garded as homologous with the jointed appendages. They 
originate as expansions of the integument of the body, sup- 
ported by numerous tubular ribs or "veins" containing branches 
of the blood vessels, tracheae, and nerves. Wings, when 
present, comprise one (flies) or two pairs (bees). Often the 
anterior pair is hardened and serves merely as a protection 
for the second pair. Either pair, more often the second, may 
be aborted. 

317 Ccelom. The development of the arthropods shows that the spaces in 
the body are not truly coelomic as a rule, but are, so to speak, much enlarged 
blood spaces containing the corpuscle-bearing fluid. The pericardial sinus is 
one of these. Such a body cavity is known as a hamoccele. 

318. Integumentary Structures. The arthropod skin has 
an epidermal layer of cells which secretes the chitinous cuticle 
constituting the external skeleton. The chitin may be mixed 
with salts of lime. Beneath the epidermis is a layer of con- 
nective tissue, the dermis, containing nerves and blood 
vessels. Still within these are the longitudinal muscles of the 
body wall. This chitinous covering serves for protection 
and support of the soft parts, and for the attachment of the 
muscles of locomotion. When the secreted shell becomes thick 
and hard, further growth is necessarily more difficult. This diffi- 
culty is usually overcome by moulting, in which process the old 
cuticle is separated from the epidermis, rupturing along some 
line of weakness, and allowing the escape of the animal. This 
moulting extends not only to the minutest of the external 
organs, but to the stomodaeum and prqctodaeum as well. A 
new cuticle begins to be secreted at once, but this "soft-shelled" 


condition is one of great danger and helplessness to the animal. 
The process besides is exhausting, and to these facts we may 
attribute, in part at least, the small size of most arthropods. 
The cuticul$ is laid down very thinly at the joints. Thus is 
secured the flexibility necessary in locomotion. 

319. Muscles and Locomotion. The muscles are well 
developed, and many of the arthropods are very powerful in 
proportion to their size. The circular muscles characteristic 
of the Annulata are lacking in the arthropods. The chief body 
muscles are the longitudinal which cause the flexion and ex- 
tension of the segments. There are in addition the muscles by 
which the appendages are moved. These fibres are of the cross- 
striate type. Less massive groups of fibres are found in the 
walls of portions of the digestive tract. 

The paired, segmented appendages are primarily organs of 
locomotion. They are variously used for crawling, walking, 
jumping, and swimming. Many have wings, and the quality 
of the nervous and muscular control of these can be realized from 
the fact that the bumble bee can make some 240 wing strokes 
per second and the fly 330 per second. Some forms, as the 
crayfish, have the power of using segmented portions of the 
body in a powerful backward swimming stroke. Some types 
have the power of skating round on the surface of water. 
Locomotion in the phylum is remarkably varied and remarkably 
well done. 

320. Digestion and the Digestive Organs. The alimentary 
tube is typically rather complex. It commences with a mouth 
which is usually supplied with three or more pairs of external 
appendages assisting in the capture, transfer, and preparation 
of food. This is followed by an esophagus either with or 
without a crop ; a stomach frequently consisting of several 
regions: viz. (a) a proventriculus or gizzard in which food is 
further broken to pieces physically, and (fr) a ventriculus or 
stomach proper in which chemical digestion occurs ; an intestine 
which is not always clearly marked off from the stomach; and a 
posterior opening, the anus. The development of the gut shows 
both stomodseum and protodaeum (see 93). The former is 
often very extensive, embracing even the pro ventriculus, in 


.which chitinous grinding plates may occur (crayfish, cockroach). 
The "salivary" glands when present open into the esophagus 
or mouth cavity. Into the mesenteron important digestive 
glands may open, as the pyloric caeca (many insects), or liver 
(crayfish and spiders). The Malpighian tubules (see dissec- 
tion of the grasshopper) associated with the hind gut are 
believed to be excretory rather than digestive in function. 
The digestive system as a whole is strikingly correlated with 
the character of food used, which is exceedingly diversified 
in this phylum. This results in the fact that the details of 
structure are scarcely the same in any two species. Indeed the 
digestive process and structures are very different in the larval 
and adult stages of the same individual, as in the caterpillar 
that feeds on green leaves and the adult that lives on nectar or 
fruit juices. This can be appreciated only by extended ob- 
servation and comparison. The student is urged to com- 
pare such figures of these organs as he may be able to find in 
the reference texts at his command. 

321. Respiration. In some instances the Arthropoda ob- 
tain their oxygen directly from the air, in others from the 
water. In the latter the exchange is effected through the body 
wall, or by gills. These are essentially thin outgrowths of 
the body wall, with the cuticula much reduced or absent, 
into which the blood passes (e.g., the majority of Crustacea). 
In the former it takes place wholly by means of tubular air- 
passages or trachea (insects), or these may be supplemented by 
thinned folds of the body wall book-lungs (spiders). By 
these devices the oxygen of the air or water and the blood 
are brought into intimate relations. In the water-breathing 
forms the gills are either the modified appendages (Limulus, 
Asellus}, or specialized outgrowths from them or from the 
general body wall (crayfish; Fig. 126, g). The gills vary 
widely in number and position, but are found especially in 
connection with the thoracic and abdominal appendages. 
The air-breathing forms possess a system of interbranching 
tubes which may open to the exterior by a pair of stigmata or 
pores in each somite. These tubes unite, branch and penetrate 
to every portion of the body. The air is carried to the blood 



rather than the blood to the air. The tubes are lined by a 
thin layer of cuticle, and are kept open by a spiral thread of 
the same material reinforcing the wall. The book-lungs when 
present lie within a sac which opens to the exterior by a stigma 
or pore, and consist of a series of pleatings, within which the 
blood circulates and between which the air circulates. 

The larvae, especially of air breathers, are often developed 
in conditions very different from those chosen by the adults. 

FIG. 126. 

PlG. 126. Diagrammatic cross section of Crayfish in the thoracic region, to show relation 
of circulation and respiration, a, appendage; c, carapace; e.f., flap of carapace overhanging the 
gills; d, digestive tube; g, gill; h, heart; /, liver; m, body muscles; m', muscles of the appendages; 
n.c., nerve cord; p.s., pericardial sinus; r, reproductive glands; st, sternal artery; r.a., ventral artery; 
v.s., ventral blood sinus in which the nerve cord lies. Modified, from Lang. 

Questions on the figure. What is the relation of the gills to the body wall? 
Follow the course of the circulation by the arrows. It leaves the heart by definite 
arteries and comes back by less definite blood sinuses. What is the function of 
the valves? What gain is there in the position of the ventral nerve cord in the 
blood sinus? 

This fact may make necessary very important changes in the 
respiratory organs in the metamorphosis. Some forms are 
even water breathers in the larval stage and air breathers in 
the adult (dragon-flies). 


322. Circulation. The heart or pulsating organ when 
present is dorsal and may be much elongated, with an enlarge- 
ment in each somite. It lies in a membrane-bounded cavity 
called the pericardial sinus (Fig. 126, ps.), which is a part of 
the haemoccele or secondary body cavity (317). The blood 
comes to the pericardial cavity and enters the heart by means 
of slit-like openings, with valves. Definite arterial vessels leave 
the heart and pass to capillary regions and thereupon open 
into irregular spaces in the tissues without definite walls (lacuna) . 
The hsemoccele is in reality an enlarged lacuna. In insects 
there is an anterior artery only; in spiders and Crustacea, pos- 
terior and lateral arteries also occur. The return of the blood 
takes place through the irregular haemoccele spaces (lacunae). 
These become more definite in form as they near the pericardial 
chamber, or as they approach the gills in aquatic forms. One 
of the more important blood spaces is the ventral, in which the 
nerve cord lies (Fig. 126, v.s.). The blood corpuscles are color- 
less and amoeboid. The plasma may be variously colored by 
pigments which seern to assist in the work of respiration, as 
haemoglobin does in vertebrates. 

323. Excretion. The importance of excretion increases 
with the activity of animals. Except in Peripatus it is not 
conclusive that any of the adult excretory organs in this phylum 
are homologous with the segmental organs of Annulata. In 
insects and spiders there are excretory tubules communicating 
with the hind gut. In the crayfish and related forms a pair 
of excretory glands "green glands" open at the base of the 
antennae. It is of importance to remember that the exoskeleton 
of the Arthropoda is an excretion, which is incidentally protective 
and supportive. 

324. The Nervous System- consists essentially of the same 
parts as have been described for the annelids. It is, however, 
on the whole, more fully developed. This development accords 
with the differentiation which we have seen in the somites and 
body regions. The brain and sub-esophageal ganglia, for 
example, have become more pronounced with the differentia- 
tion of the head; accompanying the fusion of the body seg- 


ments there is a massing and fusion of the corresponding ganglia; 
and in general, everything considered, those ganglia are best de- 
veloped which lie in the best-developed somites. The concen- 
tration of the ganglia of the ventral cord may continue until 
they form practically one mass. Nerves arise from the brain, 
from the connective about the gullet, and from the ventral 
ganglia, and pass to sensory organs and to the muscles of the 
body and the appendages. 

325. Organs of Special Sense. As the thickened cuticular 
covering of the arthropods develops, it is apparent that much 
of the sensitiveness of the surface to external conditions must 
be lost unless special structures are produced to compensate 
for this by the reestablishment of connection between the 
internal organs and the outside world. Such structures we 
find in the chitinous hairs of various shapes which project be- 
yond the surface and in pits or canals which pierce the skeleton. 
These all have nervous connections and have been variously 
interpreted as tactile, taste, auditory, and olfactory organs. 
They are especially abundant in the more movable portions, 
particularly those about the mouth, though they may be found 
over the whole surface of the body. Figures illustrating the 
great variety of forms of such hairs should be sought in the 
reference texts. 

It has been shown that the crayfish and many insects are 
sensitive to chemical stimuli. These sensations have to do with 
selection of food and with testing the character of the water 
or air. They are analogous to our sensations of taste and smell. 
The organs may be scattered over the body or more commonly 
localized on the appendages about or in front of the mouth. 

At least three classes of organs have been described as audi- 
tory among the arthropods : (a)* vibratile hairs, as in the case 
of the male mosquito (Fig. 42); (b) otocysts, as in many aquatic 
forms; (c) a tympanum or membrane in connection with which 
are special nervous cells for the reception of the vibrations (as 
in the grasshopper and other insects) . The otocysts of the Crus- 
tacea may be open or entirely closed. In the former case the 
animal itself may place the otoliths in the otocyst in the form 


FIG. 127. 

of grains of sand. Recent investigations, however, show that 
the function of this organ is not hearing, so much as that of in- 
forming the animal of its relation to the pull exerted by gravity, 
thus enabling it to keep its equilibrium. 

There are two classes of eyes in the group: (a)' compound 
eyes, made up of numerous similar elements, as 
in the insects and Crustacea, and (b) simple 
eyes ocelli found alone in spiders and in many 
larvae, or in connection with the compound eyes, 
as in many insects. 

The compound eye is made up of elements 
(ommatidia) radially arranged about the end of 
the optic nerve. Each ommatidium is probably 
capable of forming an image of a limited portion of 
the field, and consists of (i) a cuticular cornea, 
appearing externally as a "facet," (2) a cellular 
lens or cone which directs the rays of light, (3) 
sensory retinal cells which receive the light, and 

FIG. 127. An ommatidium or eye-element from the eye of the Lobster 
(after G. H. Parker), c, cornea (cuticle); c.h., corneal hypodermis, which 
secretes the cuticle; co., cone cells; cr., crystalline cone; n, nuclei; n./., nerve 
fibres; r.d., distal or outer retinula cells; r.p., proximal or inner retinula cells; 
rh., rhabdome. 

Questions on the figure. Identify the following regions: 

(1) protecting part including the cornea and hypodermal cells; 

(2) focusing portion, the crystalline cone and the cone cells; 

(3) the pigmental elements of the retina (distal and proximal 
retinular cells), the former of which prevent rays of light en- 
tering one ommatidium from passing obliquely into adjacent 
ones; the proximal cells may be more immediately connected 
with (4) the nervous elements which unite the eye with the 
nerve centres. Define an ommatidium. Is it known whether 
the image is inverted in such an eye as this? 

(4) pigment cells which separate the retinal elements of adjacent 
ommatidia, and play an important, though not fully understood, 
r61e in vision (see Figs. 44 and 127). 

326. Library Exercise. If time allows some student might be required to make 
a more detailed report of the structure of the compound eye in Arthropods and its 
method of image formation. Other reports may be made, in which drawings of 
the various sense-organs in arthropods are presented to the class, especially the 
various types of auditory organs. 


327. Reproduction and the Reproductive Organs. Repro- 
duction in arthropods is sexual. With few exceptions the 
sexes are permanently separate. There is often much differ- 
ence in the size, color, structure, and activity of the two sexes. 
The males are often smaller, more active, and more highly 
colored than the females (see "sexual dimorphism," 149). 
Sometimes the members of a single sex are dimorphic, as in 
the workers and queens among the bees. This is correlated 
with individual division of labor in the social insects. 

The sexual organs are usually paired, and in the female 
consist of the ovaries (which may be subdivided into ovarioles) , 
oviducts, receptacula seminis, in which spermatozoa are stored 
at copulation, accessory glands, sometimes external copulating 
and egg-depositing organs. The male has testes; vasa deferentia, 
which may have special enlargements for the storing of sper- 
matozoa and the formation of sperm masses ; and external copu- 
latory organs. See figures of the sexual organs of the honey-bee 
or other representative insect in the reference texts. Compare 
them with those of the snail. 

328. Parthenogenesis. In several insect types the eggs 
have the power of developing without being fertilized by the 
male element. Its occurrence is determined primarily by the 
absence of males, but even when males are present the female 
may often deposit unfertilized eggs. She is influenced to do 
this possibly by the special conditions of temperature, nutrition, 
and the like, to which she is subject. 

The individuals resulting from parthenogenesis may differ 
very materially from those produced by the normal sexual 
method. In the case of the bee, the males or drones come from 
unfertilized eggs, and the workers and queens from fertilized. 
The cause of the differences between workers and queens is 
apparently one of nutrition purely. Biologically, partheno- 
genesis is to be considered as a modified or abbreviated form 
of sexual reproduction, in which the stimulus to cleavage comes 
from some source other than the male cell. 

329. Development. After fertilization the nucleus divides 
as described for other forms, but usually, on account of the 



abundant yolk which the eggs contain, complete segmentation 
of the cell is not effected. After a series of divisions some of 
the free nuclei assume a superficial position where they become 
surrounded by protoplasm, and form the blastoderm (Fig. 13, 
D, 3).. This is described as partial and peripheral segmenta- 
tion. On the side of the egg where the embryo is to lie, a 
thickening called the ventral plate is formed. From this area 
of the blastoderm there arises, by specialization, by insinking, 
and by multiplication of the cells, the three-layered condition. 
The presence of yolk so obscures and complicates the process 
that the student must be referred, to more comprehensive books 
for even an outline of it. 

330. The Later Development may be either direct or in- 
direct. That is to say, the young when hatched may be the 

FIG. 128. 

PIG. 128. The Z6ea of Cancer irroratus. From Verrill. X 15. 

Questions on the figure. Compare with the larva of lobster (Fig. 131) and 
with the Megalops (Fig. 129), and note likenesses and differences. 

adult in miniature, possessing its form and habits, or may 
have a very different form and assume the adult condition by 
one or more metamorphoses. The differences between the larval 
and adult conditions may be slight or very great. To effect 



the change from larva to ^adult a series of moultings of the 
chitinous covering is usually necessary; these may be accom- 
panied or preceded by periods of rest, in which important in- 
ternal changes take place. The metamorphosis is more common 
among insects (Figs. 142 and 148), although a similar thing 
happens in many of the Crustacea (as crabs, Figs. 128 to 130). 
In spiders the development is direct. The eggs of many insects 
hatch as worm-like larvae (grubs, maggots, caterpillars). These 
are usually active, voracious, fat-storing animals, which after 

FIG. 129. 

FIG. 129. Megalops of Cancer irroratus. From Verrill. X 15. 

Questions on the figure. Compare with Pigs. 128 and 131, and make note of 
the chief points of contrast. Compare also with adult crab (Fig. 130). What 
differences are to be noted between the development in the lobster and in crabs? 
Is the larval or adult crab more like the lobster? 

a period pass into a resting condition, often surrounding them- 
selves with protective coverings (cocoons) . During this quiescent 
stage they are described as pupcz. In the pupal stage the accu- 
mulated fat is used by the organism in forming the new organs 
of the adult or imago. The internal larval organs may be torn 
down completely by the aid of amoeboid cells and be made to 
contribute material to rebuild the new. The extent of these 


changes can only be realized by a comparison of the structure 
of a caterpillar and of the butterfly into which it develops. The 
larvae may be suited to aquatic life, the adult to aerial ; the larva 
may be carnivorous or herbivorous, the adult may live on the 
nectar of flowers. These changes of habit are closely correlated 
with the changes of structure noted in the metamorphosis. 
The reproductive organs are not mature until the imago stage 
is reached. Frequently the imago only survives long enough 
to insure the laying of fertilized eggs. In general the length of 

FIG. 130. 

FIG. 130. Violet Land-crab. After Shufeldt. 

Questions on the figure. Compare the crab with the lobster (Fig. 132) as to 
the development of the body-regions, segmentation, appendages, etc. Compare 
all the figures of crabs available and note in what respects they vary externally. 

331. Library References. Make a report on the metamorphosis in Crustacea. 
What is meant by an incomplete metamorphosis? Illustrations. 

life in insects is short, although it is claimed that queen bees 
may live five years and queen ants thirteen. The Crustacea 
are much longer lived. 

332. Ecology. When we remember the great number of 
species and of individuals in the group of arthropods we are 
forced to realize something of their importance in their rela- 
tion to other forms of life on the earth. Their numbers and 
their enormous power of reproduction make it inevitable that 
they become pests and threaten the existence of the plants and 
animals on which they prey, and likewise that they become 


important elements in the food supply of animals which prey 
on them. It is only by their great reproductive power that 
they can hold their own against their many enemies, the birds 
and other insectivorous animals, and the accidents of climate, 
etc. The Crustacea furnish a large part of the food of fishes. 

From a human point of view they may be the greatest pests 
or the greatest helpers. In the voracious larval stage they 
devour waste material as scavengers, strip vegetation, spread 
disease, produce silk, and furnish food to the higher animals. 
In the adult stage they may destroy crops; cross-fertilize 
flowers in their search for nectar, which they may store for 
themselves and their young to be intercepted in the case of 
the bee by man; may spread contagious diseases; may devour 
stored grain, or by their mere presence become a nuisance to 
man and the domestic animals. In both stages they may be 
parasites on man and other animals. Few of the arthropods 
are directly useful as food to man, though lobsters, crayfish, 
shrimps, etc., are important items in our food supply. 

Many special devices of structure and of instinct have arisen 
making their continued existence in the presence of their enemies 
possible. Indeed there is no group of animals in which so 
many and such interesting adaptations to the special conditions 
of life are found as among the arthropods. All are provided 
with some degree of external protective covering. Many are 
so colored and shaped as to be inconspicuous in their natural 
environment. Some are endowed with offensive odor and 
taste, some with stinging organs. Others which are them- 
selves perfectly harmless are so much like forms which are 
repulsive or dangerous, as to be preserved thereby from their 
enemies (see Chapter VIII). 

Many insects as ants, bees, and wasps are strikingly social 
in their habits, and show a high degree of differentiation among 
themselves. Among the bees a special class of females the 
queens lay the fertilized eggs, the other females the workers 
being sexually immature. In the ants, still further division 
of labor occurs among the workers. Some individuals act as 
soldiers for the protection of the ordinary workers. Some 
species of ants make slaves of other species of ants which do the 


work of the colony, or of other animals (aphides) for the purpose 
of feeding on their secretions. A high order of social instincts 
and skill is shown by certain members of this group, the 
highest, apparently, shown by any of the invertebrate animals. 

333. Behavior. In the Arthropods we are dealing with 
organisms more complex in their nervous organization and with 
a greater variety of sense organs than we have yet found, 
with the possible exception of some of the cephalopods. Their 
reactions to stimuli and general behavior in the presence of the 
conditions of life, and probably their mental life, are correspond- 
ingly rich and varied. We feel that their activities are reflex 
and instinctive rather than intelligent, although it has been 
definitely shown by experiment that the crayfish and some of 
the insects learn to accomplish things by the trial and error 
method. That is, they modify behavior through experience. 

The nervous system, scattered as it is largely through the 
segments of the body, is suited more to reflex activities than to 
intelligent activities. The latter become more pronounced as 
the brain comes to contain a larger portion of the nervous 
matter, with more complex connections. 

Crayfish prefer darkness and are positively thigmotropic 
(that is, like to touch solid objects). Both these reactions are 
valuable in bringing them into hiding. 

Some species of insects show striking powers of recognizing 
one another, and of communicating their mental states. This is 
done apparently through tactile and chemical sense organs in 
the antennae and other appendages and by the making and 
hearing of sounds. The whirring and buzzing of wings in flies 
and bees and mosquitoes, the scraping of legs and wings by which 
chirping is done, the sustained stridulations of the cicada are 
examples of these. The lazy hum of the bee is very different 
from the angry buzz, and is quickly recognized by other mem- 
bers of the hive. In others as the mosquito the song of the 
female is probably a sex call. Probably many of them, such 
as the tappings of some beetles, are without meaning except as 
evidences of nervous activity. Others are merely incidental to 
necessary activities, as flying. 


The complex instincts of many insects as yet defy our anal- 
ysis. They are often set off, so to speak, by external conditions, 
as the laying of eggs on decaying meat by flies; but we do not 
know to what extent they are controlled by external and to 
what by internal conditions. The most remarkable instincts 
are connected with the place and time of egg laying, the building 
of homes, the capturing of prey, and the complex social life of 
the higher forms. The student can well afford to seek illustra- 
tions of these. 

In some of the Crustacea there is an interesting power of 
breaking off an injured leg at a definite point. This is not because 
of particular weakness at this point, but is the result of definite 
muscular contractions. It is a reflex and instinctive response 
to certain stimuli. The regeneration of the leg occurs from the 
stump. Many crustaceans have the power of regenerating 
any of the appendages which may be lost. This is true also of 
the eyes, which are on stalks. Antennae, however, may develop 
in place of the eyes. 

334. Library Exercises. Reports on the social life of bees and ants; on the 
animals captured and utilized by ants; on power of flight in ants; on queens among 
ants and bees; on myrmecophilous (ant-loving) insects; on intelligence among 
insects and spiders. 

335. Classification. 

Class I. Crustacea (with shell; Crayfish, Crabs, Barnacles, etc.). Arthropoda, 
chiefly inhabiting the water and breathing by means of gills or through the body 
wall. The head typically consists of five segments more or less fused and bearing 
two pairs of antennae or feelers, one pair of mandibles, and two pairs of maxillae. 
The thorax or second region of the body may be separate from or fused with the 
head (cephalothorax) . It possesses a variable number of segments, which usually 
bear the locomotor appendages. The remainder of the body (abdominal segments) 
is normally of distinct segments in which the appendages are much reduced. The 
chitinous skeleton is ordinarily well developed. 

Subclass I. Entomostraca. Crustacea, small and simple in organization, with 
a variable number of segments of .which the appendages are simple and less diverse 
than in the next subclass. Many of them are parasitic and degenerate. A meta- 
morphosis occurs. The group embraces many small free forms, found both in 
fresh and salt water, some fish parasites, and the sedentary barnacles. Here 
belong Cyclops and Daphnia, which occur abundantly in fresh- water pools and feed 
on the algae common there. They constitute an important portion of the food of 
the fresh-water fishes. They multiply very rapidly and keep closely up to the 
limit of the food supply. The eggs of many of them can resist drying to a re- 
markable degree. This is of manifest importance when we remember that they 
frequent pools in which drouth is not uncommon. 


The barnacles (Cirripedia) are Crustacea which in adult life become attached 
to the rocks near' low water-mark or to floating objects of various kinds. The 
bottoms of ships become foul with them. The group is especially interesting in 
that it points to the giving up of free motion, which its ancestors possessed, for a 
mode of life wholly different, and demanding marked changes of structure. They 
possess, besides the organs for attaching themselves, bivalve shells similar to those 
of Mollusca, for protection; they are often hermaphroditic, which is a very un- 
common thing in arthopods. The advantages gained by their special habits are 
evident. The waters near the shore contain a great deal of organic de"bris, and any 
organism which can attach itself here and yet be protected from destruction by 
the waves is fortunate. Those attached to floating objects are brought, without 
their effort, into constantly changing localities. 

Subclass II. Malacostraca. Crustacea of larger size and more highly organized. 
Segments, except in one order, twenty, and well differentiated. Nineteen of 
these segments bear appendages. The first stage in the metamorphosis (the 

PIG. 131. 

FIG. 131. Larva of Lobster (Homarus americanus) removed from egg shell. From Herrick. 

Questions on the figure. Compare with the adult (Fig. 132) and note similari- 
ties and differences? Examine Dr. Herrick's figures (Bull. U. S. Fish Commission 
for 1895) and notice the gradual change to the adult condition by successive 
moultings. What structures can you identify? 

nauplius) is usually passed before hatching. The group embraces (i) the Deca- 
poda, or the lobsters, crabs, crayfishes and shrimps, which agree in the possession 
of ten walking feet, eyes on movable stalks and a carapace covering the thirteen 
fused segments of the cephalothorax ; and (2) the Tetradecappda, comprising 
numerous smaller types such as beach-fleas, sow-bugs or wood-lice, in which head 
and thorax are not fused, the eyes are not movable, and the walking appendages 
are fourteen. 

The crayfish and lobsters have well-developed abdominal segments, whereas 
in the crabs the abdomen is reduced and bent under the thorax, which becomes 
broad and massive (Figs. 129, 130). The larger Crustacea are omnivorous, almost 
all organic matter, dead or living, being acceptable. Lobsters are known to attack 
and devour fishes. The lobster (Homarus, Figs. 131 and 132), of which there are 
two species, an American and a European, is economically the most important 
member of the group, and stands next the oyster as the most important invertebrate 


FlG. 132. 

FIG. 132. The American Lobster (Homarus americanus) . From Herrick. 

Questions on the figure. What body-regions are distinguishable in the 
lobster? Compare by actual measurement the size of the crushing claw with that 
of the body. How many segments in the abdominal region? Compare with 
Fig. 130. 

food species. It is estimated that as many as one hundred million lobsters have 
been taken in a single year in New England and Canadian waters. There is no 
doubt that the lobster is in immediate danger of extinction as a food animal, as is 
shown by the fact of greater difficulty in obtaining them and by the decrease in 
the average size of the animals put on the market. This decrease occurs in the 
face of the fact that the mature female produces from ten thousand to one hundred 
thousand eggs. These are carried under the abdomen of the mother until hatched, 
which requires a period of ten or eleven months. After hatching the young 
undergo a series of moultings during which time they are the prey of many kinds 
of enemies. Such is the mortality that, on an average, not so many as two of all 


the young of a female reach maturity. This is another way of saying that 
a species is losing ground. Two general methods have been tried to make good the 
decline in the supply: first, legislation forbidding the taking of animals under the 
size, which indicates sexual maturity (eight to twelve inches), and forbidding 

FIG. 133. 

FIG. 133. Palcemonetes vulgaris. From Verrill. 

Questions on the figure. Compare the appendages of Palaemonetes with those 
of the lobster, the crab and Gammarus. What seem to be the functions of the 
various appendages, so far as position and form may indicate? 

FIG. 134. 

FIG. 134. Gammarus ornatus. From Verrill. 

Questions on the figure. How does this form compare with the lobster and 
the crabs in differentiation of the segments, in fusion of the segments and in the 
differentiation of the appendages? 

the capture of females carrying the developing embryos; and, second, attempts on 
the part of the national government to hatch artificially and care for the moulting 
young under such conditions that they will be protected from their natural enemies. 
The problem is not yet solved, and in the meantime another source of food is likely 
to be destroyed through overfishing. 

The crayfish is prized for food in European countries, but is little used in 
America as yet. Shrimps, prawns, the "soft-shelled" or blue crab are all of 
considerable importance in this regard. The smaller Crustacea are a very im- 
portant element in the food supply of the fishes, both in the fresh waters and in the 



Class II. Onychophora. Peripattis is the best known genus of this class. It 
is interesting chiefly because it is, in some degree, intermediate between the 
Annulata and the higher arthropods. They are remarkable for a wide distribu- 
tion out of proportion to their numbers, and are found in moist places, under 
wood, stones, and in rotting bark. They agree with the chaetopod annulates 
(see 276) in the possession of segmental organs (nephridia), a dermo-muscular 
sac, and poorly developed appendages. The segments are also homonomous 
(see 264) as in the worms. The relationship to arthropods is indicated by the 
possession of tracheae, by the substitution of haemocoele (the enlarged lacunae in 

FIG. 135. 

FIG. 135. Caprella geometrlca. From Verrill. X 4. 

Questions on the figure. In comparison with other Crustacea what are the 
aberrant or peculiar features of this form? See also figures in reference texts 
(e.g., Parker and HaswelTs Zoology, Vol. I, p. 546). 

which circulation occurs) for the true coelom, and by the differentiation of some of 
the anterior segmental appendages as mouth parts. The Onychophora resemble 
the larval condition of those insects which undergo a metamorphosis much more 
than the adult stages. This suggests that they are more closely related to the 
ancestral types from which the insects have sprung than to the insects themselves 
(Fig. 136). 

Class III. Myriapoda (many feet; Centipedes, etc.). Tracheate arthropods 
with*a worm-like body. Segments numerous, and much alike, one (or, in Dip- 

FIG. 136. 

FIG. 136. Peripatus capensis. From Nicholson after Moseley. 

Questions on the figure. Externally in what respects is this form like the An- 
nelids ? In what respects different from them ? Of what special zoological interest 
is this genus? What are its habits? In what respects is it like and in what unlike 
the centipede (Fig. 137)? 

lopoda, two) pair of appendages to each segment. The head is distinct and bears 
antennae and mouth parts. The eyes are numerous and simple (ocelli). In 
fundamental structure and development the myriapods resemble insects. There 
are two principal orders. One embraces the centipedes which are carnivorous, 
have biting jaws, have one pair of appendages to each segment, and are poisonous. 
The second includes the millipedes which are vegetable feeders and possess man- 


dibles suited to chewing vegetable matter. They are wholly harmless. They 
have two pairs of legs to each of the numerous segments except the first four. 
Both centipedes and millipedes inhabit the land, and frequent dark places. Many 
are nocturnal in habit (Fig. 137). 

Class IV. Hexapoda (six feet; Insects). Tracheate arthropods with three dis- 
tinct body regions, head, thorax, and abdomen. The head has four segments 
with appendages, a pair of antennae and three pairs of mouth parts. The thorax 
has three segments (pro-, meso- and meta-thorax), each of which bears a pair of 
legs; the meso-thorax and the meta-thorax may each bear a pair of wings. The 
abdomen has a variable (often obscure) number of segments. Its appendages are 
usually entirely wanting or much reduced. A metamorphosis frequently occurs. 
The larval condition often suggests the annelids and the myriapods in the similarity 
of its segments, and in the numerous appendages. 

The student is referred to more comprehensive works for an exposition of the 
numerous orders of this enormous group of Hexapoda. Only the more important 
are suggested below. 

Fig. 137. 


FlG. 137. Centipede (Scolopendra heros). Photo by Folsom. Four-fifths natural size. 

Questions on the figure. What differentiation of segments is apparent? Are 
there any fusions into body-regions? What is the law of the occurrence of ap- 
pendages? What diversity is there among them? 

Order Aptera (without wings}. This order embraces a number of minute, 
wingless insects which do not undergo any metamorphosis. The body is covered 
with scales or hairs. The spring-tails and snow-fleas are examples. These make 
their leaps by suddenly straightening out a tail-like structure which is bent under 
the body when at rest. They are not the only wingless insects and hence the 
name is somewhat misleading. See Fig. 138. 

Order Orthoptera (straight wings}. In this order the metamorphosis is incom- 
plete or lacking. There are usually two pairs of wings, the anterior often some- 
what thickened, serving as a cover for the posterior. Mouth parts are adapted 
to biting and chewing. Here belong the cockroaches, grasshoppers, crickets, 
locusts, katydids, walking-stick insect. The order is of considerable economic 
importance. Most of its members are vegetable feeders and when they are 
gregarious are often very destructive. The Rocky Mountain locust, so named 
because it breeds on the plateau at the eastern base of these mountains, in 1873 
and again in 1878, migrated eastward over Nebraska and Kansas in search of food, 
literally stripping fields of vegetation. Since the settlement of the regions where 



they breed, with the ploughing up of the eggs and the destruction of the young, 
there is reason to hope that these migrations are at an end. Accounts of similar 
migrations of locusts are recorded in the history of the old world. These migra- 
tions and their effects illustrate how climatic conditions in one locality may change 
the balance of life in another. The second chapter of the prophet Joel gives a 
vivid account of a visitation of locusts. See Fig. 139. 

FIG. 138. 

FIG. 138. Campodea, a Thysanuran. Magnified 30 times. By J. W. Folsom. 

Questions on the figure. In what respects does this form seem intermediate 
between the Myriapods and the higher insects? How does this compare with the 
larvae of insects which undergo a metamorphosis? Can you distinguish head, 
thorax, and abdomen? 

Order Neuroptera (nerve-wings}. The members of this order have a more or 
less complete metamorphosis. Usually there are two pairs of netted membranous 
wings. The mouth parts are suited to biting. This group includes many diverse 
forms which the entomologists distribute into nine or ten orders. These are: 
the Ephemerida, or may flies whose adults may live for only a few hours, although 
the larvae may require from one to three years to develop; the Odonata, or carnivo- 
rous dragon-flies, and damsel-flies; the Plecoptera, or stone-flies; the Isoptera, or 
termites (white ants) which are social, polymorphic forms; the Corrodentia, book- 



lice and bark-lice; the Mallophaga, parasitic bird-lice which eat hair and feathers; 
the Physopoda, or thrips; the Dermaptera or earwigs; the Neuroptera, the Dobson 
flies, ant-lions, and lace- winged-flies; the Trichoptera or caddice flies, which in the 
larval stage cements around itself a protecting case made of sand or other particles 
of matter. 

FIG. 139. 

FIG. 139. Katydid (Cyrtophyllus perspicitlatus), natural size. Photo by Folsom. 

Questions on the figure. How many pairs of appendages are visible in the 
figure? How many pairs are present ? To what order of insects does the Katydid 
belong? What are its feeding habits ? What can you find of its development? 

FIG. 140. 

FIG. 140. Periodical Cicada. Natural size. Photo by Folsom. 

Questions on the figure. To which order of insects does Cicada belong? 
Which -of its habits are most familiar to you? What are its nearest relatives 
among the insects? 

Order Hemiptera (half -wing}. Hexapoda with an incomplete metamorphosis, 
and having two pairs of wings, or none. Mouth parts are modified for piercing 
and sucking. Here are included the true bugs, as the squash bug, chinch bug, bed- 
bug, the water boatman, etc.; the lice; the plant-lice; the scale insects ; and the 
cicadas (sometimes called "locusts"). These should not be confused with the 



beetles, which are often called "bugs." See Fig. 140. The Hemiptera furnish 
some really serious pests, as the scale insects, aphids, chinch bugs, etc. 

Order Diptera (two wings}. These Hexapoda undergo a complete metamor- 
phosis, having the anterior pair of wings developed (not in fleas). The posterior 
pair is very much reduced or wanting. The mouth parts are well adapted for 
piercing and sucking. The order is very large in species and includes such common 
forms as the flies, mosquitoes, gnats, fleas. Many members of this group are of 
great importance to man. The maggots of the true flies are scavengers, developing 
in decaying organic matter and assisting in its destruction; the adults, on the other 
hand, besides being unpleasant companions and demanding a share of our com- 

FIG. 141. 

FIG. 141. Larv ae of the Bot-fly (Gastrophilus equf) in the stomach of the horse. One-half natural 
size. From Luggar, after Heller. 

Questions on the figure. What do you 'know of the habits of the bot-fly? 
Where are the eggs deposited? How do the larvae come to have the position 
figured above? How do they pass from this to the adult condition? See also 
Fig. 142. How does it retain its position in the stomach of its host? 

forts, spread disease. Other species suck the blood of man and domestic animals, 
producing disease and death. The bot-flies are most important in their larval 
stage. The eggs, deposited on the exterior, are taken into the digestive tract and 
there develop, often migrating into other organs and producing definite diseases. 
Mosquito larvae devour the decaying organic matter in stagnant pools. The adult, 
especially the female, is a blood-sucker and is, through the parasitic protozoa 
which may infest it, the chief instrument of the spread of malaria and yellow fever 
among men. They are all very prolific and develop rapidly considering the fact 
that they undergo a metamorphosis. The fly, for example only requires a few 


3 OI 

hours for hatching into the maggot stage. If food and temperature are favorable, 
this maggot may grow to full size in a week, when it passes into the resting or pupa 
stage, from which another week or more is required for the young fly to emerge. 

The eggs of mosquitoes are deposited in water, where they hatch into active 
larvae called " wigglers." These breathe the air by means of a tube on the next to 
the last abdominal segment. Their common position with the end of the tail at 
the surface of the water is thus explained. The mosquito larva does not cease 
to be active, but by a series of moults comes to the so-called pupa stage from which 

FIG. 142. 

PIG. 142. Stages in the development of the Bot-fly (Gastrophilus equi). From Parker and 
rfaswell, after Brehm. a, adult insect; b, egg attached to a hair; c.d. and e, stages in^he development 
of the larva. (See also Fig. 141.) 

by an early moulting the adult mosquito emerges, balancing itself on the floating 
pupal skin until its wings are hardened sufficiently for use. See Fig. 143. 

The Hessian-fly deposits its eggs in the tissues of growing wheat and corn; the 
clover-gnat and others produce galls which interfere with the growth of the plant, 
often very seriously. In the case of the Hessian-fly great damage to the wheat 
crop often results. See Fig. 144. 

The fleas are to be looked upon as degenerate. They are often placed in a 
separate order (Siphonaptera) . The adults are external parasites without wings. 
They are flattened laterally and thus pass readily between the hairs of the host. 
The larvae are not parasitic, but live on decaying organic matter. 

3 02 


Order Lepidoptera (scale-wings}. These are Hexapoda which pass through a 
complete metamorphosis, in the adult possess sucking mouth parts, and have two 
pairs of large membranous wings covered with scales. The moths and butterflies 
are the representatives of the order. The larvae are known as caterpillars, which, 
with a few exceptions, are vegetable feeders. The adult butterfly differs from the 


143. Two stages in the metamorphosis of the Mosquito. From Packard A, larva; B, 
pupa; C, ventral view of the oar-like appendages of the last segment of the pupa. 

FIG. 144. 

PIG. 144. 

The Hessian Fly (Cecidiomya destructor). From Standard Natural History, 
adult; b, larva; c, pupa; d, larvae in position on stalk of wheat. 

a, the 

Questions on the figures. Give names to all the structures apparent on the 
adult. In which stage does the insect do its damage? What is its economic 
importance? What is the origin of its common name? 

moths (typically) in the fact that the former fly by day, hold the wings erect when 
at rest and have antennae with a club on the end. The butterflies share with the 
birds the preeminence in beauty among animals. They present many points of 
interest in their metamorphosis, in their habits, their coloration their distribution, 
and their economic effects. 



The caterpillars are usually voracious and may strip their food plant of its 
leaves and buds. The majority of the larvae have become highly specialized in 
their food habits, becoming restricted in some instances to one species or to a few 
related species (as illustrated by the tomato worm, which feeds on tomato, potato, 

FIG. 145. 


FIG. 145. The Cabbage Worm (Pieris rapcz). Natural size. Photo by Folsom. A and B, larvae; 

C, pupa. 

*". Questions on the figure. What is a larva? What is a pupa? Which is the 
earlier stage? What is the color of this caterpillar in nature? See the next 
figure for the adult. 

FIG. 146. 

FIG. 146. The adult Cabbage Butterfly (Pieris rapce). Natural size. Photo by Folsom. 

Questions on the figure. Why is the larva of this animal called the cabbage 
worm? Why is the adult called the cabbage butterfly? What are its feeding 

and tobacco leaves; or the cabbage worm which eats the leaves of certain of the 
cruciferous plants). The distribution of such species is thus clearly determined 
by that of their host plants. The most injurious to vegetation are the "tent- 
caterpillars" which occur gregariously and spin a web-like nest; the army- worm 
so-called because it appears and moves from its hatching grounds in great numbers; 



the cotton-boll worm; the canker-worms and fruit-borers. The silk- worm seems 
to be the only useful member of the order. The clothes-moth lays its eggs in 
woolens or furs, its larvae thus being exceptional in preferring animal diet. 

The adults are usually short-lived and some do not eat at all. The majority 
of them suck nectar from flowers and juices from ripe fruits and other objects by 
means of special tubular mouth parts which are modified paired appendages. 
They carry pollen from flower to flower, effecting cross-fertilization, in some 
instances. The color of the larvae and the adults is very varied and has close 
relation to the environment and habits of the animals. We have already noticed 
in the chapter on adaptations (Chap. VII) how the coloration may be protective. 
This is the more needed since the group has many enemies, especially in the larval 
stage. The power of reproduction is great. Several broods per year may be 
produced. There are 25,000 known species of Lepidoptera, 7,000 of which occur 
in North America, north of Mexico. The species are more numerous and striking 
in the tropical regions of South America. 

FIG. 147. 

PIG. 147. The Army Worm (Leucania unipuncta). After Riley. A, caterpillar; B, adult moth. 

Questions on the figures. What are the principal facts concerning the habits 
and economic importance of the army- worm? Why is it so called? 

Order Coleoptera (shield-wings}. In this group there is a complete meta- 
morphosis. The mouth parts are suited to biting and chewing. The front wings 
(elytra) are hardened and serve as covers for the true membranous wings when the 
latter are not in use. These are the beetles, falsely called "bugs." Although a 
well-defined order the beetles are very various, as will be seen from the fact that 
there have been described over eleven thousand species for this continent north of 
Mexico. There are said to be more than one hundred thousand known species of 

The larvae are commonly known as grubs. The feeding habits are almost as 
diversified as the form. Many are scavengers and lay their eggs in carrion and 
other decaying matter ; others bore into wood and bark, as the long-horned beetles ; 
some frequent grain, nuts, fruits; others are leaf-eaters; a few devour other insects. 
The Colorado potato-beetle, the weevils, the museum pest, the locust-borer or 
the hickory-borer will serve to illustrate some of the more hurtful representatives 
of this immense order. 

Some especially interesting forms are the fire-flies, the scarabeids, including 
the sacred dung-beetle of Egypt, and the ladybird-beetle. The latter is useful to 
man owing to the fact that it preys on certain hurtful insects. In California the 
cottony-cushion scale, which in some way had been imported from Australia, 



promised at one time to destroy totally the orange industry. The Australian 
ladybug, which keeps it within bounds in its native home, was imported, and the 
increase of the ladybugs was such that the cushion-scales were all but destroyed. 
This species of ladybug feeds exclusively on the cottony-cushion scale, and there- 
fore the destruction of the latter led in turn to the rapid decline of the ladybugs 
from the loss of their food supply. Indeed it was necessary to keep colonies of 
the scale insects protected in order to furnish food and to prevent the entire 
destruction of the imported beetle by starvation. In Australia where both are at 

FIG. 148. 

FIG. 148. Swallow-tail Butterfly (Pafiilio machaon), larva, pupa, and adult. From Nicholson. 

Questions on the figure. Which is the larva and which the pupa? Which 
of these is the earlier stage? What are the chief characteristics of the three stages 
in the metamorphosis of butterflies, the larva, the pupa and the imago? 

home the natural conditions and the adjustment of the two species are such that 
this scale-insect does not become a pest. The discovery of the biological relations 
of these species, and the relief of the orange industry furnish a sample of the ex- 
cellent work being done by the U. S. Department of Agriculture in connection with 
the economic aspects of biology. 

Order Hymenoptera (membrane-wings}. Hexapoda with four membranous 
wings; mouth appendages adapted for sucking or for biting; metamorphosis com- 
plete. This is the most highly developed division of Insecta, and embraces such 



forms as bees, wasps, and ants. The most important habits of the group, which 
are those growing out of their social life, have been referred to in the chapter on 
adaptations (Chapter VIII). The chief economic value of the order is in the 
honey of the honey-bee, the fertilization by bees of certain plants, as clover, and 
the reduction of more hurtful species of insects by certain parasitic members of 
the order, as the ichneumon-flies and chalcid-flies. Some of the larvae are leaf- 
eating, as the rose-slug, and others produce galls on the oak and other plants in 
depositing their eggs. These are harmful to human interests. 

FIG. 149. 

PIG. 149. Hornet's nest, sectioned. Photograph from life by Shufeldt. 

Class V. Arachnida (spiders, scorpions, etc.). Arachnida are arthropods in 
which the head and thorax are typically fused and represent about seven segments 
with six pairs of appendages. There are no antennae. The abdomen is often 
segmented but usually without paired appendages. Respiratory organs are con- 
fined to the abdomen, and may be of three types: book-gills , associated with 
appendages (king-crab) ; trachea similar to those of insects; and book-lungs (spiders). 
Development is usually direct. 

Order I. Xiphosura (the king crab). This order contains only one genus, 
Limulus, a marine form with book-gills, and a cuticular test like that of the Crus- 



tacea, with which it was formerly classified. Numerous related forms flourished 
earlier in the world's history but are now extinct (Trilobites). 

Order II. Scorpionida (scorpions). Arachnids with a much elongated and 
segmented abdominal region closely connected with the thorax. They are air- 
breathers, with four pairs of book-lungs in the abdomen. The posterior abdominal 
segments form a tail the last segment of which bears a sting. See Fig. 150. 

FIG. 150. 

FIG. 150. Scorpion (Buthus). Photo by Folsom. 

Questions on the figure. Compare the scorpion with figures of Crustacea, 
insects and spiders, noting the chief differences and likenesses. Of what use is 
the long, segmented abdomen in the scorpion? 

Order III. Araneida (spiders). The Araneida are air-breathing arachnids, 
with book-lungs alone or in connection with tracheae. Poison glands are common 
in connection with the first pair of (mouth) appendages. The abdomen is un- 
segmented and without appendages, unless the spinnerets represent reduced 
appendages. On these latter, open the ducts of the numerous glands secreting 
the fluid which hardens on exposure to the atmosphere and makes the silk of the 

Spiders may be classified on the basis of the type of web which they make. 
The "orb-weavers" construct webs of great regularity and beauty; others, as the 

3 c8 

FlG. 151. 

PIG. 151. Spiders (Epeira marmorea). After McCook. Male on left; female on right. Natural 


Questions on the figure. What differences do you note with respect to the 
sexes? What habits of the spiders are correlated with this difference in size in the 

FIG. 152. 

FIG. 152. Web of Epeira slrix, an Orb-weaving Spider. After McCook. 

Questions on the figure. By reference to other texts or by observation 
determine if there is any regular order in which the parts of the web are produced. 
To what is this form of web an adaptation? Evidences? What other forms of 
webs are constructed for similar purposes? 


cob-web spider, make a complex and irregular mesh-work of fibres running in all 
directions; others spin a web similar to the last with the exception that at one point 
it is continued into a tube into which the spider retreats for hiding. The webs of 
these spiders are for the purpose of catching flies and other insects on which the 
animal feeds. The trap-door spiders make a tunnel in the ground which they 
line with their secretion; a door is woven which is so covered with materials like 
those about the nest that its presence is effectually hidden. A considerable number 
of spiders do not spin proper webs, but use their secretion merely in forming cocoons 
for their eggs, or in binding together objects to make a home. This wonderful 
secretion is used by the spider in many other ways than in the capture of prey and 
the making of a nest. By means of it some of the spiders make a near approach to 
flying. A spider may bridge the space from one object to another either by fasten- 
ing one end of the strand and hanging at the other, or by sitting still, he may allow 
the free end to float out until it becomes attached. In some cases at least it is 
known that, by spinning thus loose silk in abundance, the weight of the spider 
may be readily carried by the action of the wind upon his silken sails. 

The chief economic importance of spiders lies in their habit of preying on various 
insects, of which they destroy considerable numbers. 

The Arachnida embraces a number of other orders including less important 
or less easily observed animals, as the mites, certain ticks, harvest-men or "daddy- 
long-legs," and many parasitic or otherwise degenerate forms. 

336. Suggestive Studies, for Field and Library. 

1 . Dimorphism and polymorphism in insects. 

2. Protective adaptations in insects. 

3. What senses seem most used among the insects? 

4. Report on observed signs of intelligence among arthro- 

5. Is there any evidence of power of communication among 
the social insects, as the ants ? 

6. Courtship among the spiders. 

7. Spiders' webs: form, position, efficiency, mode of con- 

8. There are some insects which have wings during a por- 
tion of their life but lose them later. Investigate the condi- 
tions and find an explanation. 

9. Report an observed instance of insects pollinating flowers 
(i.e., transferring pollen from one to another). How is it 
effected ? Why does the insect do it ? Is the pollination of 
flowers by insects deemed a common and important phenome- 
non by botanists ? 

10. Can you find any recorded instances of what may be 

3 io 


called symbiosis (see 160) between insects and other organ- 

1 1 . Have you any experimental evidence as to how growth 
can take place in forms with a firm external skeleton such as 
that of the crayfish ? 

12.- Do you have any reason for thinking that a metamor- 
phosis is advantageous to any of the Arthropoda ? Is it in any 
respect disadvantageous ? 

13. How is moulting effected ? 

14. Habits of the "hermit crabs." 

15. The lobster and its habits. 

1 6. Silkworm culture, value and methods of. 

17. Insects introduced from foreign countries. 

1 8. The history of the efforts to find enemies to some of the 
more important noxious insects. 

19. Relation of insects to the culture of figs. 

20. The structure and habits of the king crab (Limulus). 
Why is it not to be classed with the true crabs ? 


337. This phylum includes, beside the typical Vertebrata 
to be described in later chapters (fishes, amphibians, reptiles, 
birds and mammals), several groups of much more simple 
organization. These latter forms may be included under the 
general head Protovertebrata, not because they all show close 
relationship among themselves, but because of their primitive 
character, considered as Chordata. They are of very great 
interest to the biologist on account of the hints they may 
offer concerning the ancestors of the vertebrates. For detailed 
description of the manner of life and the structure of these 
primitive chordates the student must be referred to advanced 
text-books of zoology. 

338. General Characters of the Chordata (Protovertebrata 
and Vertebrata). The Protovertebrata are allied with the 
typical vertebrates and separated from the invertebrates by 
the possession, either in the larval or adult condition, of the 
following features : 

1. A mid-dorsal, longitudinal rod of cells (noto chord) de- 
rived from the entoderm, but often surrounded by mesodermal 
structures (see Fig. 157). This lies ventral to and supports 

2. The central nervous system, a mid-dorsal cellular tube 
with thickened walls derived from the ectoderm. 

3. Gill-slits or perforations connect the cavity of the pharynx 
with the outside directly or through an atrial chamber. 

4. The heart is typically ventral to the digestive tract. 

339. In the Group of Protovertebrates may be placed: 

i. Balanoglossus, a soft-bodied, worm-like form whose claim 
to a place among the Chordata rests upon the fact that an out- 
growth of the gut extends into the proboscis, where it forms a 
solid rod which in its origin suggests the notochord ; a portion of 


the nervous system is dorsal; and gill-slits occur. On the other 
hand there is a connective around the esophagus and a ventral 
nervous thickened band as in Annulata, and it shows little or 
no signs of segmentation (see Fig. 153). 

2. Tunicates (sea-squirts, ascidians, etc.) comprising a 
variety of forms which may be said, on the whole, to be de- 
generate in the adult condition. It is in the larval or tadpole 
state particularly that their relation to the Chordata is sug- 
gested. -In the larva they possess a notochord especially in the 

FIG. 153. 

FIG. 153. Balanoglossus (Male). After Bateson. a, anus; m, mouth; p, proboscis; po., pores, 
the openings of the gill-slits; ts., tester. 

Questions on the figure. Make reference to other texts and figures and de- 
termine what features of Balanoglossus tend to ally it with the chordates. What 
are the habits of the animal? Where do the earlier zoologists class Balanoglossus? 

tail region, a dorsal nervous system, and gill-slits. The adult 
forms are usually attached, many of the larval organs becoming 
much changed or even wholly lost in consequence of the 
changed mode of life. The adults have been variously classified 
as worms, mollusks, etc. Many of the tunicates multiply by 
budding and form colonies from the fact that the buds remain 

3. Amphioxus (lancelet) possesses the characters above 
mentioned as belonging to all the Chordata, except that there 
is no true heart. In addition it has a fish-like body, and the 
muscles are arranged in segments which appear externally. It 



lacks paired fins, but has a median dorsal 
fin which continues over the tail and for- 
ward a part way on the ventral side. In 
front of this is a thick fold (metapleure) on 
either side the body, at the junction of the 
side with the belly of the animal. The 
metapleure is thought by some zoologists 
to be the forerunner of the vertebrate ap- 
pendages. Amphioocus is without a definite 
brain ; that is to say, the anterior end of the 
nervous tube is not highly specialized. It 
has no skull, eyes, nor ears, such as charac- 
terize the head of true vertebrates. Amphi- 
oxus is a small semi-transparent animal 
about two inches long (Fig. 154). It bur- 
rows in the sand with only the fringed mouth 
exposed. It may vary this by swimming 
about for short periods. 

4. Cyclostomes (Lampreys). These are 
eel-like animals usually classed with the 
fishes, and are doubtless more closely related 
to them than to the forms before mentioned, 
but because of their primitive qualities they 
may be placed for the purposes of this course 

FIG. 154. Diagram of the anatomy of Amphioxus, drawn as a 
semi-transparent object (after Perrier "Traiti de Zoologie"). a, 
anus; a. p., atrial pore; c.f., caudal fin; dr., cirri, on the edge of the 
vestibule leading to the mouth; d.f., dorsal fin; r, fin rays; g, gill 
or branchial structures consisting of alternate slits, through which 
the water passes, and supporting plates, in the walls of which are the 
blood vessels; in., intestine, from which as a diverticulum springs 
/., the liver; m, the mouth surrounded by a fringed velum; my., 
myotomes or muscle segments; n.c., notochord; o., ovaries; s.c., 
spinal cord; v.f., ventral fin. 

Questions on the figure. What elements of struc- 
ture appear in the figure suggesting the chordate charac- 
ter of Amphioxus? What is the relation of the spinal 
cord, notochord and digestive tract ? How much of the 
length of Amphioxus possesses gills? What is the posi- 
tion, extent and function of the atrium? (Refer to more 
extended texts.) What structures show evidences of 
segmentation? What fins has Amphioxus? Compare 
with fins of fishes. 

FIG. 154. 


among the Protovertebrata. They have a round sucking mouth 
destitute of jaws ; they lack paired appendages and the external 
skeleton. There is only one nostril, which may or may not 
communicate with the pharynx. The cyclostomes possess a 
true brain, a cartilaginous internal skeleton, and gills (usually 6 
or 7 pairs) in pouches. They differ from the true fishes in the 
fact that the notochord is not constricted, i.e., the mesodermal 
sheath does not, by its growth, compress it by the develop- 
ment of distinct vertebrae around it (see 344). See Fig. 64. 

340. Library Exercises. By reference to all the available 
literature make a report on the general structure, habits, and 
important adaptations of each of the above types? How do 
the larvae and adults of the tunicates compare? How is the 
degeneration accounted for. To what extent is colonial life 
represented among these types ? Are any parasitic ? Examine 
particularly for figures of these groups in the standard refer- 
ence zoologies. 




For general illustration of the vertebrates the author is 
convinced that no form is superior to the frog for use in ele- 
mentary classes, although some teachers prefer a fish. In 
a course arranged for one year it is not desirable to make 
elaborate dissections of more than one or two vertebrate types. 
Directions are given both for the fish and the frog for the con- 
venience of those teachers who prefer the former. Supplemen- 
tary studies for the other classes of vertebrates will be found 
in connection with the chapters devoted thereto. 

341. Fish. Any common fish will serve perch, sucker, trout, smelt. Speci- 
mens eight to ten inches in length are of most suitable size. If convenient one- 
half the class might take one species and the remainder another. 

A. The Living Animal. Place in a tub of water, or better in a vessel one side 
of which is glass. Watch the locomotion and notice all the accompanying motions 
of the various parts. What is the rate of the tail stroke? How far, on an average, 
does one stroke of the tail carry the fish? Compare these points when the fish is 
in very rapid motion. What part do the anterior fins play in locomotion? Bind 
one of them flat against the body with a string. Bind both. Results? Experi- 
ment similarly with the other fins and see if your first conclusions are strengthened. 
Do you find any variations in the above respects by comparing several species? 

How does the temperature of the fish compare with that of the water? Allow 
one specimen to remain for an hour or more in water at a temperature of 70 F. ; 
another in cooler water (50 F.): compare results. 

Can the fish detect in the water the presence of substances which have a decided 
taste to us? Use colorless solutions, acid, sugar, quinine. Can you get the 
animal to show any choice as to food? - 

Note the motions of mouth and eyes. Can the fish see any point with both 
eyes at once? 

B. External Anatomy. (Make careful outline sketches showing all points of 

The Topography of the Body. Note the symmetry; indicate the degree of 
differentiation of anterior and posterior ends, and of dorsal and ventral surfaces, 
as shown by the shape, special organs, etc. What structures appear paired? 
To what degree are head, trunk, and tail clearly distinguishable? Locate and 



identify all the external openings. How would you describe the general shape of 
the body? What are the external evidences of segmentation? 

The A ppendages. How many paired ? Unpaired ? Locate : the dorsal, caudal, 
anal, the pectoral, and the pelvic or ventral. Does the skin of the body extend 
over the fins? What seems to be the nature of the fin rays? Number? Are the 
upper and lower lobes of the caudal fin equal or unequal ? 

The Covering. Does the specimen possess scales ? Is there any regularity in 
their arrangement? Is this constant among several specimens? Is it the same 
in different species? Are any parts of the body free of scales? Are the scales 
covered with skin? What is the shape and nature of the free margin of the scales? 
Examine with a hand lens or low power of microscope. Is there any color? How 
does this appear under the microscope? Do you distinguish a line (lateral line} 
along one of the rows of scales on the side of the body? Examine one of these 
scales under the microscope. How does it differ from the others? How many 
rows of scales above the lateral line? Below? 

The Head. What goes to make up the head of the fish? Note position, shape 
and size of mouth. Find the bony framework: upper jaw (premaxillaries in front 
articulating with maxillaries behind); lower jaw (dentaries). Are both jaws 
movable? Locate all the bones which bear teeth? How are teeth arranged? 
Is there a tongue? Do the nostrils communicate with the mouth cavity? 

Eyes: number, position, coverings (lids?), iris, pupil. 

Are there any ' ' ears ? ' ' Evidences ? 

Gills and Gill-coverings. How many bones in the gill cover (operculum)? 
Describe the structure at its inner, lower margin. How wide is the isthmus 
between the right and left gill-openings? Identify and number the gill arches, 
which bear the reddish gill-filaments, and the gill slits between. With what do the 
passages between the gill arches communicate ? Determine by extending a probe 
into mouth. How does the inner or pharyngeal side of the gill arches differ from 
that which bears the filaments? Examine the gills for parasites: 

C. Internal Structure. With forceps and sharp scalpel remove a strip of skin 
an inch wide from one side of the fish, from the belly to the dorsal fin. Note the 
muscular segments (myotomes). Is the line separating two of these a straight line? 
In what direction do the muscle fibres run ? 

With scissors cut the body wall along the middle line of the belly from just in 
front of the anus to the isthmus. Be very careful not to injure any of the organs 
within the coelom. A portion of the side muscles may be removed on one side 
or cuts may be made perpendicular to the first so that the sides may be more readily 
opened. Notice the lining of the body cavity (peritoneum). Color? How is 
the heart separated from the abdominal cavity? (False diaphragm.) Sketch 
the cavities thus laid open and represent the organs as they appear before dis- 
turbing them. Has the liver lobes? 

Examine more in detail, turning liver to one side. 

I. Digestive Organs. Extend probe through the mouth into stomach, and 
locate: esophagus; stomach, form and position; intestine, its point of origin, course 
and outlet. About the junction of stomach and small intestine look for finger-like 
projections from the surface of the gut (pyloric c&ca). If present, cut one: is it 
solid or hollow? Examine the membrane (mesentery) which holds the intestine 
in place. To what part of the body wall is it attached? The spleen, a dark-red, 
ductless gland, occurs close to the intestine. Cut intestine an inch from the anus 


and the esophagus in front of stomach. Remove, open, and examine interior. 
Figure differences in different portions. Look for parasites in the tract. 

2. Reproductive and Excretory Organs. Find the whitish testes or the yellow 
or pinkish ovary (or ovaries). Do they possess ducts? Where is the outlet? 

Observe position and dimensions of the air-bladder (if present) ; pierce it and 
discover dorsal to it the red kidneys. Number, shape, and dimensions of these? 
Can you find their outlets? 

3. Pericardial Cavity and its Contents. 
Shape and boundaries of the cavity. 

Heart: position; portions; ventricle (ventral) and auricle (more dorsal). 
From the ventricle the whitish bulbus arteriosus passes forward and 
narrows into the ventral aorta. Posterior to the heart is the thin-walled 
sinus venosus which communicates with the auricle. 

[The teacher may supplement this work by a demonstration of the ventral 
aorta with its branches passing to the gills, by means of a larger fish in which these 
vessels have been injected with a colored mass. See appendix.] 

4. The Nervous System. Cut off the head and remove the muscles from the 
back and top of the skull. Use a strong cartilage knife and gradually slice and 
pick the bone until the cavity within is well uncovered. Note the loose tissues 
covering the brain. Remove this with great care. 

Beginning in front, identify as you pass backward: olfactory lobes, tapering 
toward the front and communicating with the nasal cavities; cerebrum, two oval 
prominences meeting in the middle line; the two large, rounded optic lobes; the 
cerebellum, a single median lobe; and the medulla oblongata, which tapers backward 
into the spinal cord. Is there any real boundary between the spinal cord and the 
medulla, or is the distinction arbitrary? What is the size of -the cord where it 
emerges from the cranium? What is its position in relation to the vertebrae? 
Have you found any nerves leaving the medulla or the cord? If so how many? 
Are there any cavities in the brain lobes? 

5. The Eye. Remove the bone from above the eye and examine it in position. 
How is the eye moved in life ? Can you discover any of the muscles effecting these 
motions? How are they attached? What is the shape of the eye? Split it 
open, and find the lens. Is the lens more or less nearly spherical than you expected ? 

6. The Skeleton. The general shape and character of the skull and its bones 
may be seen by boiling the head of another fish and scraping and picking away the 
flesh. The principal regions are the cranium or brain case, the opercular bones of 
the gill covers, and the facial bones. Notice the loose way in which the lower jaw 
is articulated. 

Boil a two inch block taken from the tail of the fish until it becomes tender. 
Notice incidentally the shape of the individual myotomes or muscle segments as 
they fall apart. Clean the vertebrae of flesh, and study the structure of one of 
them. Note the centrum; the dorsal or neural arch and spine; the ventral or hamal 
arch and spine. What is the shape of the centrum? What structures occupy 
the arches? Prepare a trunk vertebra and compare in all respects with the caudal. 
How are the ribs related to the vertebra? Can you find any evidence whether 
they are homologous with the haemal processes ? 

Are there any bones connected with the fins? 

D. General Questions. What internal organs show segmentation? Do they 
show it equally in all parts of the body? Do the internal organs show bilateral 


symmetry as completely as the external? How do you account for the fact? 
Compare the relative position of the anterior and posterior appendages in as many 
species of fish as you can secure? What are the habits of the species you have 
been studying? Feeding habits; spawning and breeding habits? What are its 
nearest relatives among the fishes? 

E. Studies in Nature. Many most interesting and instructive problems present 
themselves for the student of fishes in their native haunts, if the school is favorably 
situated. What species are found in the nearby waters? How do they differ in 
appearance? How in habits of feeding; of swimming; of spawning? What kind 
of habitat does each prefer? What knowledge must a successful fisherman of a 
given species have? 

342. The Frog.* (Rana). Any species of frog will serve. 
For internal anatomy as large specimens as possible should be 
used. The frog is especially suitable to represent the verte- 
brates because of its metamorphosis from a water-breathing 
or fish habit into the air-breathing condition, and the readiness 
with which the main facts of this metamorphosis may be fol- 
lowed even by an elementary class. Frogs may be kept alive 
almost indefinitely, even through the winter, by putting them 
in a deep box covered with netting, in which a pan of water 
is placed. The bottom of the box should be covered by sod 
or moss which must be kept moist. Change the water in the 
pan every few days. Do not place large and small frogs in 
the same box, as the small ones are more than likely to disap- 
pear. Unless living animals, as grasshoppers and the like, 
can be given them it is scarcely worth while to try to feed 
them. They seem to do quite as well without food for a 
reasonable length of time. 

A. The Living Animal (chiefly physiology). One or more 
exercises may well be given to observation of frogs in their 
native haunts, if the study comes at such time as will allow. 
You will know enough about the frog to set for discovery a 
number of .interesting things. Keep careful notes of all your 
questions and your discoveries. If this is not possible, much can 
be gained by studying living specimens in the laboratory. Record 
what you know from observation of the animal's general haunts 
and habits. To what extent is it a terrestrial animal? 
Aquatic? What is the natural position when at rest? What 
are its modes of locomotion on land? Place on the floor, and 
test. Describe its motions in water, and the use made of the 


parts of the body in swimming and in its other methods of 
locomotion. Can it rest at the surface of the water? How 
much of the body protrudes from the water? How does it 
dive? Can you find any evidence that it does anything to 
increase its specific gravity when diving ? 

Feed by placing living grasshoppers or flies in the vessel 
with a frog, or by dangling a piece of meat in front of it at 
the end of a string. Note the action of the tongue in making 
the capture. Examine the mode of attachment of tongue, 
and suggest its possible advantages. 

Watch the animal while floating at the surface of water or 
out of water. Can you detect any signs of breathing? Note 
carefully the nostrils, the cheeks and the sides of the abdomen,, 
and determine how it gets air into its lungs. Determine 
what senses are represented in the frog. How does it react 
to salines, acids, sweets, bitters? Judging from the position 
of the eyes and from experiment, can a frog on the ground 
see objects in all directions? Can it do so while floating on 
the surface of the water ? Are the eyes movable ? Can the 
frog see any point with both eyes at the same time? What 
happens if a frog is placed on his back? Explain. Does the 
frog seem to have any sense of elevation' when he comes to the 
edge of a table? How do you estimate this? By experiment 
and by reference determine whether the frog's actions are at 
all modified by experience. 

Select a small frog and chloroform it until quiet, but not 
enough to kill it. Wrap it in a wet cloth, and place on a support 
of such height as will allow the web to be stretched over the open- 
ing in the stage of the microscope. With the lower power note 
the pigment cells and blood vessels. Determine which are 
arteries and which veins; present your evidences. By placing a. 
little water and a cover-glass on the web the high power may be 
used, and the behavior of the corpuscles studied as they pass 
through the capillaries. Similar studies may be made on the: 
gills of very young tadpoles. 

B. External Anatomy. What is its symmetry? Compare 
carefully the structure and form of the dorsal with that of 
ventral surface; similarly those of the anterior and posterior 


ends. Compare several individuals as to shape, color markings, 
size, etc. 

General form. 

Head, trunk, limbs. Is there any neck ? 

Anterior appendages: arm, forearm, hand (including 
digits). Compare with your own hand, and deter- 
mine which is the first digit, or the thumb side of 
the hand. 

Posterior appendages: thigh, shank, ankle, foot. How 
many digits ? Which is the first ? How many joints 
in each? What other peculiarities are noteworthy? 
Special head structure. 

Mouth; position, dimensions, degree of extensibility; 

tongue ; teeth, where located ? 

Sense organs: position of eyes, ears, nose. Do the nasal 
openings communicate with the mouth? Pierce the 
tympanic membrane in a dead frog and discover with 
what the opening communicates. 
Cloacal opening. 

C. Internal Anatomy. Make a slit in the skin of the ventral 
surface from a point just in front of the cloaca forward to the 
throat, a little to one side of the middle line. Make incisions 
perpendicular to this and turn the flaps back to show the 
muscles beneath. Is the skin as closely attached to the muscles 
as in the fish ? Do you find myotomes as in the fish ? Draw 
in outline some of the more important muscles of the chest and 
abdomen. Cut the muscular wall in the same way, passing to 
one side of the breast bone. Turn back the flaps and sketch 
and identify the organs in their position in the ccelom. 

i. Digestive Organs. Pressing the liver aside, identify the 
following parts of the digestive tube: esophagus, stomach, 
small intestine, large intestine. Are there any sharp boundaries 
between these parts ? 

Compare the lengths of the different portions. Find the 
mesentery, and show its relation to the intestines. What is 
the relation of the liver to the digestive tube? Find the gall- 
bladder : does it have any duct Reading to the tube ? What is 


the position of the light colored pancreas? Of the darker 
spleen (this organ has no duct) ? 

Cut the large intestine about an inch from the anal opening 
and the esophagus in front of the stomach; remove the tract 
from the body. Split it from end to end, wash it of its contents 
and describe and make drawings of the interior. Do you find 
any internal parasites ? 

2 . Urino genital Organs. Without removing any other organs 
identify : 

The kidneys: color, form, position. Do they have any 

outlet? The bladder; position and general structure. 
The fat bodies : position ? With what connected ? 
In the male: 

Testes; yellowish, rounded bodies. With what organ 

connected ? 
How in a fresh specimen can you be sure you have found 

the testes ? 
In the female : 

Ovaries, which vary much in size and appearance with 
the time of year. What are their position and at- 
tachments ? 

Oviducts. Do these communicate with the body cavity ? 
How do they communicate with the exterior ? Is there 
any trace of the oviducts in the male ? 

3. The Lungs. Open the mouth, find the glottis, insert a 
blow-pipe, and inflate the lungs: number, position and shape? 
Cut them open and examine the inner surface. 

4. The Circulatory System. 

The heart: Does this organ lie free in the body cavity? 
What is the shape of the heart ? To what is it attached ? 
Identify the auricles in front, and the ventricle behind. Can 
you recognize the aorta arising from the ventricle; and the 
venous sinus dorsal to the heart and receiving the large veins? 
How many chambers to the heart? Their relation to each 
other ? 

Further study can be pursued successfully only by injecting 
the vessels with a colored mass. If time allows each student 
may well inject the arterial system and trace its course. At 



least a specimen thus injected and dissected by the teacher 
should be used to demonstrate the three aortic arches (carotid, 
systemic and pulmonary), the dorsal aorta and its chief branches. 
5. Muscle. Strip the skin from the leg like a stocking. 
Without cutting, separate the muscles from each other, demon- 
strating their general shape and the tendons at the ends by 
which they are connected with the bones. The end attached 
to the least movable bone is the origin, the other, the insertion. 
What is the origin and what the insertion of the large muscles 
of the thigh? Are the muscle fibres plain or cross-striate ? 
(Examine a small bit under the microscope after teasing it 
apart as much as possible.) These studies may very well be 
extended to include the muscles of the throat, chest, arm, ab- 
domen, and leg. In all such studies the most important ques- 
tion the student can ask and answer is this: "What does this 
muscle do when it contracts?" To answer the question one 
must find the origin and insertion of the muscle, as well as its 
relation to other muscles. 

6. Nervous System. Remove with great care the skin, muscles and bone from 
the roof of the skull so as to expose the brain. Continue backward and expose the 
anterior portion of the spinal cord. Sketch, as it appears from above, and identify, 
beginning with the anterior end: 

Olfactory lobes. 

Cerebral hemispheres; number, size, form. Are they separate? 

Optic lobes. 

Cerebellum; a narrow transverse band. 

Medulla oblongata, tapering into the spinal cord. 

Examine the nerves arising from the spinal cord. Look within the body cavity 
also for these. What is their position in relation to the vertebrae? How many 
pairs can you discover? Does each arise by a single or double root? Find the 
large nerve (sciatic) which is the chief nerve of the hind leg. How many spinal 
nerves enter into the formation of it ? Seek a similar plexus in connection with the 
front leg. Seek the sympathetic ganglia in body cavity on either side of the 

Dissect the bone and muscle from one side of the skull, showing the cranial 
nerves. Begin at the anterior end and identify: (i) the olfactory nerve; cut, and 
lift the brain slightly, showing (2) optic nerves. Cut these as far from the brain as 

Note other smaller nerves and cut these. How many are there? From what 
part of the brain do the majority of them arise? Do the optic nerves join at the 
point where they enter the brain? 

7. Skeleton.- Pick the bulk of the flesh from the bones of an uninjured skeleton. 
A few minutes of boiling will be of advantage in the final stages of cleaning. 



Identify the axial skeleton and the appendicular . Do the appendages unite directly 
with the axial skeleton? Count the vertebrae. To what extent do they differ? 
Can they be grouped into regions? Select a typical one, and draw from various 
positions to show structure. Do any bear ribs? Describe the posterior, bone in 
the series. Identify the parts of the anterior and posterior limbs and girdles by 
referring to Fig. 161, and see to what extent they depart from the type described 
there. Make outline sketches of all the bones of the right girdles and appendages. 
What is the nature and action of the various joints of the limbs? In the skull 
notice how small a portion is brain-case. How is the great width of the head 
secured? How is the lower jaw related to the skull? Make 'a sketch showing 
the proportions of these various parts. In what position are teeth borne? Ex- 
amine the sternum or breast bone. How related to the girdle? Of what parts is it 
composed. How much of it is cartilaginous? 

8. Development. Eggs of frogs and toads may be found in the early spring in 
ponds or sluggish streams, floating or attached to submerged objects. They 
occur in slimy strings or masses, each egg enveloped in a jelly-like covering. 
Transfer these to the laboratory, and keep covered with water in a shallow vessel. 
Change the water frequently, and keep a close watch on the changes which they 
undergo. After hatching keep a few water plants in the vessels for the tadpoles to 

Note appearance of the egg (with low power of microscope). 
Gelatine: outer layer, not really a part of the egg. 
Fertilized ovum; the darker interior sphere, of protoplasm and yolk. 

If the eggs are recently laid, the beginning of segmentation will furnish an 
interesting demonstration for the class. How are the first cleavage planes related 
to each other? 

If more advanced, note especially: the gradual elongation of the embiyo, the 
enlargement of the head, development of the tail, hatching, the external gills. 
What becomes of the gills? Do you find any trace of mouth, eyes, nasal openings? 
Where do the legs first appear? What becomes of the tail? Prove. Tadpoles 
of all ages may usually be found in the shallow ponds late in spring. These should 
be compared with those reared in the laboratory. Dissect one of the larger tad- 
poles, and examine particularly the intestine and the gill chamber. 

343. Compare with the frog any other amphibian types which can be found, 
as the toad, the newt, or the salamander. Note especially the differences in habits, 
haunts, external form, appendages, method and time of depositing eggs, the form 
of the tadpoles, etc. 


344. General Characters. In common with the simpler 
Chordata thus far considered the Vertebrata are bilaterally 
symmetrical Metazoa with a ccelomic cavity, a notochord de- 
rived from the entoderm, gill-slits at some stage of life, dorsal 
nerve tube and a ventral heart. In addition, the following 
points may be given as distinguishing the true vertebrates: 

i. The notochord comes to be surrounded by a sheath of 
tissue derived from the mesoderm. This produces around the 


notochord the internal skeletal axis, the centra of the vertebrae, 
composed either of cartilage or bone (Figs. 156-158). 

2. Outgrowths from these centra pass dorsally to protect 
the nerve tube, and ventrally to protect the viscera (Fig. 159). 

3. Several sets of organs show varying degrees of meta- 
meric segmentation: e.g., vertebral column; muscular system; 
nervous system. 

4. Jointed appendages having a central skeleton never ex- 
ceed two pairs; one pair or both of them may be rudimentary 
or wanting. 

5. The respiratory system is developed in connection with 
the anterior end of the digestive tract. 

6. The heart always has as many as two chambers and the 
blood contains red corpuscles. 

345. General Form. While varying greatly in form, ver- 
tebrates are typically elongated animals with the mouth at or 
near the anterior extremity of the long axis. The position of 
the anus is variable. It may be one-half the length of the 
body from the posterior end. The body is roughly divisible 
into head and trunk with or without an intervening neck. The 
neck is more pronounced in the land than in the water forms. 
Posterior to the trunk containing the body cavity, there may 
be a tail into which the skeleton is continued but which is 
destitute of a body cavity. 

Bilateral symmetry is shown by the paired condition of the 
eyes, ears, and other external and, to a less degree, internal 
organs. Metamerism on the contrary is much more evident 
from the internal than from the external organs. There are 
usually two pairs of lateral appendages for support and loco- 
motion: the thoracic at the anterior end of the trunk, and the 
pelvic, ordinarily occurring near the union of the trunk and 
tail. These are variously modified as to their form and in- 
ternal structure (e.g., fins, legs, arms, wings), but are looked 
upon as homologous. In many water forms there are median 
appendages (dorsal, ventral, and caudal fins) also assisting in 
locomotion. The ccelom or body cavity is well represented in 
the trunk region, and arises by a splitting of the mesoderm into 
an inner layer which comes to unite with the digestive tract 


and an outer layer which unites with the ectoderm (Fig. 156). 
In this the visceral cavity beside the mesoderm-covered 
digestive tract to which reference has already been made, lie 
the principal organs of respiration, of excretion, of circulation, 
and of reproduction. Dorsal to the notochord the nervous 
system occupies a cavity within the mesoderm, which is not, 
however, a part of the ccelom. This is described as the dorsal 
or neural cavity and is protected by a sheath of cartilage or 
bone. In the anterior region this is much enlarged to accom- 
modate the brain. This condition of a dorsal and ventral 
cavity is very characteristic of vertebrates. In mammals the 
ventral cavity is further divided by the diaphragm into an 
anterior or thoracic and a posterior or abdominal cavity. 

346. Protective and Supportive Structures the Integu- 
ment. Covering the body of vertebrates is the skin, which 
consists of two layers; the outer, or epidermis, which is de- 
rived from the ectoderm, and the dermis or true skin which is 
mesodermal in origin. The epidermis consists of from two 
to many layers of cells in thickness, and in the higher forms 
the differentiation into layers becomes very pronounced (Fig. 
155, E). The outermost cells of the epidermis frequently be- 
come hardened for the better protection of the parts within. 
This is especially true of the terrestrial forms. The inner layer 
of the epidermis is usually columnar in form, and from this 
layer the outer cells are renewed, and all special epidermal 
growths arise (Fig. 155, c.e.). The dermis consists largely 
of connective tissue, but contains in addition nerves and blood 
vessels beside such ingrowths from the epidermis as glands, 
hair-follicles, etc. Fat is deposited in the lower layers of the 
denims in many vertebrates. 

347. Special Products of the Integument often occur in the 
form of outgrowths or ingrowths. Glands are examples of the 
latter, and are frequent in connection with the epidermis. They 
may be simple and unicellular (mucous glands in fishes) or 
multicellular, penetrating deep into the dermis (sweat and oil 
glands, Fig. 155, sg). The mammary glands of Mammalia 
are modified forms of the oil or sweat glands. The outgrowths 



may be purely epidermal, as in hair, feathers, nails, hoofs, claws, 
and the scales of some reptiles ; or in other instances the principal 
structures are formed in the dermis, usually with an outer layer 

FIG. 155. 

FIG. 155. Diagram of the skin in Mammals, showing the multiple layered condition, together 
with outgrowths and ingrowths. Drawn by Dr. J. W. Folsom. E, epidermis; D, dermis; a, adi- 
pose tissue, fat deposited amid the connective tissue; b, blood vessels; c.e., columnar epithelial 
layer of the epidermis;/, hair follicle; h, hair; n, nerve; n.e., nerve ending (sensory corpuscle); p, 
pore of sweat gland; seb., sebaceous or oil gland of hair; s.g., sweat gland; s.c., horny layer of epider- 
mis; s.m., mucous layer of epidermis. 

Questions on the figure. What suggests that the columnar layer of the epi- 
dermis is the most vitally important layer? Are the hair and glands dermal or 
epidermal growths? Which structures found in the dermis seem to be invasions 
of that layer by outside structures? What are the functions of the various layers 
of the skin? Which parts are ectodermal and which mesodermal in origin? 

contributed by the epidermis, as in the teeth or the scales and 
bony plates which form in many instances (turtle, armadillo, 
etc.), a very complete external skeleton. Some of the bones 



of this external, or dermal, skeleton persist even in the highest 
forms (e. g., man) and unite with bones of the internal skeleton, 
as in the formation of the cranium and the facial bones. 

The most apparent function of the skin is protection. The 
outgrowths (hair, scales, claws, etc.), evidently increase its 
adaptation to this function. In addition, the skin is partly 
respiratory and excretory. The glands represent a specializa- 
tion of this latter function. It is also sensory, and in an indirect 
way assists in regulating bodily temperature, especially in the 
warm-blooded types. 

348. The Skeleton. Attention has already been called to 
the exoskeleton as the derivative of the skin. The endoskele- 

FIG. 156. 


FIG. 156. Diagram of transverse section through embryo of a Vertebrate, showing the mode 
of origin and the relations of the notochord, nervous cord, ectoderm, entoderm and mesoderm (see 
also Fig. 15). coe., ccelomic pouches; eel., ectoderm; ent., entoderm; g, lumen of the gut; IP., invagi- 
nation of ectoderm which forms the nerve cord (see c, in succeeding figures) ; mes 1 ., somatic or body 
mesoderm; mes*., splanchnic mesoderm, that portion of the mesoderm which becomes allied with 
the entoderm; n, the nerve (spinal) cord; n.c., notochord, arising by an outpocketing of the entoderm. 

ton is surrounded by muscles separating it from the integument. 
In general it may be said that these two bony systems supple- 
ment each other. In the higher forms where the internal 
skeleton is best developed the exoskeleton is usually reduced 
to a minimum. Elements from both sources may become fused 
fn the formation of a single structure (the skull; the carapace 
oi the turtle). A difference between the internal and the 


external skeleton is in the fact that bone of the former is typically 
formed in and around cartilage, whereas in the latter there is no 
cartilage. The internal skeleton consists of two portions, (i) 
the axial, embracing the vertebral column, and (2) the appen- 
dicular, or that supporting the appendages. 

As was seen in the arthropods the chief functions of skeletons 
are to protect and support soft parts, and to furnish hard lever- 
ages for muscular attachment. It is clear that any rigid skeleton 
which entirely surrounds an elongated animal must have some 
breaks or articulations in it. We found this to be true in 
Crustacea and insects. Similarly in the internal skeleton in 
vertebrates, the attached muscles would be useless if there were 
no joints. It is clear that solid bones coming together in a joint 
with the muscles external to them will make a stronger articula- 
tion than hollow shells coming together like the segments of a 
stovepipe. The arthropod joint is effective for small animals 
but would not meet the needs of the large vertebrates. 

349. Axial Skeleton. In its simplest condition this consists 
of the notochord which it will be remembered is derived from 
the entoderm and lies between the alimentary canal and the 
spinal cord (Fig. 156). In the true vertebrates, cells arising from 
the mesodermal pockets on either side (Fig. 158) produce a 
continuous skeleton-forming sheath about the notochord. 
From the cells of this sheath are developed, finally, rings of 
cartilage or bone about the notochord (centrum; pleural, centra, 
Fig. 159, c) and about the spinal cord (spinous processes, Fig. 
159, no). These, with certain other growths, constitute the 
typical vertebrae. In this process the notochord often becomes 
obliterated by the developing vertebrae. To each vertebra 
may be attached a pair of ribs, which protect the ventral 
structures, somewhat as the neural arch protects the nerve cord. 
The ribs of fishes and of the higher forms are not considered 
to be homologous structures (Figs. 159, 160). 

The axial skeleton varies from this typical condition in dif- 
ferent parts of its course. In the head region, for example, 
the nervous cord is immensely enlarged and the neural arches 
are much modified, being replaced by plates and supplemented 


3 2 9 

by the dermal bones. The following regions may be described 
as typical : 

1. Head region (skull) embracing the cranium or brain case 
and its associated ventral arches including the bones of the face. 

2. Cervical vertebrae, located in the neck and lacking ribs. 
Usually the anterior one or two are considerably modified. 

FIG. 157. 

FIG. 158. 



FIG. 157. Diagram similar in position and lettering to Fig. 156, at a later stage, c, central canal 

of spinal cord. 

Pic. 158. Transverse section of an embryo Vertebrate at a stage later than Fig. 157. m, 
mesentery; sk., the beginning of the mesodermal skeleton which surrounds the notochord (n.c.), 
and in part the spinal (nerve) cord, n. 

Questions on the figures 156 to 158. How does the mesoderm originate in 
vertebrates? Trace its gradual growth and differentiation in the figures. What 
two principal portions are to be distinguished? How does the notochord arise? 
How the spinal cord ? What is the source of the cavity of the spinal cord ? From 
which of the three layers does the protecting skeleton arise? What does the 
mesentery connect? What other organs might be expected in the ccelom, if it 
were the purpose to make a complete diagram of the visceral organs? 

3. Dorsal vertebrae, in the thoracic region and bearing well- 
developed ribs which may unite with a ventral bone, the 

4. Lumbar vertebrae, following the dorsal vertebrae and not 
bearing ribs. 

5. Sacral vertebrae, usually a small number of vertebrae, fre- 
quently fused into one piece with which the girdles of the 
posterior appendages unite. 



6. Caudal vertebrae, posterior to the sacrum and possessing 
no ribs. 

The number of bones in these regions is very variable in 
the phylum as a whole, but, in the higher forms particularly, 
individuals of related species present remarkable uniformity. 

(The discussion of the condition of the skull and the origin of its parts is entirely 
too technical for an elementary text. The student should be referred to more 
advanced works.) 

FIG. 159. 


FIG. 159. Diagram of veterbrae of a bony fish. A, caudal; JB, trunk, c, centrum or body of 
the vertebra; ch., the notochord; h.a., haemal arch; h.c., haemal canal; h.s., haemal spine; h.z., haemal 
tygapophysis, or articulating facet; m.b., inter-muscular bone; n.a., neural arch; n.c., neural canal; 
.$.. neural spine; M.Z., neural zygapophysis; r, rib. 

Questions on the figures. What is the meaning of haemal? Of neural? In 
life what occupies the neural canal ? What occupies the haemal canal in the caudal 
region? In the trunk region? Is there anything to suggest that the ribs in fishes 
are homologous with the processes which form the haemal arch (h.a.)? 

350. The Appendicular Skeleton. Here are embraced the 
skeletal parts of the appendages proper, together with the bones 
binding them to the axial skeleton (girdles). Each girdle may 
be said to consist typically of three bones, uniting to form a joint 
with the first bone of the limb. One of these is dorsal and the 
others ventral (Fig. 161, B; il, is, p.}. The appendages are 
much alike both as to their girdles and the limbs proper. The 
posterior is, in higher forms, more intimately fused with the 
axial skeleton, thus securing greater strength at the expense of 
freedom of motion. The first joint of each appendage consists 
of one bone (arm or thigh); the second, of two (forearm or 


shank) ; then follows a region of several small bones (wrist or 
ankle), succeeded by the hand or foot with five (usually) bones, 
and then by five digits (fingers, toes) of a varying number of 
joints. The accompanying diagrams (Fig. 161) will make 
clear these relations, as well as the names of the bones. Bones 
may disappear or fuse with others in such a way as to cause a 
wide variation from this type; indeed the type is perhaps never 
realized in any single animal. In fishes the appendage and the 
girdle are often very simple, the limb being little more than 

FIG. 160. 

FIG. 160. Diagram of a trunk vertebra in a Mammal, c, centrum; ch., position originally 
occupied by the notochord; h., head of the rib; h.c., haemal cavity; n.a., neural arch; n.c., neural 
canal; n.s., neural spine; r. t rib; st., sternum; s.c., sternal cartilage uniting ribs and sternum; t.p., 
transverse process of vertebra; tu., tubercle of rib. 

Questions on the figure. Compare all the parts here with corresponding ones 
of Fig. 159: ^ B, and note the differences. What is gained by the articulation 
of ribs with a sternum? What is lost? In which groups of Vertebrates is a 
sternum found? In which are fully developed ribs found? 

radiating fin-rays covered by a membrane (Figs. 176, 177). 
Yet it is believed that from some such primitive condition the 
more specialized appendages have arisen. 

351. The Digestive Organs. As in many of the inverte- 
brates which we have studied, the alimentary canal in the 
vertebrates possesses an anterior, ectodermal portion (stomo- 
dasum), a mid-gut lined with entoderm (mesenteron), and a 
posterior ectodermal part (proctodaeum) . The tract is lined 
throughout with a mucous membrane. Outside of this are the 


m.c. 1 - 


, fa.-* 

M ct -~n<> 



PIG. 161. Diagrams of the girdles and appendages in a typical Vertebrate. A, anterior; B, 
posterior, ac., acetabulum, articulation of the humerus with its girdle; c, coracoid; ca., carpals; 
c.e.. centralia; d.c., distal carpals; d.t. t distal tarsals; ^..elbow joint;/, fibula;/*., femur;/*., fibulare; 
gc. t glenoid cavity, articulation of arm with girdle; h, humerus; il., ilium; in., intermediale; is. 
ischium; kn., knee joint; m.c., metacarpals (1-5); m.t., metatarsals (1-5); P, pubis; ph., phalanges 
(J-S); Pr-c., precoracoid; r, radius; ra., radiale; sc., scapula; /, tibia; la., tarsals; <*'., tibiale; ., ulna; 
ul., ulnare. 

Questions on the figures. Compare the two appendages throughout and note 
the corresponding bones. How much is girdle? How much appendage proper? 
How many carpals? Tarsals? Which are proximal? Which distal? How do 


the phalanges differ? Which is the thumb? How can you be sure? Compare 
this figure with figures (in reference texts) of the appendages, both front and rear, 
of the frog; of some bird; or some carnivore; of the horse; of man. Where are the 
greatest variations, i.e., which bones depart most from this typical condition? 

layers of unstriped muscle fibres, circular and longitudinal, 
by which the food is forced onward. The muscles are espe- 
cially developed in certain regions, as in the stomach. Outside 
of all these, in the portion passing through the body cavity, is 
the serous membrane derived from the mesoderm, a portion 
of the lining of the body cavity. The mucous surface which 
is, -naturally enough, the important portion in digestion and 
absorption may be increased by the lengthening of the tube as 
a whole or by means of outgrowths (the glands) or by ingrowths 
(folds of various kinds) . The highly nourished condition of the 
entodermal sheet of cells presumably leads to their rapid growth 
and foldings. The folds are often so arranged across the axis 
of the tube as to retard the progress of the food through the 
tract, thus making digestion and absorption more complete, by 
increasing the time during which the food is exposed to the 
action of the digestive juices, and to the absorbing surface. 

352. The Divisions of the Tract. The mouth, which may be 
either terminal or ventral, opens into the buccal cavity, which is 
bounded dorsally by the floor of the brain case, on the sides and 
in front by the jaws, and ventrally by a muscular floor from 
which the tongue arises as a fold. The jaws are made up of 
bony elements from two sources; a core of bones from the internal 
skeleton (the first visceral arch) and a covering of dermal bones 
which fuse with it. The latter are the bones which (typically) 
bear the teeth. Teeth however occur in the lower vertebrates 
in the roof of the mouth or on the tongue. Their place may 
be taken by horny epidermal structures, as in the beak of birds. 
When present the salivary glands open into the mouth cavity. 
Posteriorly the buccal cavity communicates with the pharynx, 
wliich may be. defined as the part of the digestive tract in con- 
nection with which the lungs or gills are developed. In fishes 
and in the embryos of higher forms there are slits in the side 
walls connecting the pharynx with the outside. Gills are 
developed in the walls of these slits. In forms above fishes the 



slits become closed as the embryo develops. Above the 
Amphibia they never bear gills. 

The esophagus is a narrow muscular tube of varying length 

FIG. 162. Stomach of Dog (A) and of Rat (U). c t cardiac portion; p, pyloric portion; o, 

esophagus; *', intestine. 

FIG. 163. 

Pic. 163. Diagram of the stomach of a ruminant, o, esophagus; r, rumen or paunch; re., reticu- 
him, or honeycomb; p, psalterium or many plies; a, abomasum or rennet; i, intestine. 

Questions on figures 162 and 163. Taken as a series, what is illustrated by the 
three diagrams? What do the arrows indicate? What is known of the function 
of the various portions of the ruminant stomach? 

leading to the stomach. In birds an enlarged portion of it 
(the crop) may serve for the temporary storing and softening of 
the food. 



The stomach is usually well differentiated and may consist 
of one chamber or of several. In the latter case there is a 
division of labor among the parts. One portion may be highly 
muscular and supplied with a hardened internal lining for 
grinding the food (gizzard of fowls, Fig. 164); in such in- 
stances another portion is glandular. In the ruminants (ox, 
deer, etc.), there are four chambers in the stomach (Fig. 163). 
The gastric glands produce an acid secretion which contains a 
ferment acting chiefly on proteid foods. 

FIG. 164. 

PIG. 164. Diagram of the stomach and esophagus of the Fowl, o, esophagus; c, crop; p, pro- 
ven triculus or glandular stomach; g, gizzard or grinding stomach; *', intestine. 

Questions on the figure. Compare this figure with that of the stomach of 
ruminants as to complexity* What are the functions of the various portions? 
What changes take place in the gizzard of flesh-eating birds if they are forced to 
live on grain? Why is the crop located outside the cavity inclosed by the ribs? 

The food is retained in the stomach by means of a circular 
(sphincter) muscle at its posterior end where it narrows into 
the intestine. This latter portion is the principal digestive and 
absorptive portion of the tract and varies much in length in 
the various groups in accordance with the nature of the food 
used, the vegetable feeders for the most part possessing the 
longest intestines. Numerous circular or spiral folds of the 
mucous membrane occur in the intestine. Special absorptive 
organs (mlli) supplied with blood and lymph vessels may cover 
these folds. Near the anterior end the ducts of the liver 
and pancreas open into the intestine. The liver is the largest 


of the glands, and the pancreas one of the most important in 
digestion. The intestine may open directly on the exterior 
(most mammals), or into the ectodermal pocket (cloaca) which 
also receives the excretory and genital products (reptiles and 

353. Exercises for Field and Library. 

1. What difference have you observed in the number, position, and kinds of 
teeth in the various vertebrates of your acquaintance? 

2. Can you cite from your observation any evidences of adaptation of the 
digestive tract to the peculiar food and habits of the animal possessing it ? Supple- 
ment by library references. 

3. To what extent is food prepared in the mouth, i. e. t antecedent to swallowing, 
in the various vertebrates whose habits you have observed? 

354. Respiration. As in all higher animals there are two 
things to be considered in the respiration of vertebrates: (i) 
the exchange, between the blood and the external medium, 
air or water, of carbon dioxide for oxygen, which may be called 
external respiration, and (2) the exchange by which the blood 
gives the cells of the body oxygen and receives their carbon 
dioxid, or internal respiration. The former is usually meant 
when the simple term respiration is used, though the latter is 
really the vital process. A certain amount of respiration takes 
place through the skin in almost all vertebrates. Beside this, 
special devices both gills and lungs are developed by which 
the blood and the medium containing the oxygen are brought 
into closer relation. In fishes and larval amphibians gills are 
present; in most adult amphibians and in reptiles, birds, and 
mammals, only lungs occur. 

Gills are thin-walled external folds or groups of filaments 
bounded by a mucous membrane, in which the blood circulates 
freely. In vertebrates they are found on the wall of passages 
leading from the pharynx to the outside (gill-slits). The water 
passes into the mouth and out over the gills, through the thin 
walls of which the gases are exchanged. The walls between the 
slits may be supported by cartilages or bones (visceral or gill- 
arches). The gill-slits vary from four to eight in number. In 
the higher, air-breathing vertebrates traces of the gill-slits 
appear in embryonic development, but they never bear gills. 
(See Figs. 32, 33.) 


355. Lungs arise as out-pocketings of the ventral wall of 
the pharynx. These may persist as relatively simple sacs, as 
in the frog, or by great growth and folding they may become 
very complicated, and thus increase their surface to a wonder- 
ful degree. They are lined throughout with the entodermal 
epithelium. The blood capillaries are in contact with this 
layer and through these thin walls the gases are exchanged. 
The outer surface of the lung is covered by the pleura, the 
lining of the body cavity. The tube connecting the pharynx 
with the body of the lung is known as the trachea. The upper 
or anterior end of the trachea is modified into a chamber 
known as the larynx in the air-breathing vertebrates. The 
epiglottis closes the opening (glottis) from the pharynx into 
the larynx, whenever food is passing from the mouth through 
the pharynx into the gullet. On account of the presence 
of currents of air passing in and out and capable of produc- 
ing vibration, certain portions of the tract are used in mak- 
ing definite sounds whereby the animals are put into 
communication with their kind. The parts so used are the lips, 
teeth and vocal cords. The latter are membranous folds in 
the mucous lining of the larynx, which may be brought into 
such a position as to close that organ, in part, to the escaping 
current of air. The tense edges of the membrane are put into 
vibration. The resulting sound, reinforced or otherwise modi- 
fied by the other organs, is voice. Voice has considerable evolu- 
tionary significance in animals ; very much more in man. 

356. Supplementary Exercises for Library. Where does 
the "swim-bladder" in fishes occur? Is anything known of 
its function? Is anything like a lung known among the 
fishes ? 

What are the most important differences between the ' ' voice- 
box" of mammals and that of birds? Have all the vertebrate 
groups vocal organs? Do they all have voice? What is the 
difference between voice and speech? What are the uses of 
voice to animals possessing it ? 

357. Circulation. The blood in vertebrates contains both 
colorless and colored (red) corpuscles. The red coloring 
matter (hemoglobin) has an affinity for oxygen and thus be- 



comes a vehicle for transporting it. The colored corpuscles 
have no motion of their own, but are carried by the blood cur- 
rents. The colorless cells are much less numerous than the red 
and have power of independent motion (amoeboid). The fluid 
in which the cells float is called the plasma and carries the 
food and waste materials of the body in solution. 

The muscular heart always consists of at least two cham- 
bers, (i) an auricle which receives blood from the veins, and 

S. V. 


FIG. 165. Diagrams of the structure of the heart in the lower Vertebrates. A t primitive con - 
dition; B, the position of the parts in the fishes, a, artery; au., auricle; c, conus arteriosus with 
valves; s.v., sinus venosus; t, valves; ve., vein; vent., ventricle. The dorsal portion of the heart is 
toward the bottom of the figure. 

Questions on the figure. Which side of the figure represents the anterior? 
Compare the walls of the vessels. Where are the valves located? How is the 
term "sigmoid flexure" appropriate to the form in B? Notice how it results in 
what is morphologically the posterior portion of the heart becoming anterior. 
Trace the course of the flow of the blood. 

(2) a ventricle which has thick walls and propels the blood 
into the arteries. Morphologically the auricle is the posterior 
portion of the heart (Fig. 165, A), but in development the heart 
has undergone an s-shaped bending which has brought the 
auricle in front of the ventricle (Fig. 165, B). The veins and 
arteries are often specially enlarged and modified in the region 
of the heart. The main trunk leaving the heart is called the 
aorta. As the vessels are followed from the heart they branch 
and become smaller and the walls become thinner. The final 



divisions are the capillaries through the thin walls of which 
the blood exchanges materials with the tissues (Fig. 34, c.s.; 
c.r.). The capillaries unite to form the smaller veins and these 

FIG. 1 66. 

FIG. 167. 

c. v. r._ 

c. v. I. 

FIG. 166. Diagram of the heart, the branchial arches, and the principal veins in the Teleosts. 
Ventral view. The heart is represented without the sigmoid flexure; that is, with the auricle 
posterior. The same is true of Figs. 167 to 170. a, aorta; au., auricle; 6r.o., branchial arches of the 
aorta (1-4, numbering from the front); c, carotid; c.v., cardinal veins (right and left); d.a., dorsal 
arteries; j, jugular veins; d.c., ductus Cuvieri; s.v., sinus venosus; v, ventricle. Only four arterial 
arches are shown. 

Questions on the figure. Refer to the table on page 343 and identify the parts 
there described. Compare this figure with those following (Figs. 167-171). 
Compare also with Figs. 181 and 182, Ch. XX. Which is the anterior and which 
the posterior portion of this and the following figures? 

FIG. 167. Diagram of heart and branchial arches in Ceratodus (one of the Dipnoi). Position 
and lettering as in Fig. 166. a.b., air bladder (lung); p.a., pulmonary artery; p.c., post caval vein 
(right); p.v., pulmonary vein. 

Questions on the figure. What organs appear in this diagram which are not 
present in Fig. 166? What changes of the various portions do you note in com- 
paring the two figures? 

uniting, complete the circuit back to the heart. It is evident 
that the capillaries are the most important portion of the 
system, the part for which the rest in reality exists. 



In all the vertebrates there are certain structures called 
ductless glands. These are always well supplied with blood 
vessels. They seem to manufacture and to pour into the blood 

FIG. 168. 

FIG. 169. 

C -- 

pre. c 



FIG 1 68. Diagram of the heart and branchial arches in Protopterus (one of the Dipno). 
Position and lettering as in the preceding, pre.c., precaval vein, made up of right and left jugulairs, 
subclavians, etc.; post.c., postcaval, made up of the cardinals, right and left. 

Questions on the figure. What are the chief differences between the con- 
ditions here and in the preceding figures: (i) as to the heart: (2) as to arteries; (3) 
as to veins; (4) as to lungs? 

FIG. 169. 

Diagram of the heart and branchial arches in the Frog. 
/., lungs; l.a., left article; r.a., right auricle. 

e.g., carotid gland; 

Questions on the figure. How does the heart differ from that of the Dipnoi? 
How many branchial arches of the aorta are shown? What evidences can you 
find by comparison that the pulmonary arch is derived from the 3d or 4th branchial ? 
What evidences that the carotid and systemic are the first and second respectively ? 
Compare with the table on page 343. Is there anything to indicate that the im- 
purest blood in the heart will go to the lungs? 

directly certain secretions that have effect on even remote parts 
of the body. These glands may properly be discussed as a 
part of the circulation since they have no other outlets. These 



secretions are called hormones or internal secretions. The 
spleen, the thyroid, the thymus, adrenal bodies, and pituitary body, 
are the chief ductless glands. 

FIG. 170. 

FIG. 171. 



FIG. 170. Diagram of the heart and branchial arches in a Reptile. Position and lettering as in 
preceding figures, l.v., left ventricle; r.v., right ventricle. 

Questions on the figure. Compare this with figures 166-169 and make a note 
of the differences. How much communication is there between the two sides of 
the heart? What tends to insure that the purest blood in the heart shall go the to 
head? That the least pure goes to the lungs? 

FIG. 171. Diagram of the heart and the branchial arches in Mammals. A dotted outline of 
the arches of the Fish is drawn for ready comparison. The auricles are represented in a posterior 
position, as in the preceding figures. 

Questions on the figure. What changes in the heart are shown in this as conv 
pared with former figures? In the systemic branchial arch? Remember that the 
heart is not represented in its normal position; the auricles are really at the anterior 
of the heart (see Fig. 166). Compare this condition with table, page 343. What 
are the grounds for believing that the auricles are, morphologically, the posterior 
part of the heart? 

The spleen is a dark body on the mesentery near the begin- 
ning of the small intestine. In its cavities old red blood cor- 


puscles are broken down, and probably white blood cells are 
produced. The thyroid and thymus glands are paired glands 
which are developed in the neck in relation to the embryonic 
branchial arches. The function of the thymus is uncertain. 
There is iodine in the thyroid secretion. In man the disease 
or removal of the thyroid produces serious disturbances in 
development. It may produce physical degeneracy and defor- 
mity, and idiocy. 

The adrenal bodies are placed near the kidneys, as the name 
indicates. They secrete into the blood a substance known as 
adrenalin which among other things causes the muscles around 
the blood vessels to continue in a state of contraction. In 
this way the blood pressure is controlled. Removal produces 
derangement of functions and death. 

The pituitary body is in part a ventral outgrowth from the 
floor of the brain (see inf., Fig. 172, B). This fuses with tissues 
arising from the pharynx. Excess or deficiency of the secretion 
of this organ causes changes in the growth rate of certain organs 
and in a number of the metabolic processes. Removal of the 
organ in dogs causes functional disturbance and death within 
a fortnight. 

358. In Figs. 1 66 to 171 will be found diagrams of the 
circulation in the principal groups of vertebrates. It will be 
seen that there is a progressive complication of the structure, 
involving the heart, veins, and arteries, as we ascend the scale. 
These changes accompany and are partly caused by the change 
from gills to lungs. See also the accompanying table. Locate 
the vessels and trace the changes indicated by means of larger 

359. In fishes the blood passes through the heart only once 
in making the circuit of the body. In all the air-breathing 
forms at least a part of it returns twice, passing from the 
heart to the lung, then back to the heart, and thence to the 
system and back to the heart again. In amphibia and reptiles 
the blood from the lung and from the system mix somewhat 
in the heart, because the partition between the right and left 
sides is not complete, but in birds and mammals the two sides 




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Hepatic portal; veins from 
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capillaries of the posterior 
extremities breaking up in 
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of the heart are completely separated and the pure and impure 
blood are not allowed to mix (see Figs. 169, 170). 

360. Excretion. We have seen that carbon dioxid, one 
of the waste products of the protoplasmic activity, is eliminated 
through the lungs and skin. Water is similarly excreted. The 
most important remaining waste (e.g., urea) contains nitrogen. 
This is taken from the blood by means of the kidneys, a pair 
of organs very complicated both as to structure and develop- 
ment. They lie near the middle line of the body at the back 
of the body cavity. Each gland represents a large number 
of nephridia or tubules similar in some respects to the seg- 
mental organs of worms (Fig. 35), but much more compli- 
cated. The kidneys are always well supplied with blood. 
In fishes, amphibians, reptiles, and birds both arteries and veins 
carry blood to the kidney; in mammals, only arteries. The 
excretion, more or less in solution in water, is poured by the 
tubules into a duct the ureter which may be the final outlet; 
or the ureters may empty first into a urinary bladder, which 
has its own outlet (the urethra). 

361. Reproduction. With a very few exceptions among 
the fishes the sexes are separate in all the vertebrates. The 
sexual elements are derived from modified portions of the 
lining of the body cavity (germinative epithelium). This layer, 
supported by connective tissue, forms the essential part of 
the ovaries and testes, of which there is usually a single pair. 
The eggs vary in size from Jf oo of an inch in mammals to 5 
inches (ostrich), or more in some extinct birds. The outlets 
for the ova and spermatozoa (oviducts and vasa dejerentia) are 
modified portions of the embryonic excretory and kidney ducts. 
Throughout the group there is a close connection between the 
excretory and the reproductive organs. The oviducts may have 
special glands for depositing nutritive or protective material 
about the egg before or after fertilization (as the albumen and 
shell in egg of birds). Fertilization is external in most fishes 
and some amphibia, and internal in the higher groups. Natur- 
ally it must be internal in all forms in which large masses of food 
or shells come to surround the ovum proper, since the sperm 


could not penetrate these after they are once formed. Similarly 
internal fertilization would be necessary in all cases where the 
young are developed within the body of the mother. In all 
such there are special organs and special instincts that lead 
to the introduction of the sperm into the body of the female 
(copulation). This however must not be confused with fer- 
tilization. The uterus is a special portion of the oviduct where 
early embryonic development may occur. (See Figs. 205, 206.) 

362. Development. Those eggs which are fertilized out- 
side develop principally by means of the yolk of the ovum. 
Those internally fertilized may receive, after impregnation, 
additional materials for the further nourishment of the embryo, 
as above noticed for reptiles and birds. The fertilized ova 
may be retained for a longer or shorter time in the oviduct 
or in some modified portion of it (uterus, in mammals) and 
undergo development there. Where this internal develop- 
ment is slight (as in birds) the animals are described as oviparous; 
where it is considerable, as in mammals, and the young are 
free at birth they are described as viviparous. 

The table on page 346 will give some of the facts concerning the early develop- 
ment of vertebrates. It will be found an excellent exercise for the students to 
verify the data collected in this and the preceding table (p. 343). It can readily 
be supplemented by a demonstration of figures from more advanced texts. 

363. The Muscular System. We have seen above (345) 
that the internal layer of the mesodermic pockets comes to 
be united with the digestive tract and furnishes- the non-striped 
muscle fibres of its walls. The external portion, which be- 
comes associated with the ectoderm, gives rise to the muscles 
of the body- wall and those which move the skeleton. The 
fibres of these muscles are cross-striped or voluntary (Fig. 
30). It is by means of them that locomotion is effected. The 
skeletal muscles may be classed as (i) axial, and (2) appen- 
dicular. The axial are well shown in Amphioxus (Fig. 154) and 
the fishes, where the whole body is made up of repeated seg- 
ments (myotomes) of muscle fibres. The muscles segments 
alternate with the segments of the spinal column, as one would 
expect. The appendicular muscles are those which move the 






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limbs. Their general arrangement will be seen from the study 
of the frog. In the higher vertebrates the segmentation of the 
axial muscles becomes less conspicuous especially in the head 
region, and the appendicular muscles become relatively of 
greater importance because of the greater use of the appendages. 
The muscles associated with the entoderm are unstriate and not 
under conscious control. 

364. The Nervous System. The nervous system in verte- 
brates consists of two portions, the central and peripheral. 
The central nervous part embraces the deep-seated organs, the 
brain and spinal cord, and has for its most characteristic feature 
numerous ganglion cells. From these central cells the cell-proc- 
esses or fibres pass to the various tissues of the body, terminat- 
ing in a manner appropriate to the special case whether it be a 
muscle, sense organ, or gland. These nerves and their endings 
constitute the peripheral part of the system. 

365. The Central Organs. The central nervous system origi- 
nates from the ectoderm as a longitudinal groove-like depres- 
sion in the mid-dorsal line of the embryo. The union of the 
edges of this fold produces a tube and an overgrowth of the 
ectoderm separates it from the outside world (Fig. 156). It 
becomes surrounded by mesodermal elements (bone and con- 
nective tissue, Fig. 158), and itself undergoes numerous and 
complex changes.- At the anterior end of the tube occur three 
distinct enlargements (Fig. 172, A). These are known as the 
primary vesicles of the brain, and by the later growth and 
differentiation of their walls they give rise to the five brain- 
regions of the adult. The brain must be considered merely as 
the specially modified anterior portion of the spinal cord. 

Three sets of changes occur in the development of the adult 
vertebrate brain from this primitive condition : 

1 . The axis becomes more or less curved, the concavity being 

2. Out-pocketings of the walls occur, in special regions, 
whose cavities (ventricles) retain connection with the central 
cavity (e. g., the hemispheres). See Fig. 172, h, pi. 



PIG. 172. Diagrams of the brain in Vertebrates, bowing the primitive regions a d their chief 
modifications. A, early stages, showing the anterior enlargement in three lobes: I, II, andJIIt the 
primary vesicles. B, a sagittal section, showing the more fundamental modifications of the walls 
of the primary vesicles. C, frontal section of same. D, lateral view of the brain of the Frog. The 
vertical lines indicate corresponding points in the different diagrams, i, 2, 3, and 4 are the ven- 
tricles of the brain, c.c., crura cerebri; cer., cerebellum (hind brain); h, hemispheres (cerebrum or 
fore brain); inf., infundibulum; med., medulla oblongata; mes., mesencephalon or mid brain; met., 
metencephalon or hind brain; my., myencephalon or medulla; n 1 , olfactory nerve; n*, optic nerve; 
ol., olfactory lobe; op., optic lobes;, optic thalamus; pr., prosencephalon, or fore brain; .., 
pons Varolii; s.c., spinal cord; th., thalamencephalon or "between" brain. 



Questions on the figures. What portions of the adult brain are produced 
from each of the three primary lobes? Where are the principal outgrowths, 
thickenings and thin portions of the wall? In comparison with figure D what 
portions of the brain become highly developed in the higher Vertebrates? Make 
a diagram based on D, which will show the general relation of these parts in man. 
Compare the diagrams with the table on page 350, and verify the statements there. 

3. Thickenings or thinnings of the roof, sides, or floor of 
the tube may produce lobes and affect the size of the cavity of 
the tube. The accompanying diagrams and table will furnish 
an outline from which the teacher may, if he desire, pursue 
the details somewhat further. 

FIG. 173. 


FIG. 173, Diagram of head and brain of human foetus six weeks old (heavy boundaries). The 
dotted line indicates the outline of the brain of a foetus three months old. Note the great growth 
of the hemisphere (A), cer, cerebellum; med, medulla oblongata; mes, mesencephalon; p, pituitary 
body; pr, prosencephalon; s.c., spinal cord; th, thalamencephalon; i, olfactory nerve; a, optic nerve. 
Compare with Fig. 172. 

Questions on the figure. Locate point by point the corresponding regions in 
the diagrams in Fig. 172. What are the chief points of modification? Note par- 
ticularly the great flexures of the organ. Where is the cerebellum of the older 
embryo? What is the nature of the pituitary body in the adult? 

These diagrams do not give the exact proportions between 
the various parts of the vertebrate brain. The student is 
urged to examine figures less diagrammatic in their nature in 
the larger texts. See, Fig. 173; also, Edinger: "Anatomy of 



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the Central Nervous System in Man and Vertebrates," Figs. 21, 

366. That portion of the central nervous system not enclosed 
in the skull is called the spinal cord. It is surrounded and pro- 
tected by the dorsal arches of the vertebrae. The cord is nearly 
circular in cross section, is somewhat enlarged in the regions of 
the appendages, tapers toward the posterior end and is divided 
into symmetrical right and left lobes by a dorsal and a ventral 
longitudinal groove (see Fig. 174, df.). It possesses a central 

FIG. 174. Diagram of a cross-section of the spinal cord through the roots of spinal nerves. 
Drawn by Folsom. c, central canal; d.f., dorsal fissure; d.r., dorsal root of spinal nerve arising from 
the dorsal horn of the gray matter (g); gn., ganglion on the dorsal root; n, spinal nerve; r./., ventral 
fissure; p.r., ventral root of the spinal nerve, arising from the ventral horn of the gray matter; w., 
white matter. (The dorsal fissure in the diagram is broader than it should be.) 

Questions on the figure. What is the structural difference between the white 
and gray matter in the cord? Describe their arrangement. How are the two 
halves of the cord united? Which are sensory and which motor roots? What 
structural differences do you notice in the roots? 

canal continuous with the cavities of the brain. The outer 
part of the cord (Fig. 174, w.) is composed of the white matter 
(longitudinal nerve fibres) and the interior portion, of gray 
matter (a mixture of nerve-cells and fibres) . This is somewhat 
the reverse of the condition found in the brain. 

367. Peripheral Nervous System Spinal Nerves. Groups 
of nerve fibres spring from the gray matter of the cord and pass 
to the organs of the body. These nerves arise in pairs one 
pair to each body segment and pass out between the vertebrae. 
Each nerve has two "roots," a dorsal and a ventral, from each 
of which some of its fibres come (Fig. 174, d.r., v.r.). The roots 


differ in appearance in that the dorsal has an enlargement 
(ganglion) containing nerve cells; the ventral has none. The 
fibres from these two roots combine to form the nerve, but each 
fibre remains independent throughout. It is known by experi- 
ment that the fibres of the dorsal root carry impulses toward the 
spinal cord ("sensory") and those of the ventral root carry 
impulses from the cord ("motor"). In certain regions the 
nerves springing from successive segments of the body may have 
nurnerous interlacing fibres, forming what is known as a plexus. 

368. Cranial Nerves. Those nerves arising from the brain, 
that is, inside the cranium, are called cranial nerves. There 
are ten to twelve pairs of these, but they are not of equal mor- 
phological value, nor are they strictly equivalent to the spinal 
nerves. Some have dorsal and ventral roots, but a much larger 
number have only one group of roots, either dorsal or ventral. 
Some are purely sensory nerves, some are motor, and some are 
mixed. How these nerves are related to the segments of which 
the head is believed to be composed is yet an unsettled question. 

The first or olfactory pair arises from the olfactory lobe of the fore-brain ; its 
fibres, which are purely sensory, are distributed to the lining of the nose, the end 
organ of smell. 

The second or optic nerve arises from the second division of the brain (thalam- 
encephalon), consists of purely sensory fibres, and is distributed to the retina 
of the eye, the end organ of vision. 

The third, fourth, and sixth pairs are purely motor and are distributed to the 
muscles of the eye. The third and fourth arise from the third division of the brain 
(mesencephalori). The sixth nerve arises from the medulla, as do the following. 

The fifth (trigeminal) comes from the anterior portion of the medulla (hind- 
brain) and, like the spinal nerves, has both dorsal and ventral roots. It is largely 
sensory, supplying the skin of the face, mouth and tongue. Motor fibres pass to 
the muscles of the jaw. 

The seventh (facial) is largely motor, is distributed chiefly to the muscles of the 
face and controls facial expression. 

The eighth or auditory is sensory, and is distributed to the inner ear, the end 
organ of hearing and of equilibration. 

The ninth or glossopharyngeal is a mixed nerve and is distributed to the muscles 
and mucous membrane of the pharynx and to the tongue. 

The tenth or vagus arises by numerous roots, has both motor and sensory fibres, 
and is the most widely distributed nerve in the body. Its fibres pass to the pos- 
terior visceral arches, lungs, heart, stomach and intestines. 

We find the cranial nerves and their nerve endings concerned chiefly with the 
higher senses, the muscles of expression, and the sensations and activities involved 
in the fundamental processes of nutrition. 


369. The Sympathetic System which is always distributed to the viscera 
organs is made up of a series of connected ganglia in the dorsal part of the body 
cavity. This system is in connection at various places with the central nervous 
system. It serves to connect the internal organs more intimately, and is reflex in 
its action. 

370. The Special Senses. The sense organs represent 
specialized terminations of the nerve fibres, or special epithelial 
cells which have become associated with such fibres (Fig. 43). 
Prom the very nature of the case they must be external. In the 
case of higher animals, the more complicated sense-organs are 
removed from the surface and are much modified, but the 
essential sensory portion is similar in all, and they retain some 
suitable connection with the outside. It is usually these ac- 
cessory structures which transmit the stimuli to the nerves that 
render the sense organ so complicated. 

371. The Skin Senses. Scattered over the body of many 
forms of animals are single cells, or groups of cells, or free nerve 
endings, which are for the reception of contact and temperature 
stimuli. These are not equally numerous or well developed in 
all parts of the body. They are often especially developed in 
connection with hairs. In the lower aquatic vertebrates, espe- 
cially the fishes, groups of snich sensory cells occur in pits or 
longitudinal grooves along the sides. These are called the or- 
gans of the lateral line. Their exact function is still in some 
doubt. They may possibly assist in the determination of the 
chemical condition of the water or in determining the equilib- 
rium of the animal in the water. 

372. The Chemical Senses Taste and Smell. The chem- 
ical senses involve close contact and a chemical union between 
the substance to be perceived and the organ itself. For that 
reason the substance must be capable of solution in the fluids 
that moisten the surfaces. In vertebrates these organs are 
located at the anterior end of the body and usually within special 
pits or cavities. The taste organs are in the mouth, especially 
on the tongue and soft palate. In some animals the sense is 
poorly developed. The end organs of the sense of smell are 
located in pits (nose), anterior or dorsal to the mouth, lined 
with folds of the mucous epithelium. In most fishes these pits 



are not connected with the pharynx, but in all air-breathing 
forms there is such connection, and the nostrils constitute the 
normal inlet for air to enter the lungs. The sense of smell is 
much more developed in the air-breathing vertebrates, if indeed 
it can be said to exist at all in the aquatic animals. It is not 
always easy to distinguish between smell and taste. The ol- 
factory organs are more sensitive than those of taste. At any 
rate smaller particles of the stimulating substances will arouse 
the sense of smell than will serve to arouse taste. This makes it 
true that smell is useful to detect chemical conditions at a greater 
distance than is possible in taste. 

373. The Ear. There is a single pair of ears in verte- 
brates, and these are located at the sides of the head behind the 
eyes. The essential sensory portion of the ear (internal ear) 
arises as an in-pocketing of ectoderm, and consists of a closed, 
fluid-filled membranous sac which is surrounded by meso- 
dermal structures often solid bone. Ordinarily this sac is con- 
stricted, being thus partially separated into two irregular cham- 
bers, the dorsal (utriculus) and the ventral (sacculus). From 
the former arise three semicircular canals which are supplied 
with sensory hair-cells in the epithelial lining and are looked 
upon as being an organ to assist in detecting direction of motion 
and maintaining balance or equilibrium. From the sacculus 
arises an outgrowth, the cochlea, which in higher forms is well 
developed. It becomes spiral, and is well supplied with sensory 
cells. It is regarded as the chief auditory organ in those forms 
possessing it. This membranous sac or labyrinth is completely 
surrounded by cartilage or bone in fishes, and lies toward the 
middle line from the spiracle. There is no external ear. In 
forms above the fishes a membrane (tympanic membrane) stops 
up the spiracle and incloses what is known as the middle ear, 
which still communicates with the mouth by the Enstachian 
canal. A bridge of minute bones is also formed from the tym- 
panic membrane across the middle ear whereby the external 
vibrations can be communicated to the internal ear. In addi- 
tion to this, particularly among the mammals, is found an exter- 
nal tube leading to the tympanic membrane. Expanded folds 



of skin supported by cartilage form a funnel to catch the waves. 
The tube (external auditory meatus) and the funnel or pinna 
constitute the external ear. 

374. The Eye. The eyes of vertebrates are a single pair of 
organs lying imbedded in an orbit of cartilage or bone, within 
which they have considerable freedom of motion. Six muscles, 

FIG. 175- 

FIG. 175. Diagrammatic horizontal section through the right eye of Man. The line a p is the 
axis of vision. The optic nerve leaves the eye on the median side of this line. a.c. t central artery; 
a. h., aqueous humor; &., blind spot, the entrance of the optic nerve; c, conjunctiva; ch., choroid 
layer of the eyeball; c.L, crystalline lens; c.m.c., circular fibres of the ciliary muscles; c.m.r., radial 
fibres of the ciliary muscles; co., cornea, the transparent portion of the sclerotic; c.p., ciliary process; 
c.s., canal of Schlemm, a lymphatic vessel; fo., fovea centralis, the point of clearest vision; o.., 
optic nerve; 0.5., ora serrata, the anterior wavy margin of the visual portion of the retina; r, the 
retinal layer; sc. t sclerotic layer; sh., sheath of the optic nerve; v.h., vitreous humor. 

Questions on the figure. Which is the essential sensory portion of the eye? 
Which parts are concerned in bringing the rays of light to a focus? How many 
refractive surfaces are present? How many refractive media? Which portions 
of the eye are primarily protective and supportive? What is the function of the 
various parts of the choroid layer? In what way is an image formed on the retina, 
of objects in front of the lens? 

four straight (rectus) and two oblique, serve to move the eyeball. 
These muscles receive the third, fourth, and sixth of the cranial 
nerves. In the higher forms muscular folds of the skin serve 
to protect the eye in front. The upper and lower lids act 
vertically, but the third (nictitating membrane) works from the 



inner angle of the eye outward. Sometimes all three lids are 
present together. In the lower groups the lids are wanting. 

The essential part of the eye is the sensory expansion of the 
optic nerve the retina which occupies the innermost posi- 

PIG. 176. 

PIG. 176. Diagram showing some of the retinal elements in their relations to each other. 
Layer I is directed toward the interior of the eye and consists of nerve fibres (/) which finally enter 
the optic nerve at the blind spot; 2, the ganglion-cell layer, made up of the nerve cells of which the 
fibres are a part; 3. the inner "molecular layer" made up of the fine, much-branched nerve fibrils 
from 2 and 4; 4, the inner nuclear layer, containing numerous ganglion cells (g) ; 5, the outer "mo- 
lecular layer," similar in structure to 3; 6, outer nuclear layer, containing the nuclei of the rod and 
cone cells (r.c. and c.c.); 7, the layer of rods and cones (r, c). This is the nervous epithelium, or 
the nerve-endings of vision. The rods and cones are partly imbedded in a pigment epithelium (8). 
It must be remembered that hundreds of elements are omitted where one is shown in the figure. 

Questions on the figure. Which portion of the retina does the light first 
strike on entering the eye ? To what point must it penetrate to arouse nervous 
activity ? Over what route must a nervous impulse pass to reach the brain from 
the point of stimulation (rods and cones) ? Compare with similar figures in other 

tion, bounding the posterior portion of the cavity of the eyeball. 
This is a very complicated layer, but a general idea of it can be 
obtained from the diagram (Fig. 176). The layer of rods and 
cones, in close connection with a layer of pigment, is the sensi- 


tive layer. Light falling directly on this from all directions 
would produce no image, just as the photographic plate exposed 
outside the camera would present a general blur. We find then 
the necessity of the same optical devices as are found in the 
camera: (i) a sensitive surface, the retina; (2) a box for sup- 
port and for keeping out the light except from one direction, 
the opaque layers of the ball of the eye; (3) an aperture for the 
passage of the light into the interior of the box, the pupil and 
the transparent cornea overlying it; and especially (4) a lens 
or series of refracting surfaces which cause all the rays of light 
coming to the eye from each external point to be brought to- 
gether again beyond the lens at a corresponding point on the 
sensitive surface. The elementary relations of these parts 
as found in the eyes of vertebrates may be gathered from a 
study of Fig. 175. 

Accommodation of the eye to objects at different distances 
is effected by means of changes in the shape of the lens through 
the action of appropriate muscles. 

375. Library Exercise. What portions of the vertebrate 
eye are derived directly from the ectoderm? Which from the 
brain (i.e., indirectly from ectoderm)? Which from meso- 
derm ? (See Fig. 45.) 

What variation occurs among vertebrates as to the condi- 
tion of the bones in the middle ear? Whence are they con- 
sidered to be derived ? What variation in the cochlea ? Study 
from figures the structure of the cochlea. 

376. Classification. The principal divisions of the sub- 
phylum vertebrata are : 

Class I. Pisces (e.g., sharks, lung-fishes, bony fishes). 
Page 3 58. 

Class II. Amphibia (frogs, toads, salamanders, etc.). Page 


Class III. Reptilia (crocodiles, lizards, snakes, turtles). 
Page 384. 

Class IV. Aves (birds). Page 398. 

Class V. Mammalia (mammals). Page 437. 



377. The class of fishes, representatives of which are famil- 
iar to all, is important not only from the point of view of its 
specialized present-day representatives but from the fact that 
it was the first successful vertebrate group of geological times. 
It represents the primitive aquatic habit from which the land- 
inhabiting types of vertebrates must have arisen, and in it 
we find the fundamental plan of structure which has been 
modified in the higher forms (as the Amphibians) in adaptation 
to their present mode of life. It must of course be borne in 
mind that the types of fishes which are supposed to be the 
ancestors of the air-breathing vertebrates were much less 
specialized in structure than the present members of the class. 
There are, however, even now some of the fishes which have 
changed less than the majority, from the primitive condition. 

378. General Characteristics. 

1. Fishes are aquatic vertebrates having gills functional 
throughout life. These consist of filaments or sheets, contain- 
ing blood-vessels and attached to bony or cartilaginous arches 
in the region of the pharynx. 

2. Paired appendages (pectoral and pelvic) are normally 
fin-like, not having a median jointed axis as in the limbs of 
the higher vertebrates. The medial fins are dorsal, ventral, 
and caudal. The last is the chief organ of locomotion. 

3. There is usually a dermal skeleton consisting of scales, 
covered with epidermis. The latter may deposit enamel on 
the dermal core of the scale. 

4. There is a two-chambered heart through which the sys- 
temic (impure) blood flows. 

5. Vertebral column either cartilaginous or bony; the verte- 
brae biconcave. 



6. There is no true (allantoic, see Fig. 208, al.) urinary 

7. A longitudinal line of sense organs ("organs of the lateral 
line") on each side of the body. 

8. Nasal pit does not (usually) communicate posteriorly 
with the mouth. 

9. Fertilization and development usually take place outside 
the body. 

379. Form. Fishes usually have a body somewhat flattened 
from side to side, though it may be quite cylindrical or flat- 
tened dorso-ventrally, and gradually tapering toward either 
end. This is readily seen to be a form well suited to motion 
through water. Those which enter crevices under rocks or 
elsewhere, as eels and catfish, are cylindrical. Some types are 
extremely freakish in form, as the spherical globe fish, angler- 
fish which is chiefly head, the strange hammer-headed shark, 
the pointed gar-pike and sword-fish, and the indescribable 

The head end, while not so specialized as in the higher forms, 
is much more cephalized than in the Protovertebrata. The 
mouth with its modifications of movable jaws, teeth, etc., the 
respiratory arches, the sense organs of sight, hearing, taste, 
and possibly smell, the brain and brain case all enter into this 

There is no neck, i.e., the head is not movable with refer- 
ence to the body. The length of the body varies very greatly 
as does the number of metameres embraced. The body may 
be distinguished as pre-caudal and caudal. 

380. Appendages. Fishes possess two classes of append- 
ages paired, or lateral, and median. These are expansions 
of the skin supported by rays of bone or cartilage. The paired 
fins are four in number and are considered to be homologous 
with the pectoral and pelvic appendages of the higher verte- 
brates. They differ much as to their position, especially the 
posterior pair, as may be seen by a comparison of the figures in 
this chapter. In its typical condition the appendage consists 
of girdle and the fin proper. In the lung-fishes there is a central 


axis (Fig. 177) through the fin, instead of the usual radiate 
arrangement (Fig. 178, J.r.) of the fin-rays. The legs of higher 

FIG. 177. 

FIG. 177. Diagram of the anterior fin (archipterygium) of Ceratodus. 

Questions on the figure. What are the chief points of contrast between this 
fin and that of Teleosts, figured in Fig. 178? How does Gegenbauer consider that 
this might give rise to the simpler type of Vertebrate legs? 

FIG. 178. 

FIG. 178. Pectoral girdle and fin of a Teleost. br.o. t branchial ossicles; c, coracoid; d., 
cliavcle; f.r., fin rays;, post clavicle; p.t., post temporal which unites with skull; sc. t 
capula;, supra-clavicle. By Folsom. 

Questions on the figure. Which girdle is this, right or left? Do authors 
regard any of these bones as homologous with similarly named bones in the higher 

vertebrates are supposed to have been derived from this type 
the archipterygium). 


The median fins consist of one or more dorsal portions which 
vary in extent, a caudal portion, and a ventral part near the 
anus. These may represent remnants of a continuous median 
fin such as is seen in Amphioxus (Fig. 154). They are sup- 
ported by fin-rays in the dermal fold, which are in turn sup- 
ported by spines imbedded in the muscles. The form of the 
caudal fin, which is much used in locomotion, differs widely. 

FIG. 179. 

FIG. 179. Diagrams of some principal forms of tails in fishes. A, protocercal fin (as in Polyp- 
erus) ; B, heterocercal (as in Sharks) ; C, homocercal (as in most Teleosts) ; D, homocercal (as in 
Amia). By Polsom. 

Questions on the figures. What is the essential difference between the sym- 
metry of D and of A ? What conceivable advantage has the symmetrical over the 
unsymmetrical type? Are the heterocercal types successful swimmers? 

These differences, correlated with modifications of the end of 
the vertebral column, have considerable importance in subdivid- 
ing the class. The following types may be noted: 

1. The vertebral column passes straight to the end of the 
tail and the fin-rays are disposed symmetrically with regard 
thereto (protocercal) ; found in lung-fishes and some primitive 
extinct forms (Fig. 179, ^4). 

2. The vertebral column is bent dorsad, and a small fin lobe 
develops from its ventral side. The tail, though two-pronged, 
is not symmetrical (heterocercal). Found in sharks and many 
ganoids (Fig. 179, B). 

3. The vertebral column may become still more bent and 


reduced; the ventral lobe develops until the whole structure 
appears symmetrical again (homocercal) . Found in bony fishes 
(Fig. 179, C, D). 

FIG. 180. Skull of Cod (Gadus morrhua). From Nicholson, after Owen, b, branchiostegal 
rays borne on c.h., the ceratohyal bone; d, dentary portion of the mandible;/, frontal; h.m., hyoman- 
dibular; i.o., interoperculum; /, lachrymal; m, maxilla; n, nasal; o, operculum; p.m., premaxilla; 
P.O., preoperculum; p.s., parasphenoid; q, quadrate; s.o., sub-operculum; s.oc., supra-occipital. 

Questions on the figure. What is the operculum? How many bones are 
associated to form it? Which bones are figured as bearing teeth? Which of 
these bones belong to the cranium proper ? What is the difference between cranium 
and skull? What do authors believe to be the origin and homology of the chief 
facial bones? 

381. Covering. Most fishes are more or less covered by 
scales or scutes of bony matter developed in the dermis and 
lying between the dermis and epidermis. The scales often 
receive a layer of enamel from the epidermis. In form they 
may be cycloid (round, with smooth margin), ctenoid (toothed 
margin), placoid (plate-like bodies often bearing points covered 
with enamel), and ganoid (thick rhomboid scales covered over 
with enamel, and often closely articulated into a coat-of -armor). 
The scales are usually placed as shingles are on a roof, and doubt- 
less protect the fish from mechanical injuries. A good many 
species of fishes are destitute of scales altogether, the skin of 
such often being supplied with numerous mucous glands. In 


many extinct forms the external covering was made up of large 
plates fused into a dense armor. 

382. The Skull. The skull in fishes is especially note- 
worthy for the looseness of the connection between the facial 
bones (i.e., the visceral or branchial arches) and the cranium. 
They are readily separated from the cranium. The lower jaw 
is not articulated directly with the brain-case but with the upper 
jaw (see Fig, 180, q). 

383. Locomotion. Fishes are aquatic and are complete 
masters of their medium. The density of water as compared 
with air makes the matter of support in the medium much 
easier for the fish than for the bird. The denser medium is how- 
ever more difficult to penetrate. The specific gravity of the 
fish as a whole does not widely differ from that of water, although 
it varies within narrow limits. Four problems are thus pre- 
sented to the fish for solution : 

1. The Regulation of Specific Gravity. This is effected in 
part at least by the air bladder. The body muscles may bring 
about the compression of the contained gas and thus decrease 
the size without change of weight. 

2. Propulsion. The chief organ of propulsion is the caudal 
fin, acted on by the powerful lateral muscles of the body. The 
resultant of the alternate strokes against the water is forward 
motion. This may be supplemented by the action of the paired 
fins, especially in slow motion. 

3. Steering. This is accomplished in part by the changes 
in specific gravity and the regulation of the stroke of the tail, and 
in part by the action of the paired fins. The semicircular canals 
probably assist the animal in appreciating changes in its posi- 
tion, its orientation, thus enabling it to choose its direction. 

4. Balancing. Since most fishes are flattened from right to 
left there is some difficulty in keeping an upright position. The 
sense of position is doubtless given by the semicircular canals. 
The paired fins are used in balancing, as are the dorsal and anal. 
Doubtless all the fins along the body of the fish are used some- 
what as the keel of a boat or the planes of an air ship or a 





FIG. 181. Diagram of the principal vessels in the circulation of a fish, ventral view, a, 
orta; OH., auricle; br.a., afferent branchial arteries; br.e., efferent branchial arteries; bit., bulbus 
(or conus) arteriosus; c, carotid; c.a., caudal artery; c.v.r., right cardinal vein; ../., left cardinal 
vein; d.a., dorsal artery; d.c., ductus Cuvieri; g, gills; h.v., hepatic vein; h.p., hepatic portal vein; , 
intestine; j, jugular vein; k, kidney; /, liver; r.p., renal portal vein; s.v., sinus venosus; v, ventricle. 

Questions on the figure. Follow the general course of the circulation, noting 
changes in the character of the blood in various capillary regions. What is the 
extent of the hepatic portal system? Of the renal portal? Where is the purest 
blood in the body? Reason for your answer? What do you mean by "impure" 
blood? Where are the chief impurities removed from the circulation? 



384. Supplementary Exercise for the Library. What is the structure and 
position of the "swim-" or air-bladder in fishes? With what organ is it related? 
Does it communicate with the outside? Are there any evidences that it is of 
value as a respiratory organ in any of the fishes? Can you conceive any use it 
might be in steering, for the purpose of rising or sinking in the water? What 
would be the effect of compressing the air-bladder at one end more than at the 

385. The Circulation. Little needs be said here in addition 
to what has been said in the general discussion of the vertebrate 
circulation (see Figs. 166-171). The heart is two-chambered. 
The auricle receives the venous blood from the system; it is 
passed to the ventricle through a valve which forbids its passage 
in the reverse direction. From the ventricle the blood passes 
through a valvular region into the ventral aorta, which carries it, 
by a series of right and left branches, to the gills. Here aeration 

FIG. 182. 


FIG. 182. Diagram of the principal vessels in the circulation of a Fish, lateral view. Lettering 
as in the preceding figure. Adapted from Parker and Haswell. 

Questions on the figure. Compare the two views (Figs. 181 and 182) and 
identify the parts common to both, tracing the course of the circulation in the 
various vessels. 

takes place, the pure blood being gathered from the gills by a 
series of efferent branches which combine (except some anterior 
branches which go to the head) to form a dorsal aorta. The 
dorsal aorta gives off branches to the body wall, to the paired 
appendages, to the liver, digestive tract and kidneys, con- 
tinuing into the tail where it breaks up in the muscles. 

The impure blood from the capillaries of the tail is brought 
back to the kidneys by the renal portal vein, where it again 
passes through capillaries; here the blood is purified of its urea 


and similar impurities. The blood supplied direct to the kid- 
neys from the aorta and that of the renal-portal circulation is 
returned to the heart by way of right and left (cardinal) veins 
which join corresponding right and left veins (jugular) from the 
head to form the veins (ductus Cuvieri) which empty into the 
auricle. The blood which was distributed to the stomach and 
intestines is gathered into a vessel (hepatic portal vein) which 
carries it to the liver, together with much of the food absorbed 
from the intestines. The hepatic portal vein here breaks up 
into capillaries. The blood from the liver and from the appen- 
dages unites with that carried by the ductus Cuvieri before it 
reaches the heart. The student should carefully follow out the 
course of the circulation in the accompanying diagrams (Figs. 
181, 182) and determine just what changes take place in the 
blood in the various capillaries through which it passes. Varia- 
tions from this typical condition are numerous, accounts of 
which must be sought in more extended texts. 

386. Library Exercise. Find description and figures in the reference zoologies 
locating the unfamiliar structures in circulation of fishes in the table on page 343 
and testing the statements found there. 

387. Habits and Food. Fishes occur abundantly both in 
fresh and salt water. Usually the whole life is spent under the 
same general conditions. The salmon and shad, however, are 
bred and partly develop in fresh water and later pass out to the 
sea. These forms return, often with remarkable precision, to 
the place of their own hatching to deposit their eggs. Use is 
made of this habit in the capture of them for commercial pur- 
poses. Unless some means are found for limiting the destruc- 
tion of the adults during the breeding period, some of our most 
valuable food fishes are in danger of extermination. Others, as 
the eels, -may generate in the sea and spend a part or all their 
adult life in fresh water. Some burrow in the muddy bottoms, 
as the eel, cat-fish, mud-fish; others lie on the bottom, as the 
flat-fish; many quaint forms frequent the depths of the sea. 
The most are active swimmers in open water near or away from 
the shore. Many forms (herring, shad, salmon, etc.), are dis- 
tinctly gregarious, moving in great shoals especially at spawning 
time or when in search of food. This fact and the knowledge 


of places and times are matters of much moment to the fisher- 
men. The food of fishes is very diverse. Some forms are ac- 
tively carnivorous, preying on animals as large or larger than 
themselves (sharks); others, and these are the most numerous 
class, depend upon small animals such as the young of their own 
or other species of fish, on Crustacea, insects and worms. The 
microscopic animals and plants occurring in immense numbers 
in the water are important items in the food of fishes. Some 
fishes are scavengers, living largely upon the dead materials 
found in the water. Fishes differ much in their energy, courage, 
and resistance to attack. Those possessing these qualities in 
high degree are denominated "game" fish and are prized for the 
difficulty involved in their capture. The family of the trout 
and salmon includes several such species. 

The deep sea habit results in most interesting adaptations. 
The conditions of life are different from anything we know. 
The darkness is complete, the temperature invariable and not 
far above freezing point, the pressure is something enormous. 
Many of the fishes are phosphorescent. Sometimes these 
luminous organs seem arranged to attract the prey. Some 
forms have very large eyes to use the weak phosphorescent 
light ; others are blind as are some of the cave fishes. The sense 
of touch is greatly developed. In form they are very varied 
and bizarre. 

388. Economic Value. From primitive times fish has been 
one of the important human foods. Probably a larger percent- 
age of the well-known species of fishes are regarded as edible 
than of any other animal group. Their rate of multiplying 
and their occurrence in schools at available points are quite as 
important factors as the delicacy of the flesh in determining the 
food value of a species. The improved devices for capturing 
fish, the development of methods of preserving them by drying 
and by canning, and the increased price of other food substances 
for which fish may be substituted have all conspired to increase 
the destruction of the more important edible fish both in the 
fresh and salt waters. In recognition of this, most nations have 
appointed commissions for the study of problems connected with 



the fisheries and for the better regulation of the same. The 
United States Fish Commission in conjunction with similar 
state boards, has done an immeasurable amount of good espe- 

FIG. 183. 

FIG. 183. Atlantic Salmon (Salmo salar). From the "Manual of Fish Culture," U. S. F. C. 

Questions on the figure. What are the names of the various fins shown in the 
figure? What is the dotted line along the side of the fish? What type of tail 
has this fish? . 

FIG. 184. 

FIG. 184. Brook Trout (Salvelinus fontinalis') . From " Manual of Fish Culture," U. S. F. C 

daily in the following particulars : 

i. In taking the spawn of our most important food fishes 
and caring for it artificially during the period of early develop- 
ment when the young animals are in the greatest danger of 


destruction. Such fish hatcheries are scattered all over the 
Union and many of our fresh waters are being restocked with 
species believed to be hardy and suitable for food. 

2. By determining the foods preferred by special fish and 
artificially encouraging its abundance. 

3 . By encouraging the destruction of species of animals that 
prey upon the food fishes, and by the study of fish diseases pro- 
duced by plant and animal parasites. 

4. By studying the habits of the fishes and by regulation of 
the time, place and manner of catching. 

The money value of the annual catch of fish in our waters 
cannot well be less than $50,000,000. The most valuable fresh- 
water forms are the lake trout, white fish, cat fish, bass, perches, 
suckers. The chief marine species are cod, haddock, halibut, 
mackerel, menhaden, herring, and salmon. The latter, though 
marine, is caught in fresh waters in its breeding migrations. 

There are numerous species of fish which are destructive to 
the food fishes, by devouring the young or in other ways. 
Among these are the gar-pike, pickerel, muscalonge, German 
carp, the dogfish and other sharks. 

389. Supplementary Exercises for the Library. 

1. Make a report concerning the principal food fishes used by the people of the 
United States: their habits and geographical range, the mode of their capture and 
putting on the market. 

2. Make a study of the methods of capturing fish from primitive time to the 
present and show how the methods have been adapted to the habits of the fish. 

3. A study of the history and work of the United States Fish Commission as 
shown in the annual reports and bulletins. Its economic side. Its scientific side. 

390. Reproduction and Development. The sexes are sepa- 
rate. The^ sexual elements are produced in great numbers. 
The ova (spawn) are usually deposited in the water, in shallows 
on the open bottom, under rocks, or in places specially provided 
for them by the parents. The sperm (milt) is poured over these 
by the male, and the fertilization and later development take 
place in the water with little or no care on the part of the parents. 
Great loss of life occurs among the young from the voracious 
habits of other species and sometimes of the parents themselves. 
It is not difficult to believe that the enormous number of eggs 
produced by the female is an adaptation to meet this risk of 


mortality among the young. In some cases (most sharks and 
a few bony-fishes) the eggs are fertilized and the young hatched 
within the body of the mother. Only a few young are produced 
in such forms. 

The eggs of fishes are usually well supplied with yolk, seg- 
mentation being partial (discoidal, see 53). The unsegmented 
portion comes to be surrounded by a yolk sac and furnishes 
nourishment for the early stages of development. 

FIG. 185. 

FlG. 185. The Smelt (Osmerus dentex}. Bull. U. S. Fish Commission. 

391. Special Adaptations. In addition to those already 
mentioned the group of fishes shows many adaptations to special 
modes of life. 

Color. Most fishes show color as the result of pigment 
buried in the cells of the skin, or of delicate markings on the 
scales. In general, the tone of color accords with the environ- 
ment. This becomes very striking in some of the less active 
forms, as the flounders, in which the colors may change more or 
less rapidly to accord with the bottom on which they lie. It 
seems probable that some degree of protection from enemies 
may thus be gained, which would be of distinct value to the 
species. In some cases the color is in sharp contrast with the 
environment, and may be very conspicuous. This is believed 
to be a warning coloration in some instances, accompanying 
some disagreeable quality. Some deep-sea forms are phos- 
phorescent. This is probably of considerable importance ,as no 
sunlight penetrates to that depth. 

Electrical Organs. In several groups of fishes (rays, eels, etc.), 
certain muscular tracts have become so modified that under 
nervous stimulus instead of producing motion by contraction 


they form and accumulate electrical energy which may be dis- 
charged at the will of the animal. This power certainly has a 
protective value, as the discharge is in some cases powerful 
enough to paralyze much larger animals than the fish itself. It 
is probably useful also in capturing prey. 

FIG. 1 86. 

FIG. 186. Young Sea-bass (Centropristis strialus). Photo from life by Dr. R. W. Shufeldt. 

Question on the figure. Locate the pelvic fin and compare with other fish as to 

Asymmetry. In the flat-fishes we find a very striking com- 
pression from side to side. In early life they have the position 
normal to other fish, but in the adult stage they rest and swim 
with the dorso-ventral plane horizontal instead of vertical on 
the left side in some species and on the right in others. The side 
that is uppermost becomes pigmented like the back, and the 
under side loses its pigment and becomes white, as the belly of 
fishes in the normal position. The eye which belongs to the 
under side changes its position until it comes to lie on the upper 
side. The bones of the cranium, especially those about the eye, 
are twisted and the right and left branches of the jaw are un- 
equally developed. The dorsal and ventral fins become con- 
tinuous and the body tends to become bilaterally symmetrical 
in the new position. We can scarcely doubt that this asym- 
metrical condition has been brought about by the position 
which the animal takes in relation to the environment, but we 
know that in some species the eye begins to migrate now before 
the fish assumes the lateral position. 


392. Classification of Fishes. 

Subclass I. Elasmobranchii (Sharks, Dog-fishes, Rays, Skates). Marine 
fishes with essentially cartilaginous skeleton; no operculum or gill-cover; mouth on 
the ventral surface of the head; heterocercal tail; external skeleton of placoid 
scales; spiral valve in the intestine; no air bladder. The elasmobranchs are 
regarded by some as being the nearest present relatives of the primitive fishes. 
They occur most abundantly and are larger individually in warm seas. They are 
powerful swimmers as befits carnivorous, preying animals. They feed on Crustacea, 
Mollusca, and fish. 

Subclass II. Ganoidei (Ganoid Fishes: Sturgeon, Gar-pike). Fishes^with 
bony cartilaginous skeleton; gills covered by an operculum; exoskeleton of ganoid 
scales or enameled plates; air-bladder present; spiral intestinal valve; tail either 
homo- or heterocercal. 

The group was very important in the early history of the earth, and the present 
species are a mere remnant of the former glory of the ganoids. They occur now 

FIG. 187. 

FIG. 187. Long-eared Sunfish (Lpomis auritus). Adult. Photo from life by Dr. R. W. Shufeldt. 

chiefly in the rivers and lakes, though the sturgeon is also found in the sea. North 
America is as well represented as any other region in the living species of this 
remarkable group. 

Subclass III. Teleostei (Bony Fishes). Fishes with well-ossified skeletons; 
body covered with cycloid or ctenoid scales; exoskeleton of bony plates in the head 
region which become associated with bones of the internal skeleton to form the 
skull; mouth terminal rather than ventral; gills covered; spiral valve lacking; air- 
bladder usually present; homocercal tail. 

This subclass embraces the great majority of the forms ordinarily known as 
fishes. There are estimated to be 6,000 or more species of teleosts, more than 2,000 
of which inhabit fresh water. The group is variously divided by different authors 
and the student must be referred to more advanced texts for fuller classification. 
The principal orders are outlined below. 

Pneumatic duct (from air-bladder to intestine) open Order Physostomi. 

(Carp, cat-fish, sucker, salmon, trout, shad, herring, eel, etc.) 
Pneumatic duct closed. 

Dorsal, anal, and pelvic fins spiny in front. 

Bones of the pharynx (branchial arches) distinct ... Order Acanthopteri. 
(Perch, sunfish, mackerel, stickleback, silverside, etc.) 



Bones of the pharynx united Order Pharyngognathi. 

Dorsal, anal, and pelvic fins without spines Order Anacanthini. 

(Cod-fish, haddock, flat-fish, etc.) 

(Two other teleost orders of less importance, embracing some very pecu- 
liar forms, are the Plectognathi (globe-fishes) and the Lophobranchii (sea 
horses, Fig. 60; and pipe-fishes). 

FIG. 188. 

PIG. 1 88. Sheepshead. Greatly reduced. Photographed from life by Dr. R. W. Shufeldt. 

FIG. 189. 

FIG. 189. Young of the Snowy Grouper (Epinephelus niveatus). 
Shufeldt: American Naturalist. 

Photo from life by Dr. R. W. 

Subclass IV. Dipnoi (Lung-fishes). Fishes with a persistent notochord and 
the internal skeleton incompletely ossified; soft cycloid scales; spiral valve in the 
intestine, the swim-bladder used as a lung, the auricle partly separated into two 
chambers, paired appendages with a central axis producing a flapper rather than 
a fin (Fig. 1 77). There are only three or four living species, but these are especially 


interesting to the zoologist from the fact that they may represent the division of 
fishes from which the air-breathing vertebrates sprang. One genus (Ceratodus) is 
found in the rivers of Queensland ; the second (Protopterus) in the rivers of southern 
Africa, and a third (Lepidosiren) in the Amazon in South America. No marine 
forms are known. From fossil remains it is evident that the ancestors of the 
present lung-fishes were very much more widely distributed. 

393. Supplementary Studies for Library and Field. 

1. What are the theories as to the origin of the paired fins of fishes? 

2. In what way do fishes change their long axis from the horizontal position 
so as to ascend or descend obliquely in swimming ? 

3. Range of size in fishes. 

4. Probable origin of fresh-water fishes. What forms are now able to pass 
back and forth from fresh to salt water? 

5. Accumulate data concerning the habitat, food, breeding habits, distribu- 
tion, economic importance (with the reasons therefor) of some of the following 
fishes: salmon, trout, white-fish, sunfish, muskalonge, herring, eel, cod, flat-fish, 
mackerel, shark, ray, sturgeon, gar-pike, bowfin. 

6. What is known of the habits of the lung-fishes calculated to suggest how the 
lung may be of value in preserving the life of the animals? 

7. Migrations among fishes. 

8. Parental care among fishes. 

9. The number of eggs produced by various species. 

10. Study figures showing the embryology of the salmon or other bony fish. 

11. The blind fishes found in caves. What are the principal facts concerning 
them, and what explanations have been offered to account for their habits and 

12. Collect all the data possible concerning the flat-fishes. 

13. Examine all the figures of fishes found in your library, and make note of 
the chief points of variation and the range of these. 



394. The amphibians are especially interesting to the zoolo- 
gist because they begin life as gill-breathers (tadpoles) , and later 
they replace the gills by lungs. This is the meaning of the name. 
The fact that the amphibian in its individual life passes from a 
fish-like condition to the form and habits of the higher air- 
breathing vertebrates is taken as evidence that the higher ver- 
tebrates have sprung from fish-like ancestors through forms 
similar to the amphibians. The change from gills to lungs is 
not equally striking in all the members of the group. The 
transition from water to air involves important changes in the 
problem of physical support, of locomotion, and of respiration, 
and in consequence, of the organs performing these functions as 
well as correlated changes in the integument and in the organs 
of circulation. The amphibia were much more abundant in 
earlier geological times than at present, and attained huge size, 
whereas the modern forms, with a very few exceptions, are small. 
There are nearly two thousand living species. The tailless types 
(frogs and toads) are much the more numerous, as well as more 
highly developed. 

395. General Characters. 

1. Amphibia are Vertebrata which possess gills during the 
larval stage and usually lungs in the adult; in some instances 
the gills are retained throughout life. 

2. Paired appendages when present conform to the general 
vertebrate type; i.e., limbs with digits (typically five) , instead 
of fins. 

3. Exoskeleton of scales and plates absent; skin glandular. 

4. Heart is three-chambered; two auricles and one ventricle. 

5. A renal-portal and hepatic-portal circulation present. 
Red corpuscles are nucleated. 



6. A cloaca occurs, into which the anus and the ducts from 
the excretory and genital organs open. 

7. Development usually by a metamorphosis. Segmenta- 
tion total but unequal. 

396. Form. Amphibia differ much as to the shape of the 
body. The newts and salamanders are elongated, slender and 
eel-like; the frogs and toads have large, flat heads, stout trunk, 
and muscular limbs. Among the former groups there may be 
as many as two hundred and fifty body segments, in the latter 
the vertebras behind the head are reduced to ten. The neck is 
usually inconspicuous, the head being poorly movable. 

397. Appendages. There may be two pairs of appendages, 
one pair, or none at all. In most forms except the Anura (tail- 
less) the limbs are small and weak as compared with the body 
(Fig. 191). The limbs have a distinct dorsal and ventral (palmar 
surface, as well as an anterior and a posterior border. The 
digits are enumerated from the anterior border which terminates 
in the first, or thumb. In many forms there is a reduction of 
the digits on the anterior appendage from five to four. The 
digits are almost universally destitute of claws. The feet are 
often webbed, and in the climbing toads the digits may end in 
discs by which they cling to objects. 

398. The skin is normally soft, and slimy by reason of a 
glandular secretion. It is composed of two layers, epidermis 
and dermis. In the frog the epidermis is in two layers, the 
outer of which may be shed at intervals. In toads, and other 
forms frequenting dry places, the epidermis may form warty 
thickenings. The skin is often highly colored owing to the 
presence of pigment cells in the deeper layers. There are two 
chief kinds, black and yellow pigment cells. The pigment 
cells are much branched, and the pigment may either be con- 
centrated or diffused through the cells. The different colors 
the due to different concentration and different proportions of 
are various pigments. In some cases the tones of color may be 
changed in accordance with the surroundings by direct action of 
light on the pigment cells, or by the reflex nervous action of the 


animal, resulting from impressions on the retina of the eye. 
Besides light, heat and cold, moisture, and probably internal 
states of the animal, may produce color changes. In many 
instances these colors are protective by making the animal like 
its surroundings. In the extinct Labyrinthodonts external pro- 
tective plates were developed in the dermis. Minute dermal 
scales are found in some of the lowest present forms ("blind- 

399. The Skeleton. The points of contrast with the skele- 
ton of fishes are, chiefly: the presence of a sternum (formed in- 
dependently of the ribs) ; the imperfect development of the ribs; 
the typical limb skeleton ; the union of the pelvic girdle with the 
spinal column; the closer fusion of the upper jaw with the 

The vertebrae of the lower forms are biconcave as in fishes, 
in the higher forms (Anura, and higher Urodela), concavo-convex. 
The vertebral column usually consists of one cervical vertebra; 
a variable number of thoracic or abdominal vertebrae ; one sacral, 
to which the posterior girdle is attached ; and a variable number 
of caudal (one, in Anura). 

400. Respiration. In early larval stages the respiration is 
effected wholly by means of the skin, and even after the develop- 
ment of special organs of respiration the skin continues to serve 
this function in a greater or less degree. Most amphibians have, 
when hatched, external gills which may be retained through 
life (as in Siren, the "mud-eel"), or may give place to internal 
gills covered by a fold of skin (as in the development of the frog). 
Typically, lungs replace both kinds of gills in the adult. The 
gill slits do not exceed three or four pairs in number. Some of 
the aquatic forms retain their gills when the lungs are developed, 
each method of respiration supplementing the other. Those 
which possess lungs alone in the adult must of necessity undergo 
profound changes in passing from the water-breathing to the 
air-breathing habit. The lungs arise as a ventral outgrowth 
from the esophagus or pharynx. From the short trachea the 
two sac-like lungs spring. The walls are in folds but the sacs 
are simple. In some salamanders there are neither gills nor 


lungs in the adult, respiration taking place wholly through the 
body surfaces. The frog breathes through its nostrils. The 
mouth cavity can be increased by muscular action, thus allowing 
the entrance of air. The nasal openings are then closed by 
flaps and the air is forced by muscular action into the lungs. 

401. Supplementary Exercises for the Library. Find as many different types 
of respiration as possible among the amphibians, and cite examples. What forms 
have gills only? What evidence is there that the environment has much to do 
with hastening or retarding the change from gills to lungs? Give the natural 
history of the Mexican axolotl as far as respiration is concerned. Are any Amphibia 
hatched with lungs at the outset? 

402. Circulation. In the gill-breathing larvae the circula- 
tion is quite similar to that in fishes (385; Fig. 181). When 
the gills are lost and lungs developed, the arterial arches (Fig. 
169) which supply the gills change their course, or suffer de- 
struction. This is an interesting instance of the modification 
of old structures to meet new demands. Coupled with these 
changes we find the separation of the auricle into two chambers 
right and left. The veins from the lungs empty into the left, 
and the systemic veins into the right auricle. While there is 
only one ventricle into which both the pure blood from the lungs 
and the venous blood from the system go, it is so arranged that 
the venous blood is chiefly returned to the lungs and the purest 
blood goes to the head and to the systemic circulation. The 
venous circulation is modified in general accordance with the 
changes in the heart and arteries. 

403. Supplementary Exercise. Compare the arterial vessels in the adult 
frog with those in the fish and the tadpole stage of the frog, and find what, in the 
opinion of various authors, is the fate of each of the arterial arches. See Figs. 
166-169. What are the most important differences in the venous circulation in 
fishes and in adult amphibians? 

404. Locomotion. In the lower Amphibia, in which the 
appendages are poorly or not at all developed, the muscles of 
the body show the segmental arrangement seen in fishes, and 
locomotion is effected by a serpentine or eel-like action of the 
body. In the higher forms, especially the Anura, the limbs 
are well developed ; and the body muscles lose something of the 
regularity of their arrangement and become more as we find 
them in the higher vertebrates. The muscles that move the 


limbs in relation to the body come to overlie and obscure the 
axial muscles proper. The Anura (frogs and toads) are espe- 
cially adapted to leaping and swimming by the great muscular 
development of the hind legs. 

405. Exercise. Are there any special advantages in the leaping habit of 
motion either in the capture of prey or in escape from, enemies? Verify from be- 
havior of toads and frogs. Can you find illustrations from other groups of animals ? 

406. Habits and Habitat. There are no marine Amphibia. 
Nearly all live in or near the fresh-water streams, swamps, or 
ponds, even in the adult stage. Some are good climbers (tree- 
toads); others burrow. The tailless forms (Anura) are found 
the world over. The Urodela belong chiefly to the northern 
hemisphere. All are more abundant in warmer climates. Their 
food consists largely of insects, worms, and the smaller animals. 
The larvae even of carnivorous forms are sometimes vegetable 
feeders. They may live for a long time without food, and sur- 
vive the winter in the colder latitudes by burrowing deep into 
the mud at the bottom of their ponds, or otherwise hibernating. 
During this time the vital processes are suspended or much 

407. Reproduction and Development. The common am- 
phibia lay rather large eggs, with a considerable amount of yolk 
which results in more or less unequal cleavage (Fig. 13, B). 
The eggs are usually surrounded by a gelatinous material, for 
their protection and adhesion, but they have no shell. They 
are almost universally deposited in the water, where impreg- 
nation takes place. In some of the Urodela impregnation is 
internal. In occasional species the young are brought forth alive. 
Ordinarily further development takes place in the water without 
any attention from the parents (frogs and toads). In a small 
South American frog (Rhinoderma) the male carries the fertilized 
eggs in his vocal sacs until hatched ; in one of the tree-frogs from 
South America the female has a pouch on the back in which 
the eggs are stored and hatched ; in the Surinam toad the eggs are 
placed by the male on the back of the brooding female, where 
they become surrounded by spongy tissue. In these pits they 
hatch at once into the adult form without having external gills. 



FIG. 190. 

PlG. 190. The metamorphosis of the Frog. (After Brehm.) Numbers indicate the sequence 

Questions on the figures. How much of the egg is really ovum? What are 
the changes which take place in passing through the various stages? In what 
order do the legs appear? How is respiration effected after stage 6? After stage 
n? What is proven by the collecting of the tadpoles as shown in 3? How do 
they retain their position. 

FIG. 191. 

FIG. IQI. Tailed Amphibians. From Nicholson, after Mi vart. A, Siren; B, Amphiuma[lridactyla; 

C, Necturus, 

Questions on the figures. Compare these three types and note all the chief 
differences of external structure. Compare also with figures you may be able to 
find of other Urodela. 



FIG. 192. Frog (Rana). Photo from life by J. W. Folsom. 

Questions on the figure. What is the round object behind the eye? What 
elements in the resting position of the frog put him in readiness for a quick spnng? 

FIG. 193. 

FIG. 193. The Common Toad (Bufo lentiginosus). Photo from life by Folsom. 

Questions on the figure. Compare with the figure of the frog and note points 
of external similarity and difference. What do you know of the habits of the toad, 
as to feeding, egg-laying, etc.? Where do they spend the winter? What of 
their development? 


This is of course a successful adaptation (by eliminating the 
metamorphosis) to a completely aerial habit. 

From this group we have beautiful illustrations of unequal 
cleavage of the ovum, of which the student should have the op- 
portunity of seeing figures in more extended works. If possible 
the cleaving eggs should be studied. 

408. Place in Nature. The amphibians are among the oldest 
vertebrates. They are not, however, a very successful group as 
measured by numbers of species or of individuals. The class 
itself and such species as the frog suggest the way in which the 
zoologist thinks the land animals arose from the aquatic. 
Among amphibians we have types that are like fish, gill-breathers 
all through life; others begin as gill-breathers, retain their gills 
and develop lungs; still others lose their gills when the lungs 
are perfected; and some are hatched out with lungs, never 
having had functional gills. This makes a very suggestive 
series of connecting links between water and air-breathing forms. 

None of the species has large individuals. The "giant 
salamander" of Japan is the largest, reaching a length of 3-5 

Amphibians are practically uniformly helpful to human in- 
terests, or at least without hurt to them. The toads destroy 
many injurious insects, and are thus particularly valuable about 
gardens. Toads are easily reared artificially and might be made 
to give us even more aid in controlling this difficult group. The 
legs of some of the larger species of frogs are eaten. The flesh 
is white and delicate. Frogs are also used in great numbers in 
the zoological laboratories, for dissection and experimentation. 
They have thus contributed no little to our knowledge of animal 
functions, particularly the functions of the nervous system. 

Profitable frog "farms" are being conducted in various 
parts of the country to meet the demands referred to above. 
This suggests possible uses for certain swamp lands not now 
suitable for other purposes. Edible frogs and turtles might be 
encouraged in such regions instead of inedible types. 

409. Special Exercises. Describe the life history and the 
stages in the metamorphosis of the frog. (Fig. 190.) What 


larval organs disappear? What new organs are introduced? 
Compare other amphibians as to the degree and facts of meta- 

410. Classification of Amphibia. 

Order I. Gymnophiona. Degenerate Amphibia with neither legs nor tail; 
body worm-like; no gills nor gill-slits in the adult; eyes more or less degenerate. 
Scales are present in the skin. Represented by the so-called blind-worms of 
tropical countries. 

Order II. Urodela. Amphibia with tails persistent throughout life; body 
elongated; usually two pairs of appendages (sometimes only the anterior are 
present), which may be poorly developed. 

The principal suborders are: 

1. Perennibranchiata, in which the gills persist throughout life (Necturus or 
water-dog, Siren or mud-eel, and certain blind forms found in caves). 

2. Derotremata, losing the gills in the adult but retaining a spiracular opening; 
in the side of the neck which represents the gill-slit. (Examples: " Congo-snake '* 
of the gulf states, giant salamander of Japan.) 

3. Myctodera, which lose all traces of water-breathing. (Examples: Newts, 
salamanders, etc.) 

Order III. Anura. Amphibia in which the tail is absorbed in the adult 
condition, if present in the embryo. Two pairs of appendages, the posterior of 
which are well developed. Undergo a metamorphosis in which the larvse usually 
have the "tadpole" form, with gills and tail but without appendages. All traces 
of gills lost in the adult. The Anura embrace the Bufonidas or common toads, 
the Ranidae or common frogs, the Hylidae or tree-toads, and other less common, 
families. The Anura include the great majority of the species of Amphibia. 



411. Specimens of reptiles are scarcely abundant enough to 
serve as satisfactory laboratory types for elementary classes, 
but instructive comparisons may be made by single students or 
by groups of students. These results should be reported to the 

Prepare three parallel columns, one for the lizard, one for 
the snake, and one for the turtle. Select a specimen of each 
and compare them with regard to their haunts; habits; food; 
general form of body; appendages, number, position, joints, 
digits; covering; manner of locomotion. 

412. Special Topics for Investigation in the Laboratory and Field. 

1. Are reptiles warm or cold blooded? Your evidences? 

2. What are the differences between the scales of snakes and of fishes? 

3. In what various ways is the tail of reptiles used as an organ? How is the 
tail to be distinguished from the rest of the body ? 

4. What special senses do reptiles possess? What are your evidences? What 
peculiarities have the organs of sense? 

5. What peculiarities do the internal organs of the snake have which seem to 
be correlated with the slender, elongate form of the animal ? 

6. What species of snakes, turtles, and lizards are found in your locality? 
Report on the special habits of each species in so far as you can determine them 
by observation. Supplement by reference to authorities. 


413. TheReptilia differ from the vertebrates we have hith- 
erto studied in the fact that at no period of life do they possess 
gills. They agree with the lower forms in being cold-blooded 
and in the incomplete separation of the heart into right and left 
compartments (except in the crocodiles) . They are, in addition 
to their air-breathing habit, similar to the birds and mammals 
in possessing the protective embryonic membranes known as 



the amnion and allantois (see 423), the latter of which is im- 
portant in embryonic respiration, that is, before hatching or 
birth. The group reached its culmination in numbers, variety 
and size in the Mesozoic age. So true is this that the Mesozoic 
is called the "Age of Reptiles." Those we have at present are 
to be looked upon as specialized and, in some instances (snakes) 
perhaps, degenerate remnants of the first vertebrate class wholly 
to give up breathing by means of gills. In the Mesozoic era 
there were immense swimming, fish-like forms (ichthyosaurs and 
plesiosaurs) which ruled the seas; powerful terrestrial dinosaurs, 
often walking on their hind legs, and including the largest land 
animals known to have lived; and others, with membranous 
wings like the bat, the first vertebrates to learn the art of flying 
(Fig. 196). With the exception of a few marine turtles, the 
boas and pythons, and the alligators and crocodiles, the living 
species are for the most part small animals. 

414. General Characteristics. 

1. Reptilia are usually covered with scales or plates derived 
from the dermis (bony), or the epidermis (horny), or from both. 

2. The (3-5) digits when present are provided with claws. 

3. The vertebrae are concavo-convex, usually concave in 
front and convex behind. 

4. The heart is three chambered; that is, the auricles are 
completely separated, but the ventricles are only partially so 
except in the Crocodilia. 

5. There are two aortic arches, a right and a left, in the adult. 

6. Gills do not occur at any period. 

7. Reptiles are chiefly oviparous; the eggs are large, well 
supplied with yolk, arid protected by a leathery shell. 

8. The embryonic membranes, amnion and allantois first 
make their appearance in this group. 

415. The Reptiles are very diverse in form. Perhaps the 
lizards may be taken as typical, with cylindrical body, more or 
less distinct head and neck, distinct tail, and usually two pairs 
of appendages, each possessing five digits armed with claws. 
They are mostly small animals, though one species is known to 
attain a length of five feet. The crocodiles and alligators are 



similar in shape but much larger. The turtles and snakes are 
most widely different from the type and must be regarded as 
much specialized, or even degenerate, forms. The turtles have 
sought protection by means of a bony box, and are ill adapted 
for motion either on land or water. Snakes, on the other hand, 
elongated and devoid of appendages, are among the most rapid 
and graceful of animals in their motions. The long tapering 
body is a successful prehensile organ. Some of the lizards agree 
with the snakes in lacking legs. 

416. Covering. The external covering in reptiles is in the 
form of scales or plates formed by the epidermis, or the dermis, 
or both. That deposited by the epidermis is horny and that 
by the dermis, bony. In snakes and many lizards the scales 
are epidermal and may be periodically shed and renewed. The 
scales usually differ in shape and size in different parts of the 
body. In turtles and their allies the horny constituent, which 
is illustrated by the "tortoise shell" of commerce, is in the form 
of plates and is reinforced by bony dermal plates beneath. 
The latter do not, in the a'dult at least, correspond in number and 
size with the former, but are closely associated with the bones 
of the internal skeleton. In crocodiles the dermal scales cor- 
respond in general with the epidermal. 

The members of the group are on the whole well protected 
by these external growths. In many instances, as in certain of 
the lizards, prominent projections are formed upon the body 
covering, giving a striking appearance to the animals. 

417. Internal Skeleton. The vertebral column, except in 
the snakes and snake-like lizards, shows the customary regions 
(see 349). In the limbless forms only two regions are recog- 
nized, the pre-caudal which bear the ribs, and the caudal or 
tail vertebrae. The vertebrae are usually concave in front and 
convex behind, thus making a kind of ball-and-socket joint. 
In snakes the number of vertebrae is very large. No sternum 
occurs in turtles and snakes. When present, as in lizards and 
crocodiles, it is formed in connection with the ventral end of the 

' The skull articulates with the first vertebra by one surface 


(condyle) instead of two as in mammals. The lower jaw articu- 
lates indirectly, by means of the quadrate bone, with the skull. 
This gives a very movable jaw and in the snakes especially, 
increases the caliber of the throat (Fig. 194). The cranium is 
more compactly fused and completely ossified than among the 
Amphibia. The ulna and radius and the tibia and fibula are not 
fused as in the frog. Rudiments of the pelvic girdles are found 
in some snakes, although the limbs are wanting. 

FIG. 194. Skull of Rattlesnake (Crotalus durissus). From Nicholson, after Huxley, or, 
articular portion of lower jaw; de, dentary portion; bo, basi-occipital; mx, maxilla, bearing poison 
fang; no, nasal; pi, palatine, the front end being represented by a dotted line as though seen through 
the maxilla; pmx, premaxilla; po, post frontal; pr, prefrontal; pt, pterygoid; qu, quadrate; sq, squa- 
mosal; tr, transverse bone. 

Questions on the figure. Which bones bear teeth? Which are cranial and 
which facial bones? What bones do you find common to the snake and the fish 
(Fig. 1 80)? How do they differ in the two forms? What is the function of the 
quadrate? How does it differ in the different groups of Vertebrates? 

418. Respiration. Functional gills never occur, though gill- 
slits are partly developed in the embryo only to close again 
before hatching. The trachea is elongated and is supported by 
cartilaginous rings as in the higher forms. It divides into two 
bronchi, each of which passes to a spindle-shaped sac the lung 
which is much simpler in its lobings than those of birds and 
mammals. In the snakes one lung (the left) is much reduced 

3 88 


or even altogether aborted. This is an adaptation to the narrow 
elongated body cavity. The ribs when present and the muscles 
acting on them are the prime agents in breathing. 

419. Circulation. In reptiles the right and left auricles are 
entirely distinct but, with the exception of the Crocodilia, the 
ventricles are only partially so. The septum in the ventricle is 
perforated. Yet in those forms in which the pure blood of the 
left auricle and the impure blood of the right partially mingle 
in the ventricle, the arrangement is such that the purest blood 

FIG. 195. 

pr. z, 

pt. z 

t. p. - 

FIG. 195. Vertebrae of a Reptile (after Huxley). A, anterior view; B, posterior view of the 
vertebra in front of A. The surface of A fits against the surface of B. c, centrum, which is convex 
in B, fitting into a concavity in A; n.s., neural spine; pr.z., pre-zygapophyses, or anterior articular 
facets, which fit against pt.z., post-zygapophyses; t.p., transverse processes; z.s., a wedge-like 
articular face on the neural arch designed to fit into z.c., a depression on the posterior face of the 
neural arch of the vertebra in front (5). 

Questions on the figures. Try to form a clear picture of the relations of the 
articulating surfaces of the vertebrae and indicate the possible advantages of the 
arrangements. Where is the neural cavity? Where do the ribs articulate? 
What is the gain in muscular attachments from the numerous bony outgrowths 
on the vertebrae? 

goes to the brain and the least pure to the lungs (see Fig. 170). 
Two aortic arches unite, giving rise to the dorsal aorta. In the 
reptiles and higher groups of vertebrates the renal-portal circu- 
lation (see Fig. 181, r.p.) ceases to be of much importance, but 
the hepatic portal is increasingly important. The red corpuscles 
are elliptical and possess nuclei. 

420. Nervous System and Special Sense Organs. The 

brain is not large in the reptiles, but is rather more highly de- 


veloped than in the Amphibia. This is especially true of the 
cerebral hemispheres. The usual senses are represented. The 
rather large eyes are provided with movable eyelids except 
among the snakes, in which a permanent transparent membrane 
covers the eye. In some reptiles (lizards) there is a remnant 
of a median eye which is hopelessly degenerate in the adult. 
It is in connection with the pineal body in the second division 
of the brain. Hearing varies. It is rather keen in turtles; 
apparently less so in snakes. Smell is well developed. Touch 
is interfered with by the scales and plates, but is represented 
in the skin over the body. 

421. Habits. The reptiles are best represented in the 
tropical regions. The larger types, as the crocodiles, python, 
boa are almost confined to the warm zones, especially of South 
America, Africa and Asia. Numerous smaller representatives 
of the lizards, snakes, and turtles are found in temperate lati- 
tudes. These usually undergo a period of hibernation during 
the cold season. This habit of hibernating and seeking warmer 
climates seems related to the cold-blooded condition. The heat- 
producing qualities of the animals are not equal to the task of 
maintaining activity during extreme cold. The variation of 
temperature is of course a more serious problem to terrestrial 
animals than to aquatic types. Although air-breathers, very 
many of the group are aquatic, as the turtles, crocodiles, and 
many snakes. The lizards are almost without exception terres- 
trial. Nearly all prey on other animals; the smaller on worms, 
insects, and eggs of various kinds, and the larger on birds, fish, 
amphibia, and mammals. The land tortoises are vegetable 

Reptiles are rather sluggish animals. They like to bask in 
the sun. They are capable often of very rapid motion; but it 
is spasmodic. 

Reptiles, especially the snakes, have a bad reputation, yet 
there is no doubt that their dangerous qualities are much ex- 
aggerated in popular opinion. The lizards are almost wholly 
non- venomous and the majority of the common snakes of this 
country are also harmless. The principal dangerous snakes 



are the cobra of the East Indies where nearly 25,000 deaths 
were caused by serpents in 1899 ; the vipers of Europe; the rattle- 
snakes, water-moccasin, and copperhead of our own country. 
There are about twenty poisonous species in this country of 
which fifteen species are rattle-snakes. The venom serves the 
snake both as a means of defense and of paralyzing its prey. 
Many forms which are not poisonous assume bodily attitudes 
similar to those of the poisonous species. This is known as 
mimicry, and is a means of protection. The dangerous species 
are being rapidly exterminated by man. 

FIG. 196. 

PIG. 196. Rhamphorhynchus merensteri, a restoration of an extinct flying Reptile. From the 
Cambridge Natural History, after Geikie. 

Questions on the figure. In what respects does a form like this differ in 
external appearance from a bird ? From a bat ? What skeletal structures would 
a palaeontologist need to find in order to believe that an extinct form had the power 
of flight? 

422. Special Exercises. Find data concerning hibernation in reptiles and other 
vertebrates: its object and advantages; preparation; place; degree in which vitality 
is suspended during the process, etc. 

Describe the poison apparatus of venomous snakes. What is the homology 
of the fang? Of the gland? 

How do different snakes capture their prey? How prepare it for swallowing? 

Describe and attempt to explain the motion of snakes from actual observation : 
in water; on land. 

423. Reproduction and Development. The ova escape from 
the two ovaries into the body cavity. As in the Amphibia the 
inner end of the oviduct opens well forward in the body cavity. 
The ova enter the oviducts, and during the descent are fertilized. 
After fertilization the glands in the walls of the oviducts add 
albumen and shell structures, as in the birds. The eggs require 


a period of incubation which usually occurs outside the body; 
though some lizards and snakes retain the eggs in a special por- 
tion of the oviduct until the embryo is hatched, thus bringing 
forth their young alive. Many forms deposit their eggs in the 
warm sand or earth or in decaying rubbish heaps, where the 
abundant heat is favorable for the developing young. 

Much yolk is present in the egg and segmentation is partial, 
being confined to a disc. The germinal layers and the im- 
portant organs develop about the axis of this disc, the outer 
margins of which spread over the whole yolk in the form of a 
sac designed to nourish the embryo. The details of the growth 
are entirely too complicated for statement here. Two important 
embryonic membranes the amnion and allantois appear for 
the first time (see also 440). The amnion consists of folds 
of the blast odermic disc which arise, surrounding the embryo 
at its margin. These folds grow dorsally over the embryo and 
ultimately fuse to enclose a space*which becomes filled with 
fluid. The amnion folds include both ectoderm and mesoderm. 
It is protective in function (Fig. 208, am). The cavity between 
the two layers of the amnion is an outgrowth of the ccelom. 
The allantois arises as a fold from the posterior portion of the 
digestive tract, and is made up of entoderm and mesoderm. 
It finally surrounds not merely the embryo but the yolk on the 
ventral side, and being well supplied with blood vessels is most 
important in supplying the embryo with oxygen. In this and 
in other features the reptiles show a close kinship with the birds. 

424. Place in Nature. Reptiles live upon both plant and 
animal food. They devour worms, insects, fish, amphibians, 
birds, small mammals, and eggs. Not many animals depend 
on reptiles for food. They are more immune from attack than 
most animals. Some of the predaceous birds eat lizards and 

The group does not aid man at many points. Certain large 
lizards, and the tortoises and turtles furnish food. The soft- 
shelled turtle, the green-turtle, and the diamond-back terrapin 
are the most prized. 

Skins of crocodiles and of some snakes, and the horny tor- 



toise-shell are used in the manufacture of bags, ornaments, and 
the like. 

Probably the chief value of the group to man is in the de- 
struction of noxious insects by lizards and rodents by snakes. 

425. Classification of Reptiles. 

Order I. Chelonia (Turtles and Tortoises}. The Chelonia are reptiles with 
short, flattened or dome-shaped bodies enclosed in a case formed by a dorsal shield 
(carapace) and a ventral (plastron). In some the carapace and plastron make a 
rather tight-box practically covering the animal. In others they are smaller and 
the edges are further apart. In these the appendages may protrude more, and 
the freedom of motion is much greater. The jaws are covered with a horny case 

FIG. 197. 

FIG. 197. 

Common Box Tortoise (Cistudo Carolina). 

Photographed from life by Dr. R 

and are destitute of teeth. The quadrate bone is firmly fused to the cranium. 
The sternum is absent. Turtles seem rather more common in the northern hemi- 
sphere. The largest species are marine and may attain a weight of half a ton. 
Some live in fresh water and others on land. The flesh of some species is much 
prized for food. The green- turtle of the Atlantic coast is one of the choicest, its 
flesh being much used for soups. The large hawkbill-turtle of the tropical seas 
furnishes "tortoise-shell," used in combs and other ornaments. The shells of 
the leather-back and other "soft-shelled " turtles are not completely ossified. The 
"snappers" are ferocious animals, the big snapper of the Southern states being 
particularly vicious. 

Order II. Lacertilia (Lizards). Reptiles in which the body is usually covered 
with small scales. Two pairs of limbs are ordinarily present; but either or both 
may be wanting. The quadrate bone is somewhat movable. The teeth are not 
in sockets of the jaw. Sternum present. The cloacal opening is transverse. 



The Lacertilia include, beside the types commonly known as lizards, the 
chameleons, horned-toads, and the glass snake a legless lizard. They subsist 
largely on insects and the eggs of other animals. Only one species is known to be 
poisonous the "Gila monster" of New Mexico and southward. The glass snake 
possesses in a high degree a power more or less common among lizards of breaking 
loose from the tail when struck, or held by that organ. In some species, at least, 
a new tail may be regenerated. Most lizards are terrestrial, though a few are 

FIG. 198. 

FIG. 198. 

Swift Lizard (Sceloporus undulatus). Adult. 

Photographed from life by Dr. R. W. 

aquatic. They run, burrow, and climb. One species (Fig. 199) has a membrane 
from the sides, supported by ribs, that serves as a parachute. 

Order III. Ophidia (Snakes'). Reptiles with elongated bodies covered by 
fold-like epidermal scales which may be shed as a single "cast." Limbs are 
wholly wanting. The mouth is capable of great extension on account of the great 
movability of the quadrate and other bones. Teeth are numerous and fused (not 
in sockets) to the bones bearing them. Sternum wanting. There are no movable 



FIG. 199. 

FIG. 199. Flying Lizard (Draco volitans). From Nicholson. 

FIG. 200. 

FIG. 200. Dorsal view of a "Gila Monster" (Heloderma suspectum}. Photographed by Dr. R. W. 


FIG. 201. 


FIG. 201. Common Garter Snake (Eutcenia sirtalis). Photographed from life by Dr. R. W. 


PlG. 202. 

FIG. 202. Blotched King Snake (Lampropeltis rhombomaculatus). Photographed from life by Dr. 

R. W. Shufeldt. 

Questions on the figure. What structural adaptations have the' snakes which 
tend to take the place of appendages? Illustrate your conclusions by citing ex- 


eyelids. The tongue is protrusible, and is doubtless much used as an organ of 
touch, and possibly hearing. 

Snakes are, like lizards, partial to warm climates, but are also found in tem- 
perate latitudes. Most are terrestrial, but some take to water readily; and there 
are some which never come to land. These bring forth their young alive. Many 
snakes are beautifully and characteristically colored. In some instances the 
coloration is deemed to be protective. 

Order IV. Crocodilia (Crocodiles, Alligators, etc.}. Fresh-water reptiles 
with elongated bodies bearing two pairs of well-developed appendages. The skin is 
armed with dermal bony scales or scutes covered by epidermal scales. Teeth 
occur in sockets. The quadrate is immovable and the sternum is present. The 
adult heart is completely divided into right and left halves. The cloaca opens by 

FIG. 203. 

FlG. 203. Head of the American Alligator (Alligator Mtssissippiensis). From Eckstein. 

a longitudinal slit. Here are included the gavial of the Ganges, the crocodiles 
of the Nile and of tropical America, and the alligator of America. They are some- 
what sluggish animals, but when hungry will attack with success the larger mam- 
mals or man. They may attain a length of twenty feet or more. Crocodilia 
are chiefly aquatic, though they rest on the shore, and deposit their eggs in the 
sand where they hatch. 

There are numerous orders of extinct reptiles which show close relationship 
with the fishes, amphibians, and birds of early geological times. This is merely 
another way of saying that the early Reptilia and the other vertebrates were much 
more generalized in their characteristics and less differentiated than those of the 

426. Supplementary Topics for Investigation. 

1 . Have the venomous snakes any characteristic appearance ? 

2. Report on the habits of the rattle-snake. Whence the 
structure giving rise to the name ? The nature of the fang and 
the poison gland. 

3. What is the degree of activity and strength of the reptiles 
and cold-blooded animals, as compared with the warm-blooded ? 


4. The characteristics of the principal groups of geological 

5. The meaning of hibernation. Illustrations of its method 
among the reptiles. 

6. The various methods of capturing prey among reptiles. 

7. The facts concerning incubation and care of young among 

8. The development of the amnion and allantois. See 
figures in reference texts. 


427. Laboratory and Field Studies. Each student or group 
of students should be encouraged to select one or more species 
of birds and to study their habits and external structure in the 
light of the following outline. 

I. Habits and Activities. 

Haunts and feeding habits. What are its favorite foods? 
How determined ? What emergency foods will it use ? 

Social habits ; solitary or gregarious ? 

Mating habits; monogamous or polygamous? Degree of 
sexual dimorphism? 

Determine and describe its powers of song. Are they equally 
developed in all individuals of the species ? Can you cite 
any evidence that the power is of any use to the animals ? 

Nesting habits; number of eggs, their size and other char- 
acters. Is their color of any conceivable use? Which 
sex incubates the eggs ? Condition of the young at hatch- 
ing and the care given by the parents to the young. 

Migrations. Are there any evidences of winter (or other 
important) migrations? If not, how is the winter spent? 
If so, at what time does it occur ? When does the species 
return ? Any other known facts. 

Power and peculiarities of flight. Other modes of locomo- 
tion ? Is the power of perching well developed ? 

What are its relations to the other animals of the locality ? 
Has it any enemies ? Is it hostile to any species of animals ? 

What is its abundance or scarcity in your locality? Can 
you assign any explanation of the facts observed ? 

II. General External Appearance. 

Regions of the body : head, neck, trunk, limbs. 

Head: beak, mouth, nares, eyes (how many lids?), ears. 
Neck: length, natural position, flexibility, etc. 
Wings: arm, forearm, hand. 


AVES 399 

Legs : thigh, shank, foot. Where is the heel ? Evidences ? 
Note further arrangement and number of digits ; form of 
the claws; covering of the tarso-metatarsus. 
Covering of the body. Compare the color of all visible parts. 
Select feathers from various parts of the body; study as a 
type one of the large wing feathers, noting : shaft (quill 
and rachis), vane (barbs and barbules). 
Compare the other feathers selected with this one. What 

would you suggest as the prime function of each kind ? 
Arrangement of the feathers. 

On wings: remiges (large), primary (on hand) and 

secondary (on forearm) ; coverts. 

On tail: rectrices, number and arrangement; coverts. 
On body (dip in hot water and pluck): note the pits 
which have borne the feathers; arrangement of these. 
Are they uniformly distributed over the entire body ? 
Sketch the plucked bird, studying more carefully the regions 
already noted. Locate 
Openings: mouth, nares, ears, cloaca. 
Queries : 

Is there any connection between the closing of the toes 
and the flexing of the leg ? Has this any use to the ani- 

Which digits are represented? Are they equally devel- 

Which digit is turned backward ? How is this determined ? 
Is there a tongue? Are there teeth? Do the nostrils 

communicate with the mouth ? 

What do you consider the function of the nictitating mem- 
brane ? Are the eyes movable ? Do they view the same field ? 
Do the two together cover the entire field of view ? 
How much external ear is present ? 
Are the scales homologous with feathers ? (See reference 

III. Internal Structures. (The pigeon is a good type for 

anatomical study.) 

The student knows what internal organs and systems of 
organs to expect in vertebrates, and may well be required 


to block out an outline of work. What are the principal 
sets of organs to be expected? How do these compare 
with the corresponding structures in the fish or frog ? Do 
you find any structures not found in the lower types. Un- 
certainties may be settled by reference to more extended 


428. Birds must be looked upon as sharing with mammals 
the first place in the animal kingdom. Even the mammals as 
a class are not so highly specialized in structure and in habits 
as the birds. Their most striking features of specialization are 
connected with the demands of aerial life which they have so 
successfully met. They share with the insects, the most 
specialized of the invertebrate phyla, the most perfect de- 
velopment of the power of flight found among animals. It 
follows from their high degree of specialization that they are 
among the most easily recognized of the vertebrates. The 
earliest geological traces of birds show that they are closely 
linked with reptiles in their origin, and the modern birds pre- 
serve many interesting likenesses to the reptiles. Some of these 
are seen in the scales on the shank and feet of birds ; in the habit 
of laying large, well-nourished eggs which hatch outside the 
body; in the structure of the egg and its mode of cleavage; the 
peculiarities of the ankle joint; in the presence of a cloaca. 

429. General Characteristics of Birds. 

1. The Aves are vertebrates in which the epidermic out- 
growths usually take the form of feathers (or scales in special 
regions, as the feet). 

2. The anterior appendages in the majority of forms are 
modified for flight. Associated with this is a large development 
of the pectoral muscles and the bones to which they are attached. 

3. A single occipital condyle. 

4. The heart is completely four-chambered; only one (sys- 
temic) aortic arch which turns to the right; red corpuscles oval 
and nucleated. High bodily temperature, 100 to 110 F. 
Renal portal circulation almost wanting. 

AVES 401 

5. Some of the bronchial tubes terminate in air spaces (not 
true lung tissue) located in various parts of the body. These 
communicate with air cavities in some of the bones. 

6. The parts of the skeleton are much fused. There are no 
teeth, the jaw being sheathed by a horny product of the epi- 
dermis (beak). 

7. The right ovary and oviduct are aborted or rudimentary. 

8. All are oviparous; yolk abundant; segmentation discoidal; 
amnion and allantois present. 

430. Form. The birds, like many of the extinct reptiles, 
are bipeds. The axis of the more or less stout body makes an 
angle of varying size with the axis of the legs, that is, the vertical. 
The sacrum and the soft parts of the body project behind this 
point of union in such a way as to balance the anterior parts. 
The anterior appendages are not always well developed but are 
much anterior to and above the centre of gravity. This results 
in a more stable position of the body in flight. The posterior 
appendages are relatively long, sometimes extraordinarily so. 
In all cases there is an interesting correlation between the length 
of the neck and that of the legs. The wading birds are especially 
endowed in these particulars. The posterior appendages usually 
have four digits. These may all be directed forward as in some 
swifts, or much more commonly the great toe (number i) is 
directed backward ; in some species two are turned backward and 
two forward. In swimming birds a web is present which 
stretches from toe to toe. The special form and arrangement 
of the web differ in different species. The digits end in claws 
which vary greatly in accordance with the habits of the possessor. 
The anterior appendages usually show traces of three much 
reduced digits. 

431. Supplementary Studies. Allow students to make a series of studies of 
the angle made by the axis of the body with a vertical line in various birds. Com- 
pare this angle in the robin when at rest and when running. Make outline drawings 
of the shank and toes of all the types of birds which can be found, and discuss the 
differences in the light of the habits of the birds. Compare these with figures in 
texts. Make figures of the varieties of webs found in the aquatic birds. 

432. Covering. The form of birds as outlined above is 
much modified by the presence of feathers. They increase the 


stretch of the wings and the surface exposed to the air, and thus 
are important as aids to flight. In addition they are protective 
in several respects. The pigment possessed by the feathers 
serves to enhance greatly the beauty and variety of the members 
of the group. That the color patterns are of distinct value in 

FIG. 204. 

FlG. 204. Diagram showing the tracts where the principal growth of feathers occurs (Upupo epops) 
From Bronn. The dotted areas are the pterylce. 

Questions on the figure. Is this a dorsal or a ventral view? Find a figure 
giving the opposite view of some bird and compare with this. Is there variety in 
the different species of birds as to the distribution of the growth of feathers? 

sexual attraction has been believed by many naturalists. The 
feathers, together with the scales of the shank, the claws, and 
the beak, are epidermal growths and represent the remnants 
of the exoskeleton so well developed in some of the lower forms. 
Feathers are not usually produced uniformly over the body, 
but are grouped in regions which differ in different species. 
They also vary a great deal in form, from the down feathers of 

AVES 403 

the young to the stiff quill-feathers of the wings and tail. Most 
birds shed their feathers either a few at a time the year round or 
within a short period. In the former case the change may be 
scarcely noticeable. When the moulting takes place rapidly, it 
may be accompanied by profound disturbance in the health 
and habits of the animal. The new feathers may differ in color 
from the old, and thus a periodic change is apparent in the dress 
of some of our birds. This is not infrequently of such character 
as to accord in color with the changes in nature outside, giving a 
real protective value. 

433. Supplementary Topics. In what various ways are the feathers of birds 
protective ? Explain how the protection is realized in each case. What varieties 
of feathers may be found in birds, and what are the chief differences in structure? 
How are the color patterns obtained? Are they made up of feathers of one color 
so put together as to form the pattern, or is a single feather of more than one color? 
Does a single feather ever show an independent complex color pattern? Where is 
the boundary between feathers and scales on the legs of various breeds of chickens? 
Do you find any evidences that feathers are highly modified scales? Are any of 
the feathers like the hair of mammals? 

Secure further data from nature and from reference books concerning the 
moulting habits of birds. 

434. Endoskeleton. The chief points of importance to the 
elementary student are as follows: 

1. There is a fusion of several vertebrae in the sacral region 
(including some of the thoracic, all of the lumbar, the sacrals 
and the caudals) with the dorsal bones of the pelvic girdle, to 
form a strong dome-shaped structure above the viscera. The 
cervical vertebrae vary much in number (eight to twenty-four) 
with the length of the neck. 

2. The cranial bones fuse closely, and the bones of the face 
are prolonged into the core for the beak (Fig. 219). 

3. The sternum is normally well developed and provided 
with a keel to which the muscles of flight are attached. Finger- 
like processes also increase its surface for the attachment of 
muscles and the support of the viscera. 

4. The ribs are double-headed, and each has a process on 
the posterior margin, joining it to the rib behind. 

5. The pectoral girdle has its clavicles fused ventrally in the 
flying birds, forming the "wish bone." 


6. In the pelvic girdle the ventral bones (ischium and pubis) 
both pass backward from the hip joint and support the viscera. 

7. The ankle region of the birds is very characteristic. The 
proximal tarsals unite with the tibia, and the distal tarsals unite 
with the fused metatarsals to form the tarso-metatarsus or shank. 
The joint is between the proximal and distal tarsals (see 
Fig. 161). 

435. Digestive Organs. The horny beak entirely replaces 
the teeth in the modern birds. In the early members of the 
group teeth are known to have been present. The esophagus, 
often of great length, is usually expanded into a non-glandular 
crop, where the food is stored and softened. The stomach often 
consists of two portions, the anterior glandular proventriculus 
and the posterior muscular gizzard. In birds which habitually 
feed on grains or other hard objects the inner wall of the gizzard 
is lined with a hard and thickened cuticle which assists in grind- 
ing the food. Fragments of rock, sand, etc., are nearly always 
swallowed by grain-eating forms to assist in the process. These 
are manifestly devices to do work usually done by teeth. The 
usual glands are found associated with the digestive tract, ex- 
cepting the salivary. The tract ends in a cloaca. 

436. Supplementary Studies. What is the exact position 
of the crop ? Advantage of this position ? Make a comparative 
study of the beak in various birds ; how adapted to the habits ? 
Is there any recorded evidence that the character of the gizzard 
in a given individual may vary somewhat in accordance with 
the food used ? 

437. Respiration. The trachea corresponds in length to the 
length of the neck. Its rings are rigid (ossified). It divides 
into a right and left bronchus which pass to the respective 
lungs. The lungs are closely applied, and even attached, to the 
dorsal wall of the thorax and are small in proportion to the 
size of the animal. Some of the bronchial tubes connect with 
air spaces (nine in the pigeon) among the viscera and extending 
even into the hollow bones. They are probably chiefly respira- 
tory in function. 

AVES 405 

Bird notes are produced not at the upper end of the trachea 
as in other vertebrates but near its lower end, where it joins 
the bronchi. The organ is called the syrinx. Its mode of action 
is somewhat similar to that of the vocal cords in the larynx of 

438. The Nervous System and Organs of Special Sense. 

The cerebral . hemispheres are relatively larger than in any of 
the groups yet studied. Their surface is smooth. The cere- 
bellum is also large and concentrated chiefly in a central or 
median lobe. By the growth of these two portions the well- 
developed optic lobes are crowded into a lateral position. The 
olfactory lobes are small and the sense of smell is not so acute as 
in many other vertebrates. The optic lobes and the eyes are 
well developed and the sense of sight is correspondingly acute. 
The eye protrudes as a somewhat rounded cone in front. This. 
is supported by a ring of sclerotic (bony) plates. The power of 
accommodation, that is, of focusing the eye upon objects at 
different distances, is very great in birds. In addition to the 
upper and lower lids a transparent fold of the conjunctiva 
(nictitating membrane) may be drawn over the eye from the inner 
corner. Hearing is acute, and the condition of the ear is inter- 
esting chiefly in the facts of the absence of the concha of the ex- 
ternal ear, and in the presence of a well-developed but uncoiled 
cochlea in the internal ear. 

The birds are ''nervous" animals. That is to say, they are 
sensitive, active, easily aroused. This nervous activity is cor- 
related with the high temperature, rapid respiration, and large 
consumption of food. 

439. Habits. None of the animal groups present habits 
more interesting, more readily studied or more suggestive of 
the adaptation of structure to the demands of the environment 
than do the birds. The student will find by observation and 
by reference to current works on natural history many inter- 
esting facts in connection with bird-life. Under the suggest- 
ive studies a partial list of such topics will be found. In the 
chapter on Adaptations and in the section on the classification 
of birds (Ch. VIII, and 441) additional facts have been pre~ 



sented. Much of the time given to the practical studies of the 
group of birds should be directed to their life and adaptations. 

FIG. 206. 

FIG. 205. Diagram of the female genital organs of a Bird, c, cloaca; *, intestine; k, kidney; 
o, ovary with ova of different size; od,, oviduct; o.f., funnel of the oviduct; o.o., opening of the ovi- 
duct into the cloaca; u, ureter; U.G., opening of ureter into the cloaca. Only one ovary and oviduct 
are fully developed in the Birds. 

Questions on the figure. What openings has the oviduct? Why must the 
union of sperm and ovum take place before the egg gets well down the oviduct? 
Define the cloaca. On which side are the sexual organs rudimentary in the female 

FIG. 206. Diagram of the urino-genital organs of a male Bird, ad., adrenal body; c, cloaca; 
c. intestine; k, kidney; /, testis; u, ureter; M.O., opening of ureter into the cloaca; v.d., vas deferens; 
v.d.o., opening of the vas deferens; v.s., vesicula seminalis. 

Questions on the figure. What is the function of the vas deferens? Of the 
vesicula seminalis? What differentiates the cloaca from the intestine? What 
are the chief differences in the excretory organs of birds and mammals ? 

These various habits and modes of life have frequently been 
made the basis of classification: for example, some fly and 
some do not; some wade, having long legs; others swim and 



have webbed feet; some capture living prey with talons and 
curved beak; some scratch and have blunted claws; some climb 
and have two digits directed forward and two backward; 
others perch and have only one toe pointed backward. The 
resort to such superficial features in classifying birds suggests 
that the members of the class are more nearly related and more 
similar among themselves in the fundamental features of struc- 
ture than is the case with the subdivisions of the other classes 
of vertebrates. 

FIG. 207. 


FIG. 207. Diagram of a longitudinal section of the embryo of a fowl, without the amnion and 
allantois. Ectodermal boundaries are in continuous lines, the entodermal and mesodermal are in 
broken lines: the entodermal of short dashes, the mesodermal of long, b, brain; b.iv., body wall; 
c.C., central canal of spinal cord; co., coelom; g, gut; g.w., wall of gut; s.c. t spinal cord; y.s., yolk sac. 

Questions on the figure. What is the relation of the yolk sac to the digestive 
cavity ? Which of the embryonic layers surrounds it ? In what way is the abun- 
dant yolk in the yolk sac brought into the circulation of the embryo (see reference 
texts) ? 

440. Reproduction and Development. Reference has al- 
ready been made to the fact that the right reproductive organs 
of the female birds are much reduced or wanting. The ovum 
is always large, containing abundant yolk. When mature it 
breaks from the ovary, enters the funnel-shaped end of the 
oviduct and as it passes outward is fertilized. It then receives 

4 o8 


a layer of albumen, and later is surrounded by a membranous 
covering and by a porous, limy shell, all of which are secreted 
by the walls of the oviduct. The protoplasm is confined to a 
small germinal disc and segmentation is discoidal, resulting in 
a blastoderm like that of reptiles. In the newly laid egg cleavage 

FIG. 208. 
aw.c anf 





FIG. 208. Diagram of a longitudinal section through the embryo of a fowl, showing formation 
of amnion and allantois and the relation of these membranes to the embryo. The boundaries are 
as in the preceding figure, am 1 , inner or true amnion; am 2 , outer or false amnion; am.c., amniotic 
cavity; al., allantois; c.c., central canal of the spinal cord; co., ccelom; g, gut; ys., yolk sac. 

Questions on the figure. Which of the three embryonic layers enter into the 
amniotic folds? Which go to form the allantois? Show that the cavity between 
the true and false amnion is "extra-embryonic" ccelom. How is the amniotic 
cavity lined ? With what is the yolk sac lined ? The cavity of the allantois is in 
reality a portion of what cavity? Which of these membranes unite in mammals to 
form the chorion (see 462). 

is well advanced. After the egg is laid,' cleavage is checked 
until the necessary temperature for further development is 
supplied either by the brooding of the parent or by some special 
device. Owing to the action of gravity on the heavier yolk 
the living disc is always directed upward, the position most 
favorable for getting the warmth of the parent's body in incuba- 



tion. For the details of further development the student must 
be referred to more extensive texts, but it may be stated that 
the blastoderm comes to consist of two layers of cells which 
have been likened to two watch glasses so placed as to enclose 
a shallow cavity. The outer layer is ectodermal and is continu- 
ous at the edge with the inner, which is composed of larger cells 
incompletely separated from the yolk beneath (Fig. 13, C, 4). 
This inner layer gives rise to both entoderm and mesoderm. The 
blastoderm continues to grow at the margins until the yolk is 
entirely enveloped by a living membrane which is well supplied 
with blood vessels and serves to extract the food for the use of the 
embryo and to aerate the blood before the lungs become of use. 

FIG. 209. 

FIG. 209. ArchcEopteryx lithographica, an early reptilian Bird. From Claus. 

Questions on the figure. What in the figure shows this to be a bird? Wha* 
shows it to be different from typical birds? What is signified by each of the term- 
in its scientific name? 

The amnion and allantois (see 423; Fig. 208) are both de- 
veloped as in reptiles. Of these the amnion appears first. By 
a study of Figs. 207 and 208, together with others in the reference 
texts, it will be seen that the amnion is an outgrowth of the body 
wall of the embryo and has a cavity continuous with the ccelom. 
The outer layer is known as the false amnion; the inner is the 
true amnion (Fig. 207, am 2 , am 1 ). Into the space between the 
amnion-layers the wall of the gut outpockets, forming the al- 
lantois (Fig. 208, a/.). The cavity of this sac is continuous 
with the lumen of the gut. The embryo thus becomes com- 



pletely surrounded by protective membranes. The cavity 
between the true amnion and the body wall (Fig. 208, am. c.) 
is the amniotic cavity and may be filled with a fluid. 

441. Classification of Aves. 

Subclass I. SaururcB (reptile-tailed). These are extinct birds 
related to the extinct reptiles the dinosaurs in having a 
vertebrated tail, and jaws bearing teeth. Each vertebra of 

FIG. 210. 

FIG. 210. Apteryx australis. From Romanes. 

Questions on the figure. What peculiarities does this bird present? What 
does Apteryx mean? What is the distribution of this species? What are its 
nearest relatives among the birds? 

the tail possessed a pair of feathers, the tail thus having a row 
of rectrices on either side. 

Archaeopteryx, of which two specimens have been found in 
the lithographic quarries of Bavaria, represents the group and 
was about the size of a crow (Pig. 209). 

Subclass II. Neornitkes (modern birds). This group is 
characterized by the reduction and fusion of the tail vertebrae 
in such a way that the tail feathers (rectrices) are arranged 
in a semicircle (or sometimes wanting). Teeth are wanting 
except in some extinct forms, which stand intermediate between 
the Saururae and the recent birds. 

Division I. Ratitcs (flat). These are running birds with a 



flat breast bone (i.e., no keel) and with all the organs of flight 
much reduced. The barbs of the feathers are not held together 
by barbules, thus producing plumes. 

The Ratitas (order Cur sores or runners) are the lowest forms 
of living birds and include the ostriches, emus, cassowaries, in 
all of which the wings are reduced, and the Apteryx or wingless 
bird of New Zealand (Fig. 210) in which they are very rudi- 

FlG. 211. 

FIG. 211. Ostrich (Struthio). From Wood's Natural History. 

Questions on the figure. Which of the types of feathers of ordinary birds 
become the plumes in the ostrich ? What is the real size of the ostrich ? 

mentary. The ostrich (Fig. 211) is the largest and most power- 
ful of living birds. Ostriches are somewhat gregarious, and 
frequent regions more or less desert. At mating time they 
unite in pairs, the male assisting in incubating the eggs, which 
are laid in holes in the sand. Ostrich culture is an important 


industry in South Africa and to a certain extent in America, 
on account of the plumes which are extensively used as orna- 
ments. Besides the types mentioned there are a number of 
extinct forms belonging to this division, some of which have 
become extinct in recent time. Mpyornis is one of these, 
formerly a native of Madagascar, where remnants of its eggs 
have been discovered showing that its volume was about six 

FIG. 212. 

FIG. 212. Wood Duck (Aix sponsa). Photographed by Dr. J. W. Folsom. 

times that of the ostrich egg, i.e., having a capacity of about 
two gallons. 

Division II. Carinatce (with a keel). Birds with the keeled 
breast bone, the wings, and the other organs of flight usually 
well developed. Barbs of the feathers have barbules. All the 
modern flying birds are embraced in this subclass. 

The further subdivisions of the Carinatse, as given in the 
recent classifications, based upon internal structure, are entirely 
unsuited for beginners. An older arrangement of the prin- 
cipal orders, based upon habits and certain superficial features, 
is presented below for the convenience of the student. It 



should be remembered, however, that the classification is not 
the best possible, inasmuch as forms in reality not very closely 
related in structure are, according to it, placed together because 
of similar habits. The student is asked to refer to other texts 
for different arrangements. 

FIG. 213. 

PIG. 213. Ross 1 Gull (Rhodostethia rosea). Upper figure adult male; lower, young female. From 
"Chapters on Natural History"; drawn by Dr. R. W. Shufeldt after Ridgway. 

Questions on the figure. What indications of structural adaptation to habits 
do you find in the figure? What sexual dimorphism is perceptible? 

The Pygopodes (feet on the rump) include the auks, penguins, 
grebes, and loons. These are all aquatic birds and are expert 
divers and swimmers. Their feet are poorly adapted for the 
land, and in consequence the birds are awkward. The auks 
and penguins have poorly developed wings. The loons however 
are good fliers. The penguins, auks, and puffins are marine 



forms. The loons and grebes are found in fresh waters. The 
penguins at the breeding season are gregarious and are often so 
thick as literally to cover the surface that is level enough to 
be occupied. They lay one or two eggs, with no pretense of 
making a nest. The loons and grebes may use flags and rushes 
for nests. 

FIG. 214. 

FIG. 214. Green Heron (Ardea virescens). Photographed by Dr. R. W. Shufeldt. 

Questions on the figure. To what order of birds does the heron belong? 
What are its nearest relatives? What can you say of the habits of the order? 

The Longipennes (long-winged) are the albatross, the tern, 
gulls, petrels, shearwaters, etc. These are aquatic birds with 
webbed feet and long, pointed wings. They are splendid fliers 
and mostly good swimmers. There are both marine and fresh- 
water types. They are gregarious, especially at the breeding 
season, when they swarm on sandy shores, in the marshes, or 
on the rocky coasts, where they lay their eggs in crude nests or 
on the bare rocks. They are the most common birds of the 
seashore and the high seas. 



In the Steganopodes (web-footed) are included cormorants, 
pelicans, flamingos, storks, the ibis, etc. These are large water 
birds in which the legs are usually long and the feet suited to 
wading and swimming. They are heavy eaters, living largely 
on fish. They are often expert fishermen, the white pelicans 
acting in concert in driving the fish before them into shallow 
water, where they capture numbers of them in a capacious pouch 

FIG. 21=;. 

FIG. 215. A right lateral view of the skull of the American Flamingo (Phcenicopterus ruber). 
Photographed from specimen by Dr. R. W. Shufeldt. 

Questions on the figure. Distinguish upper and lower jaws, comparing 
them as to massiveness. Is this the usual condition in birds? How much of the 
skull is occupied by the brain ? To what habits of the flamingo is the form of its 
beak an adaptation? Compare with Fig. 219. 

of the lower jaw. The flamingos are long-legged, wonderfully 
colored birds with most interesting gregarious nesting habits. 

The Anseres (geese). The geese, ducks, and swans are 
familiar because of their widespread migrations and because of 
numerous domestic varieties. They have heavy bodies and: 
characteristic form. Three toes are united by the web. Their- 


bills are broad and serrate. They feed upon vegetation and the 
smaller water animals. Their wings are long and broad, and 
they are good fliers. The ducks and geese spend their winters 

FIG. 216. 

X ^.-\v 

_V|P > 


FIG. 216. Pelican (Pelecanus erythrarhynchus). By Folsom. 

Questions on the figure. What is the nature and purpose of the fold beneath 
the jaw? To what division of the birds does the pelican belong? 

in the tropics and nest in the colder regions. They are much 
hunted by man during these migrations, when they frequent the 
streams and lakes of the temperate regions. They are found 
the world over. There are said to be forty species of ducks, 

AVES 417 

sixteen species of geese, and three species of swans native to 
North America. 

PaludicolcB (living in swamps). This group includes marsh 
birds which stalk about the shallow waters picking up the small 
animals to be found there. They have long legs and corre- 
spondingly long necks. The cranes, trumpeters, rails, coot, 
tern, and gallinules are classed here. Their toes are not com- 

FIG. 217. 

FIG. 217. Ruffed Grouse (Bonasa umbellus). Photographed by J. W. Folsom. 

pletely webbed. They are light birds in proportion to the reach 
of their neck, legs, and wings. 

LimicolcB (shore-residents) inhabit the margins of streams 
and lakes and may wade in the shallows. These are similar 
in many ways to the preceding group, but are smaller birds. 
The group embraces the snipes, woodcock, sand-pipers, plover, 
killdeer, avocets, and many others. They have slender legs 
and long bills. They are alert birds with quick, active habits, 

4 i8 




protectively colored, and take promptly to cover. For these 
reasons they are "game" birds, sought by hunters. 

Raptores (robbers) are preying birds with hooked beak and 
claws, sharp vision, powerful wings, and instincts to match. 

FIG. 219. 

PIG. 219. Skulls of gallinaceous Birds, as Partridge, Grouse, etc. Photographed from the speci- 
mens by Dr. R. W. Shufeldt. Adult. J natural size. 

Questions on the figures. Compare these skulls and note the points of simil- 
arity and dissimilarity. Find the position of eye, ear, and nares. What are the 
chief points of contrast between these skulls and that of the owl (Fig. 222), and of 
the flamingo (Fig. 215)? 

Eagles, hawks, kites, falcons, owls, and vultures may be classed 
here. They are widely distributed, and have a great range of 
size and adaptations. The vultures, including the buzzards 



and the great condor of South America, are scavengers. The 
owls are noted for their striking appearance and for their 
nocturnal habits and the adaptations associated therewith. 

FIG. 220. 

PIG. 220. Great horned-owl (Bubo virginianus). Adult female. Photographed from life by Dr. 

R. W. Shufeldt. 

Questions on the figure. What are the habits of owls? Does the figure show 
any structural adaptations to known habits? How does the standing position of 
the owl differ from that of other birds of your acquaintance? See also Figs. 221 
and 222. 

There is a strong human prejudice against the group, partly 
because some of the members attack our poultry and other 
domestic animals. Close observation shows, however, that most 

AVES 421 

of the species are either harmless or actually helpful because 
they destroy noxious rodents. Cooper's hawk, the sharp- 
shinned hawk, and the great horned owl are real pests. 

The GallincB (fowls) include quail, partridges, grouse, pheas- 

FlG. 221. 

PIG. 221. Great horned-owl (Bubo virginianus). Young. Photographed from life by Dr. R. W. 


Questions on the figure. Compare the young at all points with the adult. 
What are the points of difference? Of manifest likeness? 

ants, turkeys, peafowls, and the various kinds of chickens. 
These birds have a plump, characteristic form. The legs are 
mostly short or of medium length and armed with short, blunt 
claws. The beak is stout and bent slightly downward at the 



point. As a rule they do not take long flights. They nest on 
or close to the ground and they get their food there. They are 
less highly organized nervously than many birds, and more 
readily tamed. Their habits of life furthermore fit them for 
domestication. They have large broods, and in early life these 

FIG. 222. 

PlG. 222. Skull of Owl (Syrnium iifl>nh>snni). After ShufoKlt. photographed from specimens. 
Upper figure bisected, showing brain-case; the lower from a dorsal aspect. 

Questions on the figure. Is the owl a bird of prey? What is the position of 
the eyes in relation to the skull? Of the nares? Compare these figures with the 
head of the owl (Fig. 220), and with the skulls in Pig. 219. 

follow the mother bird closely. This instinct may linger into 
adult life and families often remain together in ''coveys" 
throughout the first year, or even longer. Thus large flocks may 
be formed. Their secretive instincts, their protective markings, 
and their edible flesh make them "game" birds of first quality. 



Before civilized man destroyed their haunts and persistently 
hunted them with his improved weapons, these were among the 
very successful wild birds. Most species are now really quite 
scarce. The only hope of preserving them is by legal protection 
at critical times or by domestication. 

Chief among the domesticated birds is the common fowl 
which is descended from a species native to southern Asia 

Fi*. 223. 

FIG. 223. Belted Kingfisher (Ceryle alcyon, L).. About one-fourth natural size. By J. W. 


(Gallus bankiva). By breeding and selection almost innumer- 
able varieties have been produced. The poultry and egg in- 
dustry is one of the most important in the country, amounting 
to more than 700,000,000 dollars annually. In 1901, one and 
one-half billion dozens of eggs (estimated) were produced on 
the farms of this country. This in one of the great sources of 
human food. Turkeys, guinea-fowl, and pea-fowl contribute 
in less degree. 

The Columbce (doves) are in many respects similar to the 


FIG. 224. 

FlG. 224. Arctic three-toed Woodpecker. From U. S. Dept. Agriculture, "North American 


FIG. 225. 

FIG. 225. Yellow-billed Cuckoo (Coccyzus americanus). Adult male. Photographed from life by 

Dr. R. W. Shufeldt. 

Question on the figure. What are the nearest relatives of the cuckoos among 
the birds? 



FIG. 226. 

FIG. 226. Clark's Crow. U. S. Dept. Agriculture: "North American Fauna." 

FIG. 227. 


FIG. 227. Nestling Crows (Corrus). From U. S. Dept. Agriculture Year-book, 1900. 



Gallincs. With the doves are included the pigeons. About a 
dozen species are found in America. None of these species is 
very numerous. A century ago the passenger pigeon occupied 
the region east of the Mississippi River in great flocks number- 
ing millions of individuals. In the hope of saving the species 
there has been for some years a standing offer of consider- 
able sums of money for information of a nesting pair of this 
species. The domestic pigeon, in its numerous varieties, has 
arisen by breeding and selection from the blue rock pigeon of 

FIG. 228. Gold-finch (Spinus tristis). U. S. Dept. Agriculture Year-book, 1898. 

the Old World. In cross breeding we frequently get "rever- 
sions ' ' to this type, showing that the characters are being carried 
in the germ plasm even when the combinations were not such as 
to make them appear in the body. 

In this group the young are fed with food digested by the 
parent and regurgitated into the mouth of the helpless young. 

The Pici (woodpeckers) have two toes directed forward and 
two backward in adaptation to the position they take in climb- 
ing. The tail may have stiff, pointed feathers with which they 



FIG. 229. 

FIG. 229. Mocking Bird (Mimus polyglottos). From Dept. Agriculture Year-book, 1895. 

FIG. 230. 

FlG. 230. Wood Thrush (Hylocichla mustelina). Female, one-half size. By J. W. Folsom. 



anchor when pecking or climbing. They have strong, sharp, 
chisel-like bills. By means of these they get at insects under the 
bark of trees, dig into the tender cambium of plants, and even 
excavate cavities for nests in the timber. With their long, 
protrusible tongue they pick up insects routed out by the 
hammering of their beaks. The tapping sound is also a means of 
attracting mates. Some species bury acorns in holes they have 
made in the trees, returning for them when food supplies are 

FIG. 231. 

FIG. 231. The Meadow Lark. From U. S. Dept. Agriculture Year-book, 1895. 

They are striking birds, often most interestingly colored. 
Our most common forms are the red-headed woodpecker, the 
downy woodpeckers, the flicker, the yellow-bellied sapsucker. 
The latter is an injurious species, as it opens trees to decay and 
attacks of fungi, and probably spreads such diseases. The rest 
help keep in check various insect pests. 

Near the woodpeckers are often classed the cuckoos, king- 
fishers, and toucans or hornbills. The kingfisher and rain crows 
or American cuckoos are likely to come under the observation 



of the student. Some species of cuckoos lay their eggs in the 
nests of other birds, where they are hatched and fed by the host. 
In return for this they are said often to cast the young of the 
proper owners out of the nest. 

The chimney-swifts, whippoorwill, and the humming birds 
are somewhat intermediate between the forms just spoken of 
and the next order. 

The Pas seres (sparrow-like) is the greatest order of birds. 
It has more than sixty recognized families and more than 6,000 

FIG. 232. 

FIG. 232. Loggerhead Shrikes (Lanius ludovicianus). By J. W. Folsom. 

species. About twenty to twenty-five of these families are found 
in North America. These birds are mostly small, with three 
toes in front and one behind, and adapted to perching. The 
majority are gifted with some powers of song (oscines). A 
smaller division, of which the king bird and phcebe may be taken 
as types, are known as chattering or crying birds (clamatores) . 
Among the families of oscines, which the student is most 


likely to meet, are: the crow family (Corvida; Fig. 226) including 
crows, and blue jays; the sparrows and finches (FringillidcB), 
including also snow birds, crossbills, and grosbeaks; the oriole 
family (Icterida) including orioles, bobolinks, meadowlark, and 
the blackbirds; the warblers (Mniotiltida) , small migrating 
birds with bright colors and most interesting and attractive 
qualities; the wrens (Troglodytida) ; and, finest of all, the thrush 
family (Turdidoz), including the various thrushes, the robin, 
bluebirds, and many others. 

For further description of the numerous interesting families 
of Passeres the student must refer to some special book on 
birds. The study of their habits and form constitutes one of 
the most popular and entertaining subjects of natural history 
for the recreation studies of busy people. Much good, and 
some very indifferent, literature intended for guidance in such 
studies is now being produced. 

442. Relation to Nature. It is easy to be seen from the fore- 
going discussion that birds, while very much alike in funda- 
mental structure, have succeeded in adjusting themselves to a 
remarkably varied life. In structure this adaptation is shown 
in the size and shape of the body, in the modifications of such 
external structures as neck, beak, legs, toes, claws, wings, and 
tail. Among the instincts and habits the following have a large 
place in the adaptations of birds: the food habits, instincts of 
song, nesting and breeding instincts, care of young, and response 
to climatic changes as shown in the migration impulses. 

Wings vary from the most rudimentary remnants, as in the 
ostrich and apteryx (kiwi), to the powerful ones of ducks which 
drive the birds at a rate of ninety miles an hour. The tail is 
equally variable. There may be no special tail feathers, as in 
the kiwi, or the tail may be of the most striking character, as 
in the male peafowl or bird of paradise. In these extreme styles, 
as in many birds of smaller size, the tail is a sex development, 
the male usually having greater specialization. Tails of more 
normal size serve as balancers when the birds are perched and 
as rudders when they fly. 

The legs and neck are closely related in length. If the legs 

AVES 431 

are long the neck must be of corresponding length to allow the 
head to reach the ground. Long legs are often coupled with a 
wading habit; though in the ostrich and some of the land birds 
length is associated with the running habit. 

The feet, including the number and position of toes, the 
character of the claws, and the presence of the web, are a good 
index of the manner of life. The amount and arrangement of 
the web varies greatly in the different water birds. The beak 
and the feet, though separated by the extreme length of the 
animal are really three appendages which work together. Aside 
from the prime functions of the feet in walking and swimming, 
the beak and claws work in close cooperation in getting food, in 
offense and defense, and in the nest-building operations. Both 
claws and beak are strikingly adapted to the food habits. 

By means of the syrinx (437) most birds produce sounds. 
In the higher birds (oscines) the sounds may become highly 
organized and melodious. Such songs are for the most part 
related to mating and the breeding season. The call-notes are 
more permanent though not so full of melody. By these the 
birds express such simple emotional states as fear, anger, con- 
tentment, and excitement. They are a simple language well 
understood by the other members of the species. It is probable 
that the mating songs are similarly expressions of the exuberant 
emotions of the mating time, which become sustained and 
elaborated from the pleasure they come to give the producer. 
In other words they represent a passage from the useful to the 
esthetic even for the bird himself. 

The succession of instincts which in the spring will cause 
even one-year-old birds to go with remarkable precision through 
the migration, the selection of a suitable place and materials 
for nest making, the mating, egg-laying, incubation, feeding 
and protecting the young, and finally their introduction to the 
world, is at once the marvel and despair of the naturalist. In 
all these respects there is the greatest variation. Some mate 
yearly, others mate for life. Some conceal their nests with 
greatest ingenuity and build it with great care, while others lay 
their eggs in the barest possible pfaces.^ They may lay one egg 
or a score. The hen may incubate the eggs or the male may. 


Sometimes both take part. The cock may supply the hen with 
food during incubation or pay no attention to the process. The 
young may be ready to leave the nest and run about as soon as 
they are hatched and dry (precocious) or they may be naked 
and helpless for days (altricial). 

The coloration of birds is most varied and interesting. They 
are to the vertebrates what the insects are to the invertebrates. 
As in insects, the color is due to pigment, to physical surface 
markings which produce luster, or to both. Males and females 
are often differently colored ; if so the males are the more striking. 
Young birds, before the sex qualities appear, are more like the 

Some birds are differently colored at different seasons. Where 
there is a difference the more highly colored feathers are pro- 
duced in the winter and spring as the mating period approaches. 
Color in birds is believed by some to be due to the high met- 
abolic activity they show. This activity is believed to pro- 
duce pigments as waste products. These come to be represented 
in the coloration of the feathers. The metabolism is more active 
at the mating season. Color may be useful to birds in two ways. 
It may serve as recognition marks by which members of the 
species may quickly recognize each other, and possibly prove 
attractive to mates; and in some instances at least it serves for 
concealment because of likeness to the surroundings. 

The eggs of birds are frequently remarkably colored. This 
color is in the calcareous secreted shell. It is due to* color se- 
creted by certain cells in the oviduct. The color of eggs of a 
species is often as distinctive as the color of the bird. It may 
be uniform, or in definite spots, or in irregular splotches. It is 
probable that the coloration of eggs is also frequently protective. 

None of the bird's responses to its conditions is more striking 
than the act of migrating. Some do not migrate, but most do. 
The value of the instinct is clear enough. Each species has its 
own customs. The time of starting, the route, the rate of migra- 
tion, and the termini vary for different species. Usually the 
rate is not rapid. Some fly pretty steadily for considerable 
distance. The ducks work their way along the streams and 
lakes, or from one to another. The warblers flit from forest to 

AVES 433 

forest, from tree to tree, or along the fence rows, living on the 
country. Sometimes it is just a slow retreat before the advanc- 
ing cold or progress with the advancing spring. At other times 
it may involve long flights. 

The golden plover illustrates an extreme case. They breed 
in the arctic circle in June and July. They "winter" south of 
the equator in South America from September to March; that 
is to say during the southern summer. Their course in the north- 
ward spring migration is not fully known, but it is believed that 
they follow an overland route over northern South America, 
Central America, Mexico, Central United States and Canada 
to their Arctic breeding grounds. The return journey south- 
ward is better known. From their breeding grounds they go to 
Labrador in August. Here they feed on berries and grow fat 
after their relative fast. They work their way down to the coast 
of Nova Scotia. Here they desert the land and fly across the 
ocean to the north shoulder of South America. They may stop 
temporarily in the islands of the West Indies ; or they may make 
the whole distance apparently without stop. In the course of a 
few weeks they cross the equator and reach their southern- 
most terminus in Argentine. Some other species have been 
followed with equal care and have almost equal range. 

Birds have many enemies among animals. Some of the birds 
are prey to other birds. Snakes, some lizards, and many of the 
smaller mammals feed upon them whenever they can. Many 
eggs are eaten by these and other animals. During the nestling 
stage birds are in especial danger. The parents themselves are 
often taken at this time. Man himself has probably proved the 
worst enemy of the birds, which is scarcely wise or fair, since the 
birds are among his best friends. 

443 . Relations of Birds to Man. Man has directly destroyed 
many of the wild birds for food, for their plumage, for sport, or 
because he deemed them injurious to his interests. In an in- 
direct way he has destroyed even more. Many of the birds do 
not take kindly to civilization, but prefer the quiet of unchanged 
nature for feeding and breeding. Such birds have been gradu- 
ally pushed into more limited or less favorable territories. 


Others have adjusted themselves to human surroundings and 
to our crops of grains and fruits. These have suffered less; 
indeed some of them have larger numbers than would be possible 
in the wild state. Still others have been domesticated, and by 
breeding, selection, and support have been greatly changed and 
improved for human uses. The birds that have been domesti- 
cated are chiefly the ostriches, members of the goose family 
(ducks, geese, swans), several Gallinae (chicken, turkey, guinea- 
fowls, pheasants, pea-fowls) a few of the Rap tores (as the falcon), 
the pigeons, and occasional members of other orders, as parrots, 
magpies, and occasional song birds. 

The student of Zoology should not fail to familiarize him- 
self with the main types of chickens which have been developed. 
A few of the main classes of breeds are as follows: the Asiatic 
breeds, including Brahmas, Cochins, Langshans; the Mediter- 
ranean breeds, including Leghorns, Minorcas, and Black Spanish; 
the Dutch breeds, Hamburgs and Red-caps; the French breeds, 
Houdans, and Crevecceurs; the English breeds, as Orpingtons 
and Dorkings; the American breeds, as Plymouth Rocks, Wyan- 
dottes, Dominiques, and Rhode Island Reds; and fancy breeds 
as the Bantams, game birds, etc. 

The special egg breeds are the varieties of Leghorns, Minorcas, 
Spanish, and Red-caps. The meat producing breeds are Bramas, 
Cochins, and Langshans. The Plymouth Rocks, Wyandottes, 
Rhode Island Reds, Orpingtons, and Dorkings are desirable 
breeds for general service in both respects. 

Aside from the food values of poultry, the group of birds 
renders man most notable service in two ways, by destroying 
noxious insects and by eating weed seeds. To be sure they do 
take some toll in cultivated grains and in fruits; but with very 
few exceptions it has been shown by examination of stomach 
contents of various species at various ages that the great bulk 
of the smaller birds are helpful to human interests, and often 
greatly so. An observer found 7,500 seeds of one common weed 
in the stomach of one dove. If all birds were suddenly destroyed, 
unquestionably many species of insects would at once increase 
to the point where they would be a fearful pest to mankind, 
before a new balance in nature would be struck. Mice and 

AVES 435 

other hurtful rodents are captured by owls, hawks, and other 
preying birds. 

These facts are at the bottom of the recent agitation that 
egg collecting and the slaughter of birds for sport shall stop. A 
careful investigation after seven years of close study gives the 
following list of birds which do more injury than good; the Eng- 
lish sparrow, Cooper's hawk and the sharp-shinned hawk, the 
sapsucker, and the crow. Recent investigations show that the 
robin is the most numerous bird in the United States, averaging 
six pairs to every farm of 58 acres. The English sparrow comes 
second with five pairs to that area. For every 100 robins there 
were found 49 catbirds, 37 brown threshers, 28 house wrens, 
and 26 bluebirds. 

The skins and feathers of birds have been much used for 
ornament by savage men and civilized woman. The traffic 
in skins and plumes has been a very extensive and profitable 
one, and is calculated to hasten the extermination of some 
species. Laws are gradually becoming more prohibitive and 
public sentiment is coming to support them. Shooting birds 
for sport and encouraging killing them for their plumage are 
alike barbarous. The national government, the states, and 
private individuals should be encouraged to establish havens 
where birds may not be killed. 

Because the undigested material from the digestive tract and 
the nitrogenous excretion from the kidneys are eliminated to- 
gether the manure of birds is very rich. Large deposits of this 
material, known as guano, are found along the dry coasts and 
islands on the west shore of South America. It is deposited by 
sea-birds that have lodged there for ages. The richest of these 
deposits are already exhausted. 

Of a real vital value to man are the song and color and the 
poetry of the life of birds. These esthetic features of birds and 
their wonderful habits and instincts have attracted thousands 
of people to the fields and woods. Man cannot get back to 
nature and the wild life in an open-minded and appreciative 
way without being the gainer. It is by no means a shallow 
appeal that we encourage the bird life in order that nature may 
be kept as interesting and beautiful as possible. The fact 


that birds are useful need not obscure the fact that they are 

444. Special Topics for Investigation in Field and Library. 

1. Enumerate the special structural features which seem to fit birds for suc- 
cessful flight. Compare different birds as to these features? What are the 
different modes of flight? Compare the flight of the buzzard, the woodpecker, 
the quail. What is the action of the wings in flying? Of the tail? What is the 
effect of clipping one wing? Why? The rate of flight in different species of birds. 

2. Study the group of birds from the point of view of their social and gregarious 
instincts. Are any solitary? Do any have varying social habits during different 

3. Make a general study of the migrations of birds, collecting the facts as to 
range, time, supposed causes, the effects on the species and its geographical distri- 
bution, the degree of exactness in routes and the place of return. 

4. Make a special study of the birds of the locality in which you are. Are 
there permanent residents? Summer residents? Winter residents? Migrants 
(those which stop only for a short time in the spring or autumn as they pass from 
south to north or the reverse)? Keep a record from year to year of the earliest 
dates at which migrating species are seen in your locality. 

5. What diversity is there in the mating habits of birds? Are any mono- 
gamous? Polygamous? What are the mating habits of the cuckoo? 

6. Make a report as to the nest-building habits of selected species of birds. 
How do the nests differ in location, in mode of formation, in perfection? Is there 
any relation between the character of the nest and the degree of development of 
the young when hatched? What range of variation in the number of eggs? In 
the mode of incubation? The period of incubation? Care of the young after 
hatching ? 

7. Compare the vocal powers of birds with that of vertebrates hitherto studied. 
Compare various types of birds as to the range and character of their notes. How 
are the notes of birds related to their states of mind ? Which are more vocal, the 
males or the females ? What explanations are offered for this ? 

8. What is the history of the English sparrow in this country? What are its 
habits? How do you account for its rapid spread? 

9. Make a special study of the local distribution of the species of birds known 
to occur in your vicinity. Which prefer the meadows? The marshes? The 
streams? The woodlands? Do the different species nest in the same regions in 
which they feed ? 

10. The relation of selected species of birds to man. Are they helpful or harm- 
ful to his interests? Has he been helpful or harmful to them? 


445. Laboratory and Field Work. Almost any of the 
smaller mammals may be used in the following exercise. Dif- 
ferent species may be taken with profit by the various mem- 
bers of a class. The chief points to be emphasized are the 
habits, instincts and external structure. 

I. Habits and Instincts. 

What are its natural haunts? What explanation can you 

offer therefor? 
How does it protect itself from its enemies? What are its 

enemies ? Is it active at night or by day ? Reasons ? 
What are its habits as regards food? Evidences? 
What can you say of its power and manner of locomotion ? 

Does the manner of motion differ materially, with difference 

of rate ? 
Social habits? Mating habits? Care of young and their 

condition at birth ? 

Is it scarce or abundant ? Apparent reasons ? 
What are its relations to human interests ? 

II. General Form and Structure. 

Identify the regions of the body and compare the condition 
found here with that seen in the birds. Relation of axis 
of body to appendages. Compare the anterior and posterior 
appendages at all possible points, and indicate to what ex- 
tent the work done by each is indicated by the structure.. 
Examine the claws and the soles of the feet. 

Examine the body-covering, and compare the various parts 
as to color, character of hair, etc. Does the hair com- 
pletely cover the body? What is the position and use of 
"whiskers" ? Evidences for your conclusion. 

Locate all the external openings. Study the mouth with its- 
contained structures; the eyes: position, color, lids (is 


438 ZOOLOG\ 

there a nictitating membrane ?) ; ears. To what extent is 
the external ear developed ? 
III. Internal Structures. 

(The rat, rabbit, or cat will serve if it is desired to dissect a 
mammal). The student should be encouraged to map out 
his own laboratory outline by reference to descriptive texts. 
Two fundamental questions should be kept in mind in all 
such work: How do the discovered structures compare 
with those studied in other animals ? What service do they 
render the animal and how well are they adapted to do the 
work put upon them ? 

446. The Mammalia embrace, on the whole, the most highly 
developed vertebrates. To this group man belongs. The birds 
are more highly specialized in some respects, but the mammals 
surpass the birds in the size and convolutions of the brain, and 
in the closer relations between the mother and offspring both 
before and after birth. The form of parental care seen in the 
Mammalia is an adaptation resulting in great advantage to the 
young, and has also produced a great improvement in the 
mental qualities of the parents. The class contains forms of 
very varying appearance and perfection of development, and 
suited to almost every mode of life. Many are aquatic, in- 
cluding the largest living animals, the whales; some burrow in 
the soil, as the mole and many rodents; some live largely in 
trees, as the monkeys, squirrels, sloths, etc.; a very few, as 
the bats, have acquired the power of flight; others the vast 
majority live on the dry land. 

447. General Characteristics of Mammals. 

1. Mammalia are air-breathing vertebrates in which the 
covering developed by the epidermis is hair. 

2. In the female, mammary glands occur in the skin, by the 
secretions of which the young are nourished. 

3. The diaphragm, a muscular partition, completely sepa- 
rates the body cavity into two, an anterior or thoracic and an 

4. With a few exceptions the Mammalia are quadrupeds. 

5. Heart is four-chambered; the temperature of the blood 


not determined by that of the surrounding medium; red blood 
corpuscles not nucleated; one (the left) aortic arch persists. 

6. Two occipital condyles. 

7. Chiefly viviparous (Monotremes are oviparous); foetus 
nourished during early development in the uterus of the mother, 
often being closely connected therewith by a complex structure 
known as the placenta. 

448. General Survey. There are three subclasses of 
mammals which differ in mode of reproduction and in degree 
of development. 

1. The Monotremata are the lowest and are characterized 
by the fact that they lay eggs, like reptiles and birds; there is a 
cloaca into which the alimentary, urinary, and genital canals 
open; the milk glands are poorly developed. The class is rep- 
resented by the duck-mole, an aquatic form, and the spiny 
ant-eater, both natives of Australia and neighboring islands 
(Fig. 238). 

2. The Marsupialia (pouched) possess a marsupium or 
pouch, a fold of the skin into which the prematurely born young 
are placed and nourished until able to take care of themselves. 
The period of gestation is short and the connection between the 
embryo and the wall of the uterus is slight. In the group are 
embraced the kangaroo and other Australian forms, and the 
opossums of America. It is an interesting fact that the native 
Australasian mammalia all belong to these two lower classes 
(see Figs. 52, 62). 

3. In the Placentalia (placenta) there is a placenta or mass 
of closely interwoven maternal and embryonic tissue which 
unites the foetus with the wall of the uterus, by which arrange- 
ment the young gets its food and oxygen from the blood of the 
mother. The young are retained much longer in the uterus, and 
are consequently much more mature when born. All the 

.common mammals belong to this group, which is distributed 
over the habitable part of the earth. 

449. Form. The axis of the body is usually separable into 
head, neck, trunk, and tail, though the last may be reduced 
to a very small number of segments. The proportions of these 



parts of course differ much and are to be connected with the 
habits of life. Most of the Mammalia are quadrupeds (except 
the allies of the whales, the porpoises, and the sea-cows); and 
all except man, and some of apes most like him, have the axis 
of the body in a horizontal position supported by all four 
appendages, or by the medium. The aquatic forms whales, 
porpoises, etc., become more or less fish-like in form, in adapta- 

FIG. 233. 

-- n 

FIG. 233. Diagram of Skin in Mammals, by Folsom. 

tion to the medium. There is an enormous range in size in 
the group, from the mice to elephants and whales. 

450. Supplementary Topics for Laboratory and Field Work. Compare the 
relative size, length, etc., of head, neck, trunk and tail of various types of mammals, 
using well known animals and the figures and descriptions of less familiar ones. 
Can you find any signs of connection between any of these facts and known habits 
of the animals studied ? 


Compare the anterior with the posterior appendages in a selected series of 
mammals, keeping in mind the following points: size, length, strength, uses; the 
number and character of the digits. Compare similarly the corresponding (i. e. t 
anterior with anterior) appendages in another series. Keep in mind, throughout, 
the adaptations of structure to the conditions of life, method of locomotion, etc. 

451. Integument. The skin, as in forms already described, 
consists of two portions, an ectodermal portion, the epidermis, 
and the dermis or true skin which is derived from the mesoderm 
(Fig. 233). Hair is found in the young of all mammals, though 
it may be wanting in the adult (as in whales) , or may occur only 
sparsely. Hair is produced by the epidermis, but is nourished 
by a papilla of dermal tissue (Fig. 233, /). Each hair consists 
of a central part, or pith, surrounded by a denser cornified por- 
tion, the cortex. Hair differs much in color and in structure, 
from the soft fur of the seal to the quills of the hedgehog and 
porcupine. Hair is considered as morphologically similar to the 
feathers of birds and the scales of reptiles. 

To be considered in the same connection with hair are the 
nails, claws, and hoofs, the scales on the tail of the rat or beaver 
and on the body of scaly ant-eaters, and the horny material *of 
horns of cattle and of rhinoceros. The whalebone of whale is 
also an epidermal product in the roof of the mouth. 

452. Supplementary Studies for Field and Library. What is the economic 
value of the skins of mammals? How are they prepared for the uses to which they 
are put? What animals are prized for their hairy products (fur, wool, etc.)? 
What special qualities must the hair have to be useful in making cloth? 

What instances can you adduce of advantageous coloring in the hair of mam- 
mals? What variations of color may be found within a single species? What 
changes of color are possible to a single individual? How are these changes 
brought about? What peculiar qualities have the quills of the porcupine? 

453. Integumentary Glands, derived from the epidermis, 
are common in mammals. Associated with the hairs are the 
oil glands. Over various parts of the body are long tubular 
sweat glands buried in the dermis. There are also integumen- 
tary glands producing characteristic odorous substances. These 
may be for recognition within the species, or for protection. 
The tear glands of the eye are modified cutaneous glands. 

The mammary glands, which are characteristic of the group, 
are specially developed skin glands, apparently more allied to 


the oil glands in their secretion. They are much lobed, and 
usually have teats or nipples ; but in the monotremes these are 
wanting, and the young merely lick the secretion from a "milk 
area." The glands may be distributed along the entire ab- 
dominal surface (carnivora) or confined either to the anterior 
(primates) or posterior portion (ruminants). The number of 
the milk glands is correlated in a general way with the number 
of young produced at a birth. 

454. Skeleton. Some of the more elementary facts concerning the skeleton 
may be summarized as follows. The vertebrae unite by flat faces, and the five re- 
gions of the vertebral column (see 349) have a fair degree of constancy as to 
numbers. The neck, with a very few exceptions, has seven vertebrae, the length 
of the neck depending on the length of the vertebrae and not on their number. The 
trunk vertebrae, made up of the thoracic and lumbar, usually vary within the limits 
19-23. The caudal vertebrae are most variable of all. The bones of the skull in 
the adult have their edges closely united by means of sutures (a species of close 
joint, which does not allow of motion). The lower jaw, the hyoid bone, and the 
small bones of the ear are the only movable bones in the mammalian skull. The 
lower jaw articulates directly with the cranium. The quadrate, which in reptiles 
and birds serves to articulate the jaw with the cranium, has apparently changed 
its position and given rise to one of the small bones of the middle ear. 

The pectoral girdle and arm bones are always present, but in the whales and 
sea-cows the posterior are lacking. The digits are typically five in number. In 
many carnivores these may be reduced to four, terminating in claws. In the hoofed 
form3 the toes are often reduced to four, three, two, or even one (the horse). In 
such cases rudiments of the remaining digits may occur in the form of splints. 

455. Teeth. The teeth are produced by the skin, and come 
to be lodged in pits in the bones of the jaws. While differing 
in shape, the teeth always possess the crown, the fang or root, 
the neck and the pulp-cavity. The bulk of the tooth is dentine 
deposited by the dermis. Over this is a layer of enamel formed 
by the epidermis. The teeth differ from horny outgrowths in 
that both layers of the skin are involved. In this respect they are 
like the scales of fishes and the plates of the armadillo 

The cavity is more or less filled with "pulp tissue" which is 
supplied with nerves and blood vessels. Most mammals have 
only two sets of teeth, a milk set which appears early and is 
lost and a permanent set which replaces the former. In some 
cases, however, there is only one set, and in a few (e.g., whales) 
no teeth appear above the surface of the gums. 

In the porpoises, dolphins and similar forms the teeth are 


numerous, simple, and very much alike, but in the majority 
of mammals there are at least three types of teeth. In the front 
of the upper jaw (on the premaxillary bones) are simple, chisel- 
shaped teeth, the incisors; behind these (the first tooth on the 
maxilla) is the canine tooth, usually pointed and adapted for 
tearing; posterior to the canines are the grinders or molars. 
Those grinders which replace milk teeth are sometimes called 
premolars. The true molars do not have any representatives 
in the milk set. The corresponding teeth in the lower jaw are 
similarly named. The typical number of teeth is forty-four, 
eleven in each half -jaw. This may be shown by a formula in 
which the numerator indicates the number of each kind in one- 
half of the upper jaw and the denominator a similar portion 

of the lower: i. , c. -, p. -, m. , = 44. This means that there 

are three incisors, one canine, four premolars, and three molars 

in each half jaw, both above and below. The dental formula 

2X2 3 

for adult man is: i. , c. , p. , m. , = 32. This may be written 

2 I 2 "3 

more simply '. The numbers are not always the same in 
the upper and lower jaw of mammals. 

456. Supplementary Studies. Let the student determine by examination, 
and write the dental formulae of the cat, dog, horse, cow; milk set in man. 

Compare the molars of some carnivorous animals with those of some herbivo- 
rous; similarly the canines. Describe the action of the jaws in the act of chewing in 
the dog, cow, rabbit, horse. 

457. The Digestive Organs present the same regions and 
general arrangement found in the typical vertebrates. There 
are usually fleshy and movable lips covering the teeth. Some- 
times these are much extended and in connection with the nose 
may become important organs (snout, proboscis) for the capture 
of food. The stomach varies widely but is ordinarily a simple 
sac with muscular walls. Sometimes it is partly separated into 
chambers by folds (Figs. 234, 235). This reaches its greatest 
complexity in the ruminants, in which four chambers occur 
(Fig. 235). One of these the rumen becomes a temporary 
receptacle for the food which is first swallowed without being 



chewed. This peculiar structure is correlated with the habit 
of rapid feeding and retirement to less dangerous or exposed 
locations, where the- food is forced back to the mouth in appro- 


FIG. 234. Diagram of stomach of dog (A) and rat (B). After Wiedersheim. 

FIG. 235. Diagram of stomach of Ruminant. After Wiedersheim. 

Questions on the figure. What is the significance of the term ruminant? Of 
what conceivable advantage is this form of stomach? What animals belong to the 

priate quantities and chewed at leisure. When swallowed the 
second time the food passes on to the glandular divisions of the 

The liver and the pancreas pour their secretions into the 



small intestine near its anterior end. The small intestine is 
very much shorter in flesh-eating animals than in the vegetable 
feeders. At the junction of the small and large intestine there 
is a blind pouch or sac (c&cum, vermiform appendix) which 
is large in the Herbivora, but in man it is a mere rudiment. It 
is doubtful whether it has any function in the human race. 
It is often the seat of serious inflammation. 

FIG. 236. 

FIG. 236. Diagram of the heart and chief vessels in the mammals, ao., aorta; a./., left auricle; 
o.r., right auricle; c, carotid artery; d.a., dorsal artery; p.a., pulmonary artery; p.v., pulmonary 
vein; s, subclavian artery; v.c., venae cavae (pre-caval and post-caval); v.l. t left ventricle; v.r., right 

Questions on the figure. What kind of vessels communicate with the auricles? 
What with the ventricles? What is the position of the valves? Trace the direc- 
tion of the blood flow in the various parts of the blood vessels figured. What is 
the distribution of the veins and arteries shown here, i.e., to what organs do their 
minuter branches go? 

458. Circulatory System. Mammals are warm blooded, but with lower tem- 
perature than is found among the birds. It ranges from 35 to 40 C. The heart 
is completely four-chambered as in birds, the left side containing pure blood and 
the right impure (Fig. 236). The aorta, arising from the left ventricle, has only 
one arch the left, whereas only the right is found in birds. The general compari- 
son of the conditions in vertebrates may be seen from the table on page 343. There 
is an hepatic-portal, but no renal-portal, circulation. 

The lymphatic vessels are an important part of the circulatory apparatus in 
all vertebrates. Under the pressure that exists in the arteries, some of the fluid 
portion of the blood finds its way through the walls of the capillaries into the spaces 
among the tissues. This cannot get back into the veins, and hence it is desirable 
that special vessels be provided to get it back into the circulation. Starting with 


the irregular spaces in the tissues, in which the lymph collects, we find vessels less 
regular than the veins, often running together and then rebranching, gradually 
approaching the body cavity. On their route they pass through knots of special 
tissue lymphatic glands, where colorless amoeboid cells are added. Special 
lymphatics the lacteals gather food from the intestines and, uniting with the 
general lymphatics, finally empty into the large veins in the neck region. The 
escaped lymph is thus returned to the blood. 

459. The Respiratory Structures differ from those of birds 
chiefly in the fact that they are confined to the anterior or 
thoracic cavity, in which they hang freely, suspended by the 
bronchi. There are no air-sacs outside the lungs, hence all 
the air passages terminate in the alveoli, in the walls of which 
are the pulmonary capillaries. Inspiration and expiration of air 
is affected by increasing and decreasing the size of the chest cavity 
by means of the muscles between the ribs and by the contraction 
of the muscles of the diaphragm which is normally arched for- 
ward into the chest. By its contraction the viscera are forced 
backward and more space is given to the lung, which at once 
fills the chest cavity as the result of air-pressure on the inside 
of the lung. 

460. Nervous System. The special feature worthy of note 
in the nervous system of mammals is the large size of the brain, 
especially of the cerebral hemispheres. In the higher mammals, 
particularly, these become complicated by folds and convolu- 
tions by which the surface or cortex of the brain is much in- 
creased. The brain cells, or gray matter of the brain, are espe- 
cially abundant in the superficial part, and therefore this in- 
crease of surface means that these cells are increased in amount 
as compared with other vertebrates. The intelligence of an 
animal is roughly proportional to the amount of the cortex. 
The cortex is a thin layer, varying from one to five millimeters 
in thickness. It has been estimated that there are about 9,000,- 
000,000 nerve cells in the cortex of a human brain. These nerve 
cells represent about one five- thousandth of man's total weight, 
and yet they furnish the basis for the conscious life and the con- 
trol of the body. The fibrous tracts connecting the various por- 
tions of the cortex are likewise more perfectly developed among 
the mammals, and most of all in man. 


The organs of special sense are similar to those in the birds. 
The ear becomes more complicated. There is usually a well- 
developed external ear, or pinna, in the terrestrial forms, which 
is often movable and serves to gather the sound waves. The 
membranous labyrinth of the internal ear becomes more com- 
plicated than in any of the lower forms. This is especially 
true of the cochlea, which becomes spirally coiled. The middle 
ear is bridged by a series of three bones, instead of one or two 
as in the lower groups of vertebrates where such connection 
exists at all. 

461. The Urinogenital Organs. As in the other vertebrates there is close 
connection between the excretory and reproductive organs in mammals. The bean- 
shaped kidneys communicate by ureters with a median urinary bladder, which in 
turn has the urethra leading to the outside. The urethra also serves as the outlet 
for the sperm in the male. The testes, which in other vertebrates lie in the body 
cavity, pass backward and descend into a fold of the skin, in the majority of mam- 
mals. In the female, the ovaries are in the abdominal cavity, and when the ova 
are ripe they break forth into the cavity and pass into the fringed, funnel-shaped 
mouth of one of the two oviducts. The oviducts may be completely distinct, 
opening separately into the vagina (as in most rodents), in which case each has a 
special portion in which the young are retained during early development (uterus') ; 
or there may be found various degrees of union of the uterine portions until there 
is a single uterus into which the two oviducts empty (as in man). 

462. Reproduction and Development. All the mammals 
except the monotremes are viviparous. Impregnation may take 
place in the oviduct or in the uterus. In the Placentalia the 
ova are small and have little yolk, whereas in the Monotremes 
there is much yolk, as among the birds. The segmentation in 
the placental mammals is complete but not necessarily equaL 
A solid sphere of cells is formed which becomes differentiated 
into an outer enclosing layer (thetrophoblast, Fig. 237) and an inner 
mass of cells (Fig. 237, ent.). It is the mass of cells that 
gives rise to the embryonic layers, from which are produced 
the adult organs. The trophoblast has little or no part in 
the formation of the embryo proper, but has a part in forming 
the foetal membranes so important in the group. The steps of 
embryonic development, while similar in general to those de- 
scribed for the other Vertebrata, are modified by the absence of the 
yolk and the retention of the developing egg in the body of the 
parent. The embryonic membranes amnion and allantois 



occur as among birds, but their fate is somewhat different. - The 
allantois typically fuses with the outer layer of the amnion (false 
amnion Fig. 208, am?) and the trophoblast (see above), and this 
combined tissue, the chorion, becomes connected with the wall 
of the uterus by outgrowths or villi. These become closely asso- 
ciated with the tissues of the mother. This combination of 
maternal and embryonic tissues is called the placenta, and is the 
characteristic organ of the Placentalia or true mammals. 

It is by means of these united tissues that food and oxygen 
pass from the blood of the mother into the blood of the embryo. 
In the marsupials the attachment is very slight, and for this 

... t 


FIG. 237. Diagram of Segmentation of ovum in Mammals. A, ovum; B, showing the early 
differentiation into an outer layer which produces the trophoblast (see p. 447), and an inner mass 
which produces the embryo; C, a later stage, ect., ectodermal portion of embryo; ent. % cells destined 
to produce entoderm; in., inner mass of the cells which form the embryo; o, outer layer which 
forms t, the trophoblast. 

Questions on the figures. How does this differ from the segmentation in the 
sea-urchin? What is the fate of the trophoblast? Examine reference texts and 
learn how the ectoderm, entoderm, and mesoderm of the real embryo (the inner 
mass of cells) are formed. 

reason uterine nutrition becomes insufficient relatively early 
and the young must be provided for in some other way. The 
marsupium, in which the milk glands open, presents the solution 
of the problem of later development of the fcetus. So at birth 
the immature young of marsupials are placed by the mother in 
the pouch. It is important to remember that the.blood vessels 
of the mother and the embryo are not continuous. The blood 
of the embryo is developed in the same manner as its other tissues 
and is not derived from the mother directly. The two blood 


currents interchange materials by osmosis through the capillary 
walls. There is no direct continuation of nervous tissue across 
from the mother to the young. Notwithstanding this lack of 
direct connection- each exerts a very profound effect upon the 
other. It is known that the embryo secretes materials which 
passing into the blood of the mother produce the growth of the 
mammary glands that leads finally to lactation. Proper nutri- 
tion, malnutrition, and poisons in the blood of the mother 
greatly influence the growth and normality of the offspring. 

463. Classification of Mammals. In the introductory sur- 
vey in 448 the three subclasses have already been outlined. 
Subclass I. Ornitkodelphia (bird uterus), or Monotremata. 

FIG. 238. 

FIG. 238. Duck-bill (Ornithorhynchus anatinus) . Photographed by Folsom. 

Questions on the figure. What are the peculiarities of Ornithorhynchus? 
What does the structure of its feet indicate as to its habits? 

Mammals whose mammary glands have no nipples ; they lay eggs 
with abundant yolk, which are hatched outside the body, as in 
birds. The alimentary canal ends in a cloaca. One ovary is 
sometimes incompletely developed as in birds, and the oviducts 
open separately into the vagina. The duck-bill or duck-mole 
lives in water, or burrows in the banks of streams or lakes. It 
is eighteen or twenty inches long and is covered with soft fur. 
Its eggs are laid in its burrows. Echidna or the spiny ant-eater, 
lives in rocky places and captures ants by means of its slender, 


sticky tongue. They are confined to Australasia and are in- 
teresting chiefly because of their likeness to the reptiles and 
birds (Fig. 238). 

Subclass II. Didelphia (two uteri), or Marsupialia. Mam- 
mals with nipples; these occur in the pouch on the ventral surface 
of the body in which the immature young are placed at birth. 
The young are too immature to suck voluntarily at first, and milk 
is forced into the mouth by the action of muscles about the 
gland. The pouch is usually supported by two bones attached 
to the pubis and running forward. There are two oviducts, 
two uteri, and even the vaginas may be paired (Figs. 52, 62). 

Many different types are included in this group. Some are 
rat-like in appearance, others similar to the dog, others to the 
bear. Some are herbivorous, some carnivorous, others insec- 
tivorous. With the exception of the American opossum fam- 
ily, the living species are native of Australasia. Fossil marsupials 
are found in all parts of the world, showing that they are an 
ancient type of mammals which have become extinct except in 
the places cited. Many of the fossil forms were of gigantic size. 
The largest living species is the kangaroo. 

Subclass III. Monodelphia (one uterus), or Placentalia- 
Mammals in which the young are connected to the wall of the 
maternal uterus by means of a placenta (see 462) ; two oviducts; 
uteri more or less united into one; vagina single; no cloaca; no 
marsupium. The segmentation of the ovum is total. 

The following key will assist the student to get a view of 
the principal orders of the placental mammals : 

Teeth wanting, or without enamel Edentata. 

Teeth with enamel. 

Hind limbs wanting. 

Front appendages with elbow joint Sirenia. 

Front appendages without elbow joint Cetacea. 

Hind limbs developed. 

Nails of the digits hoof -like Ungulata. 

Nails claw-like. 

The front limbs modified to form wings, 



No wings. 

Thumbs not opposable. 

Incisors and canines small, 

Incisors chisel-shape and canines 

wanting Rodentia. 

Canines large; other teeth often pointed, 


Thumbs opposable Primates. 

Order i . Edentata (toothless) . Placentalia in which the teeth 
are absent or imperfect, being destitute of enamel and true roots. 
They are found both in the Old World and in the New, especially 
in the tropics of the southern hemisphere. The chief repre- 
sentatives are the sloths and the hairy ant-eater of Central and 
South America, the armadillo of South America and as far north 
as Texas, and the scaly ant-eaters of Asia and Africa. The 
sloths are sluggish vegetarians living in the trees, on the branches 
of which they hang or climb, back downward, by means of their 
long curved claws. The ant-eaters are almost wholly devoid of 
teeth, but have narrow extensible tongues which they project 
into ant-holes, capturing the ants by the sticky saliva. The 
group is primitive and degenerate, and furnishes a noteworthy 
exception to the statement that the mammals lack an external 
skeleton. Overlapping bony scales, or plates in the form of 
rings, may furnish a complete armor by means of which they are 
kept from extermination in spite of their inoffensive, sluggish 
habits. This is best seen in the armadillo. 

Order 2. Insectivora (insect-feeders). These are small mam- 
mals with clawed digits, which feed on insects and other small 
invertebrates. The brain is small and smooth. The incisors 
are small. Many burrow, and have special adaptations for 
such a life; among these one of the most interesting is the de- 
generation of the eyes. The moles, shrews and hedgehogs are 
the chief representatives. The moles have the reputation of 
eating corn and other planted grains. Their burrows frequently 
follow the rows. Their food, however, is the insects that collect 
about the planted grain. They do much damage to turf in 
making their burrows. A single specimen has been known to 


burrow twenty yards in a day. For brief periods and in reason- 
ably firm soil they have been seen to dig a yard in ten minutes. 

Order 3. Sirenia. A small group of aquatic Placentalia, 
more or less whale-like in form. They are sluggish, ungainly, 
vegetable feeders. They have no posterior appendages and the 
anterior are flipper-like, though capable of bending at the elbow. 
They live near the shore and are represented by two living genera, 
the sea-cow of our own eastern shores (Manatee), and the 
Dugong of the Indian Ocean. 

Order 4. Cetacea (Whales, Porpoises, Etc.). The Cetacea are 
aquatic mammals with a fish-like body. There are no posterior 
appendages, and the anterior act as paddles, being without joint. 
The tail is horizontally expanded into a powerful paddle, and a 
dorsal fin is usually present. Teeth are present in the embryo, 
but may be lost or replaced by "whale-bone" in the adult. 
The stomach is chambered. The two mammae are posterior. 
Hair is very scant, but the layer of fat or "blubber" beneath 
the skin is very thick, and serves as a non-conductor of heat. 

The whales are the largest living animals. The largest of 
these, the Greenland whale, may attain a length of seventy-five 
feet or more. It must be remembered that the whales are air- 
breathers, and therefore must come to the surface to breathe 
or "blow." The Cetacea prey on the smaller swimming or 
floating animals found in the ocean, as fish, squid, Crustacea, 
etc. Whales are principally sought for their fat and baleen, 
or whale-bone. 

There are two principal suborders: the toothed whales, and 
the whale-bone whales. The former include the dolphins, por- 
poises, grampuses, narwhales, sperm whales, and beaked whales. 
The right whales, the hump-backed whale, the gray whale, the 
finback whale, and the sulphur bottom whale lose their embry- 
onic teeth and produce whalebone. Food is taken with the 
mouth open and the water is forced out through the whale-bone, 
eaving the "catch" in the mouth. The sulphur bottom may 
reach a length of ninety-five feet and a weight of 150 tons. 

Order 5. Ungulata (hoofed animals). This order includes a 
great number of animals, chiefly herbivorous, that walk on their 
toes. In these forms the horny growth which we have so fre- 


quently found in vertebrates at the end of the digits takes the 
form of a hoof. Toes are usually not more than four in number. 
The canine teeth are small or absent. The following suborders 
are important. 

Suborder (a) Artiodactyla (even-toed). These are ungulates 
with toes reduced to four or two. The third and fourth toes 
persist and bear the weight of the animal, and the second and 
fifth, if present, may or may not touch the ground. The 
mammae are distributed along the entire abdomen (hog) or are 
confined to the pelvic region (ox). 

This is a splendid group containing some of man's most 
valuable food animals. Included in it are cattle, sheep, goats, 
antelopes, elk, deer; the giraffes; the camels and llamas; the 
hippopotami ; and the hogs and peccaries. The ox, sheep, goat, 
deer, and giraffe are ruminants (see p. 444). 

The chief native American forms are llama, elk, moose, 
black-tailed deer, pronghorn antelope, Rocky Mountain goats, 
bighorn, musk ox, and bison. The bison was formerly one of the 
most abundant of our American ruminants. Now there are 
only a small number of specimens in captivity and fewer still 
wild. Most of these forms are capable of domestication, be- 
cause of their tractable disposition and their feeding habits. 
They are very valuable both for their hides and their food prod- 
ucts. In addition to the flesh foods, milk and its products of 
butter and cheese are among the most prized of human foods. 
Milk, as are eggs, is a complete food ; that is, it contains all the 
constituents, both organic and inorganic, necessary to support 
growth and development, as is shown by the fact that it is the 
sole supply of food for the young immediately after birth. 

Suborder (b) Perissodactyla (odd-toed). These are character- 
ized by the fact that the weight of the body rests on the third or 
middle toe, the others being more or less reduced. The stomach 
is simple. There is no proboscis. The mammae are few and 
confined to the pelvic region. The most common examples are 
the horse and its allies, in which the third is the only digit, and 
the rhinoceros, which has the second and fourth, as well. It 
is known that the remote ancestors of the horse had a second 
and a fourth toe where only splints occur now, and even a first 


and a fifth, where now there is no trace of either. The horse 
has been domesticated as long as we have records and there are 
as many breeds of horses as we found of chickens, some half a 
hundred. The ass and zebra belong to the same genus, Equus. 
They both cross with the horse. Tapirs are included in this 

Suborder (c) Proboscidea (with proboscis). Two living and 
many extinct species of huge Placentalia with five digits, each 
with a distinct hoof. The nose is much developed into a pre- 
hensile organ, with corresponding changes in the skull for at- 
tachment of muscles. The skin is thick and loose. The upper 
incisors grow enormously, forming the tusks characteristic of 
the group. There are no canines; molars are very complex. 
The two teats are thoracic. The largest of the land mammals, 
the elephants and the extinct mastodon and mammoth, belong 
here. They are now confined to the tropical regions of Asia 
and Africa, though in geological times they seem to have had a 
world- wide range. The tusks of species of elephants, both living 
and extinct, furnish the ivory of commerce. 

Order 6. Carnivora (flesh-eaters). The Garni vora are four- 
or five-toed animals with the digits ending in claws. The 
canines are well developed, strong and curved. The other teeth 
are often pointed and adapted to holding or tearing. Muscles 
of mastication are especially well developed. Mammas are 
numerous, occurring along the entire abdomen. There are two 
types of Carnivora terrestrial and marine. To the first belong 
the bear family, which is perhaps the least specialized group; the 
dog family, including dogs, wolves, foxes, jackals; the cat family, 
including lions, tigers, leopards; hyenas; many fur-bearing ani- 
mals as otters, weasels, minks, martens, badgers, wolverines, 
and skunks. There are more than forty species of these fur- 
bearing animals in North America. The seals and walruses 
belong to the marine group. In these forms the appendages 
have become adapted to the water habit, the digits bearing 
intervening webs. 

The principal American cats are: the wild cat (also called 
bob cat and catamount), Canada lynx, the cougar (also called 
puma, panther and mountain lion), and the jaguar of Mexico 


and South America which is the largest. Our native dogs are 
the timber wolf, the coyote, the red fox, the blue fox, the gray 
fox. We have several species of bears, polar, black, grizzly, 
and cinnamon. 

The greatest of the cats are the tigers, which may reach a 
length of ten feet, and the lions. These with the leopard are 
Southern Asiatic and African types. 

The order embraces many very powerful and intelligent 
animals which are well adapted to win in the struggle for life, 
if it were not for human interference. In the presence of man, 
however, all those which are not suited to domestication are 
gradually disappearing; some because of their dangerous quali- 
ties, others because of the value of their products. The group 
is not used to any considerable extent as food. 

Order 7. Rodentia (gnawing animals). The rodents are 
small mammals with clawed digits. They have no canine teeth, 

FIG. 239. 

FIG. 239. The Jumping Rat (Perodipus richardsoni) , adult male. Photographed from life by 

Dr. R. W. Shufeldt. 

Questions on the figure. What order of mammals is illustrated by this form? 
What explanations are offered as to the cause of the light color of the belly and the 
dark color of the backs of animals? Of what conceivable advantage is the differ- 
ence in coloration? How does the tail of this species compare with that of our 
common rat? 

45 6 


but have well-developed chisel-shaped incisors which continue 
to grow as they are worn at the extremity. The chisel edge is 
preserved by the fact that the enamel is chiefly in front, and 

FIG. 240. 

FIG. 240. The Fox Squirrel (Sciurus ludovicianus) . Photographed by Folsom. 

FIG. 241. 

FlG. 241. Porcupine (Erethizon). By Dr. J. W. Folsom. 

Questions on the figure. To what order of mammals does the porcupine 
belong? What are its peculiarities of habit and structure? 

the exposed, softer dentine behind is worn away more rapidly 
by being used. The brain is smooth. The mammae are ab- 
dominal. The rodents have world-wide distribution, and are 


especially well represented in North America. The principal 
types are the rats and mice, many of which are close followers 
of civilized man; squirrels, chipmunks, and prairie dogs, 
gophers, beavers, hares and rabbits, and porcupines. The 
rodents feed on vegetable diet, and are destructive of many 
plants and grains which man uses for food. Notwithstanding 
man's efforts to destroy them their remarkable power of repro- 
duction enables the more aggressive families to hold their own. 


FIG. 242. Flying Fox (Pteropus). U. S. Dept. Agriculture Year-book, 1898. 

Questions on the figure. What is the structure and arrangement of the wings 
in such a form as this? To what order of mammals does this type belong? 

Rats are among the worst of mammalian pests. They follow 
man closely and are very destructive of certain crops and of 
stored grains and other foods. Beside this the rats, and in less 
degree other rodents, are the carriers of the bubonic plague and 
possibly other diseases. Fleas carry the disease from rats to 

Order 8. Chiroptera (hand-winged; the bats). These are mam- 
mals in which flight is made possible by a web or fold of the skin 



stretching between the much extended fingers of the anterior 
appendages; between the arm, body, and the hind legs; and 
thence even to the tail. The thumb and posterior digits are 
clawed. The sternum has a keel as in birds. The mammae are 
thoracic. The bats are the only mammals capable of active 
flight. They feed on insects or fruits. The members of one 
family, the true vampires, live on blood of warm blooded animals. 
They cut through the skin with sharp front teeth and lick up 
the escaping blood. 

Bats are small animals and are chiefly nocturnal in habit, at 
which time they fly actively. During the day they seek retired 
and dark places where they hang head downward, holding on 
by their claws. Those in temperate regions hibernate during 
cold weather, often using caves for this purpose. (Pig. 242.) 

Order 9. Primates (first or highest). With the exception of 
man the primates are arboreal in habit. In adaptation to this 

FIG. 243. 

FIG. 243. Hand and foot of Chimpanzee. From Home and Country Magazine. 

Questions on the figure. Which is hand, and which foot? In what respects 
do they differ? How do they differ from the hand and foot of man? In which 
is the difference from the human condition greater? What is the functional 
meaning of these differences? 

the thumb and great toe are usually opposable to the other 
digits, as in the human hand. The digits are armed with nails 
which are in some cases claw-like. The cerebrum is large and 


in higher forms much convoluted. Mammae chiefly thoracic 
(abdominal in some lower forms). The group embraces the 
lemurs, monkeys of various kinds, baboons with non-prehensile 
tails, the tailless apes most like man, and man himself. While 
the primates have produced some species with the highest in- 
telligence to be found among mammals, and man places himself 
at the head of the animal series, many of the bodily character- 
istics of the primates are rather primitive. For example, the 
habit of walking on the whole foot (plantigrade), the nails, the 
teeth, and many other structures are regarded as more primitive 
and less specialized than corresponding structures in such groups 
as the carnivores or ungulates. We are more near the center 
of the shaft of evolution. (See Fig. 251). 
The chief divisions of the primates are: 

1. Lemuridae, or lemurs. 

2. New World monkeys, with broad and flat noses. 

3. Old World monkeys, having narrower nose with more 

4. The man-like apes of the Old World. 

5. Some intermediate extinct types of man. (Found in 
caves of Europe.) 

6. The Genus Homo, modern man. 

The primates below man are found chiefly in tropical regions 
and more abundantly south of the equator. They feed largely 
on fruit and insects, though some eat birds and other small 
animals. Many of them are social, or at least gregarious, and 
their habits of life are interesting and suggestive in a high degree, 
when we consider their possible relation to the human species. 

464. Additional Notes on the Habits of Mammals. We 

have seen that mammals have succeeded in occupying in over- 
whelming numbers the land, in much less degree the water, and 
least of all the air. We have classified them as insectivorous 
(moles, ant-eaters, and bats) ; carnivorous, as the beasts of prey; 
herbivorous, as the hoofed animals, rodents, and kangaroo; or 
omnivorous, as the pigs and man. They are very versatile 
and have dominated the earth since the tertiary epoch when 
they supplanted the immense reptiles of the earlier ages. 


One of the most noteworthy facts in connection with the 
group is the degree of care given to the young by the parents, 
especially the mother. This is true not merely in gestation but 
after birth in the attendance of the mother to the needs of the 
young, both in supplying food and in protecting from danger. 
It must be remembered that this is done at the expense of the 
parent's safety. It means that the species may be kept alive 
by the birth of a smaller number of young, because more will 
reach maturity than if left early to shift for themselves; and, 
further, that a higher development of the young becomes possible 
owing to the increased length of youth. The degree of develop- 
ment at birth is quite variable. In a general way it is less in the 
case of those whose parents can best protect the helpless young. 
For example the young of the Carnivora and of the Primates 
are much less able to take care of themselves at birth than the 
young of the Herbivora. Many biologists have called attention 
to the fact that the greater care of the young implies higher 
instincts and intelligence on the part of the parent. Any such 
improvement is subject to the action of natural selection as an 
advantageous characteristic. In turn a longer youth or period 
of development is demanded for the maturing of these higher 
instincts, thus in turn making a new demand on the parent for 
more care and training. 

The social instinct is well represented among mammals. 
This may vary from collection in mere shoals or herds where 
food is abundant, to groups organized for offense and defense 
and for work, as wolves, deer, beavers, and men. Indiscrimi- 
nate mating is the rule, yet in some instances strict monogamy is 
found. In many cases mates are won by force, and this tends 
to result in the selection and propagation of the strong. The 
struggle among the males is accompanied by the development 
in them of numerous structures which the females do not pos- 
sess at all or at least in such degree: as antlers, horns, tusks, 
manes, and greater size. 

Mammals do not have so great freedom of motion as the 
birds, and hence do not make as much use of migration to escape 
winter conditions. They tend to keep up an active life, like the 
rabbit and wolf, or hibernate, like the woodchuck. There are some 


forms however that do migrate. These migrations may be re- 
lated to the breeding habits or to food supply rather than to tem- 
perature primarily. Rats, the rat-like lemmings, fur-seal, and 
reindeer migrate. Wonderful stories are told of the lemmings 
of Sweden and their fierce instinctive migration, from which noth- 
ing can turn them, until they plunge into the sea and swim until 
they perish. 

The hibernating mammals must find a place where they will 
not quite freeze. All their vital activities are lowered and their 
temperature falls gradually until in some instances it closely 
approaches the freezing point. Usually this state must be ap- 
proached somewhat gradually. The larger hibernating mam- 
mals like the bear must seek caves or be satisfied with a* hollow 
log or a shallow den. Burrowing forms fare best, as they merely 
dig themselves in below the frost line. The smaller ones must 
find the safer places, since they lose their heat more rapidly. 

Different species vary greatly in the duration of their winter 
sleep. A few hibernate through the whole of the cold weather 
without interruption, as the woodchuck. Some may hibernate 
only in the coldest weather and for a brief time. Others wake 
up from time to time and seek food. Although the vital proc- 
esses are low they keep going. This uses up material. Since 
no food is taken, the fatty reserves of the body are drawn upon. 
Animals go into the sleep fat and come out thin. 

It is in the higher mammals that one finds the greatest dis- 
play of intelligence to be seen in the animal kingdom, and it 
is in man that intelligence and reason whose first beginnings in 
animals no one can mark find their culmination. That these 
high qualities are closely correlated with the great development 
of the brain there can be no doubt. The great progress of man 
in getting mastery of the earth is one of the most interesting 
aspects of the same general problem of evolution and adapta- 
tion which gives unity to the subject matter of zoology. Thus 
the sciences which pertain to man in all his various interests 
have in some measure their foundation in the science of zoology. 
(See Chapter XXV.) 

465. Mammals in Relation to Man. Mammals are not merely 
closest to man in matter of kinship, but they are his most 


valuable allies in the struggle for existence. Doubtless they were 
also his worst enemies in his earlier history. They furnish him 
meat, milk, clothing, labor, transportation, sport, and compan- 
ionship. They have always furnished his favorite game when 
he was not engaged in hunting other men. His domesticated 
animals are chiefly from this class. In most cases the beginnings 
of domestication go back to our earliest records and we can 
only guess which of the species of wild animals he began with. 
In these cases he has bred and selected them in so many differ- 
ent ways that scores of distinct varieties are now scattered over 
the face of the earth. Now that we are coming to understand 
more of the method of transmitting many of the unit characters 
(see 493) we shall be able to recombine desirable qualities 
more systematically, and doubtless new and desirable breeds 
will arise still more rapidly. 

Man has almost destroyed those fierce mammals that were 
his terror at the beginning. They are all but extinct or have 
migrated to the jungles of the tropics. There are, however, a 
number of smaller ones that he has not been able to control. 
The cat family is very much less a menace to human interests 
now than the rodents. Most of these are pests. Rats and mice 
are the most hurtful. They attack all sorts of vegetable matter 
both in the fields and in storage. Rats attack poultry. They 
gnaw and nest in all sorts of manufactured materials. There 
is no doubt that they damage hundreds of millions of dollars 
worth of materials in the United States every year. 

Rabbits in some parts of the world are scarcely less injurious. 
They attack all kinds of tender growths, and in winter may eat 
the bark fnzfm young trees as high as they can reach. These 
small rodents multiply rapidly and have become notorious pests, 
especially when introduced into new regions where their natural 
enemies were not found. Historic instances are, rabbits in 
Australia, mongoose in Jamaica and the Bermudas, and the 
rat into our own country. 

It is not always easy to classify forms as helpful or hurtful. 
Forms like wolves, coyotes, foxes, mink and weasels undoubtedly 
attack the poultry, sheep and the larger mammals that we culti- 
vate, and should be killed. In eighteen months in 1912 and 1913 


in the state of Texas alone, were killed 98,600 wolves and wild 
cats, including 53 panthers and 22 leopards. And yet when 
the domestic animals are properly protected, most of these forms 
live upon rabbits and other rodents, and insects. The balance in 
nature is so complex that we cannot prophesy all the results that 
may come from the destruction of a species or the introduction 
of a new one. Our most pressing problems relating to noxious 
animals lie in the control of the rodents, the insects, and the 

466. Supplementary Topics for Field and Library. 

1. Enumerate the native species of mammals known by you 
to be found in your locality, and determine to which of the 
orders of mammals they belong. Are you impressed that the 
number of native species of mammals is large or small as com- 
pared with other animals ? Are the individuals of these species 
numerous or not ? How do you account for the facts you have 
discovered ? 

2. Enumerate the species of domestic mammals in your lo- 
cality. Are they related to any of the native species? Trace 
the history of some of the most important domestic types. 
What do you know of the mammals domesticated in other 
parts of the earth? 

3. Make a report on the ruminants: their habits, their dis- 
tribution over the earth, and their uses to man. 

4. Make an investigation of the breeds of cattle, or horses, or 
goats, or sheep. Find out some of the strong and weak points 
of each breed. 

5. What is known of the geological history of the horse 
family ? Is the horse a native of America ? 

6. What is the history of the introduction of rabbits into 
Australia ? Can you cite any similar history of rodents in this 
country ? 

7. Report on furs and fur-bearing animals. What is the 
present state of the seal fisheries of our Pacific coast? What 
steps are necessary to the preservation of the seal? On what 
zoological grounds are these steps necessary? 

8. Make a report on the habits and instincts of the beaver. 
Describe the nature of its social life. 


9. Report on the condition of primitive man. Which of 
man's instincts have been of most use to him in his develop- 
ment ? ' What are the principal faculties separating him from 
the other primates ? Do all men possess these in equal degree ? 
What is the distribution of the principal races or varieties of 
the human species? What are the chief differences between 
these races ? 



467. General Statement. Man agrees with the higher 
primates in all the essential structures and functions, as we have 
seen. In many minor details, too, he resembles them. The 
form of the fingers and the flat nails; the number and arrange- 
ment of the teeth; the shape of the face, and numerous other 
superficial things might be chosen to show the likeness between 
man and the higher apes. It is agreed by zoologists that there 
is less difference of structure between man and the higher apes 
than between the apes and the monkeys. 

In general, man agrees with all animals in all that is necessary 
to make an organism an animal; he agrees with all the verte- 
brates in the vertebrate characteristics; he agrees with all the 
mammals in their distinctive features ; and finally he agrees with 
the primates in the qualities that separate them from the lower 

468. The Structural Distinctions Between Man and the 
Other Mammals. Man differs from the other primates in the 
fact that he is more upright in position. This brings about a 
greater development of the hind legs and the pelvic bones for 
support and leaves the hands free to serve the individual in ways 
other than locomotion. He differs also in the size of the brain 
and in the convolutions of it. The brain of the average man 
weighs from two to three times that of the gorilla, while the 
heaviest known human brain is only twice as heavy as those of 
the lowest races of men. There is a corresponding difference 
in the shape of the head (compare Figs. 244 and 245) . The great 
toe cannot be opposed to the other toes as in the case of the apes 
(Fig. 243), or as man can oppose his thumb. There is freer 
motion of the big toe, however, in newly born babes than in 
adults. The canine teeth of man are somewhat less developed, 
and the body covering of hair is not so pronounced. These 

30 465 



. differences, however, except that relating to the brain, are of 
minor importance. 

FIG. 244. 

FIG. 244. Human skulls. The one in heavy outline, with portions shaded, is a Caucasian skull. 
That merely outlined (in dotted line) is of an Australian negro. 

FlG. 245. 

FIG. 245. Skull of young orang outang (dotted outline) superimposed on that of a young child. 

469. Functional Distinction Between Man and the Other 
Mammals. While the structural differences are not sufficient 
to remove man from the group of primates, because they are 
relatively insignificant in themselves, the powers and activities 
and modes of life that have arisen from these do serve to place 
man clearly into a group of his own. These great improvements 
in powers have accompanied and are in some measure due to the 
gaining of the upright position and to the use of the hands as the 



instrument of the brain and to the development of spoken lan- 
guage. These three things the enlarging brain, the free hands, 

FIG. 246. 

FIG. 246. Skull of adult orang outang. 

Questions on Figs. 244-246. Enumerate the similarities and the differences 
apparent in these skulls. In what respects is the negro skull intermediate between 
the orang and the Caucasian? Is the difference between the orang and human 
greater in youth or in the adult? What is the probable significance of this? 
What are the zig-zag lines in some of the figures? What is the meaning of the 
shaded area above the cheek-bone and back of the eye? How does the lower 
jaw in the higher races compare with that of the lower? The meaning of this? 

FIG. 247. 

PIG. 247. The outline of the brain of an orang outang. Adopted from Gegenbauer and Vogt 
and Gratiolet. Front portion. F to O, cerebrum; C, cerebellum; M, medulla and spinal cord; F, 
the frontal lobe; P, the parietal lobe; O, the occipital lobe; T, the temporal lobe; R, the fissure of 
Rolando; S, the fissure of Silvius. 

4 68 


and the growing language would tend to train and improve 
one another. The brain working through the hands, means the 

Pic. 248. Brain of Hottentot woman. From Mill's "Text-book of Animal Physiology," Copy- 
right D. Appleton & Co. Lettering as in Fig. 247. 

FIG. 249. 

FlG. 249. Brain of Gauss, the mathematician. From Mill's "Text-book of Animal Physiology;' 
Copyright D. Appleton & Co. Lettering as in Fig. 247. 

Questions on Figs. 247-24Q.--Identify and compare the cerebrum in these 
three brains. What are the chief differences? Compare the mass in front of the 
fissure of Silvius (that is the frontal lobe), in the three. What is the reason of the 
connection between the numerous convolutions and mental development? Which 
increases more, the cerebrum or the cerebellum, as we ascend the scale? The 
meaning of this? 

MAN 469 

making of implements, of dwellings, and getting numerous forms 
of mastery over nature. The brain working through language 
and in the more intimate parental relations brought about by 
homes, makes possible education and the training of the young 
in the discoveries of the race. The increase of the cerebral 
cortex and the closer association of the different parts of this 
cortex are in some way related to the better consciousness and 
memory in man. The use of language has been the great in- 
strument whereby the power of abstract reasoning has been 
developed, and this has produced a further increase in size and 
complexity of the brain. 

470. Mental Life of Man, Habits, Instincts, Reason. 

Fundamentally, the mental life of man, no less than that of the 
lower animals, is the function of the brain and its accompanying 
systems of organs. Like that of all the other animals, much of 
man's life and activities is made up of habitual actions that have 
been acquired through trial and experience, and the crystallizing 
of these through memory. The child takes its first steps in 
learning very much as the lower animals do. It is stimulated 
and it acts somewhat at random. If satisfaction follows the 
action it is repeated. If dissatisfaction follows, some other 
action is tried. Finally it hits upon a suitable action. This is. 
the trial and error method. 

Much of it, quite as really as in the lower animals, is of those 
more mysterious impulses (instincts) that we do not get by our 
own experiences, but which belong to our make-up in some way 
as an inheritance from the past of our ancestors. We do not 
realize how much of our own life is controlled by these inherited 
instincts. These instincts predispose us to do certain things. 
The mind of man, working upon these experiences and internal 
feelings, and using language to make clear to others his states 
and to convince them of his conclusions, has reached what may 
well be regarded as the highest human power that of abstract 
reasoning: of saying, (i) this is true; and (2) this is true; (3) 
therefore, this is true. The study of this feature of Zoology is 
known as Psychology. The lower animals have their psychology 
as well as man, but it is not so far-reaching nor complex. 


471. The Social Instincts and their Result in Man. In 

the course of our studies we have found many animals that 
recognize their kind and more or less definitely associate with 
them. This reaches a very high plane in the bees and ants. 
Similar social life is to be found in all the primates, but it is not 
so well organized among the lower primates as among the ants. 

In man, even in primitive man, these social instincts are 
well shown. They are complex, just as in the bees. They 
involve the mating instinct, the care of young, the storing of 
food, the finding of shelter and safety, as well as the instincts of 
general gregariousness found in many forms. These associative 
instincts lead to the home, to marriage, to the family, and later 
to the larger family or group made up of the immediate kindred. 
Thus tribes and clans and nations come to be bound together. 
It is not claimed that this is the only way in which human socie- 
ties may be built up, but there can be no question that it has 
been so built up in many instances. 

In this growing complexity of society, customs and regula- 
tions and special institutions to accomplish certain ends spring 
up just as the division of labor arose in the bee colony. These 
demands of society gradually mould the individuals and their 
social ideals. Next to the hand and to language, probably the 
social relations of man have been important in training his mind 
and developing his brain. It is easily seen that the higher 
qualities of man, as sympathy, love, unselfishness, heroism, and 
self-sacrifice, are the qualities of mind, or "heart" as we some- 
times say, that would be given prominence in the home and the 
other really social institutions. The study of man and his 
social life is known as Sociology. 

472. Education and its Place in Human Development 

Education is, in general, the development of the individual in 
such a way that he will be able to adjust and to readjust himself 
rightly, in the light of his whole nature, to the essential factors 
of his environment. This means always to make the right re- 
sponse to the stimuli. This adjustment has come slowly in the 
race, step by step in the experience of each. Individual educa- 
tion in mankind is an effort to give the child a short cut to the 

MAN 471 

best that has been discovered by the race in its history, so that 
it will not be necessary for him to get it all by experience. Lan- 
guage is again the vehicle that makes this possible. Some edu- 
cation is possible through sight and imitation of parental actions, 
and there is probably a certain amount of such education in 
many of the lower animals. Confidence in the parents, imita- 
tion, and curiosity are important individual instincts underlying 
the education of the child. The long dependence of the child 
on the parents, the close relations of the home, the warmth of 
sympathy in the parental feeling, all enter in furnishing the 
motive and the opportunity for the training. That this educa- 
tion is efficient is shown by the fact that the individual youth 
in a period of twenty or twenty-five years may be brought to a 
knowledge of the most important experiences of the human race; 
of the great implements of human progress, as spoken and writ- 
ten language, knowledge of nature's laws, and the relations of 
numbers; and of the great, partly natural and largely artificial 
structure which we call human society, as well as of the modes of 
behavior necessary to meet its demands. 

These have required thousands of years to build. Most 
of them have not become instinctive. It is the triumph of 
human attainment that so much can be imparted in so short a 

473. Man's Relation to Nature. There is nothing in what 
we can learn about man that suggests that he is not just as 
dependent on the natural laws of his own being and of the en- 
vironment about him as any other animal in the animal kingdom. 
He starts in the same humble way, as a single cell; he has the 
same powers of growth and development, but he must have con- 
ditions favorable to them. To-day is always the child of yester- 
day, just as with the other animals. He has the like diseases; 
he is affected with similar parasites; he has enemies among 
the animals just as is true of the others. Equally, he depends 
on them for his food. The same struggle for existence and the 
survival of the fittest that we find in all of life can be traced in 
much of human history. 

Man, however, has made a mastery of nature which no other 


forms have been able to do. Probably through his wits and 
his supple hands, rather than by strength, he held his own 
against the powerful mammals which preceded him on the 
earth. By the same means, but in increasing degree he holds 
that mastery to-day. Through his wits, again, and his wonder- 
ful hands he has managed to use the inorganic forces of nature 
as no other animal has or can do. Through his wits, and most 
of all through his growing sympathies and unselfishness, he bids 
fair to build up a society, based on friendship and love, which 
will substitute cooperation for competitions in the broader rela- 
tions of life just as it has already done in the home itself. 

474. The Artificial Surroundings of Man. In what we call 
civilization man has so controlled the natural conditions as to 
create for himself an environment which is greatly different 
from that under which man first lived. Clothes, houses, cities, 
fire, seasoned foods, stimulants are terms which suggest some 
of these artificial elements that have entered into man's life. 
Just what their final effect will be on the body and mind of man 
no one can tell. Many of the most terrible diseases to which 
man is subject are the diseases of civilization. That his wits 
will continue to enable him to meet the new problems which he 
brings on himself, as he has met the natural ones, we may well 
believe. But unquestionably we must realize that his greatest 
task is to use his increasing mastery of nature in meeting the new 
difficulties which his own complex and artificial civilization is 
bringing upon him. 

475. The Age of Man on the Earth. There is no sure 
knowledge of when man, in his present form, first appeared 
on the earth. It is known that man lived in Europe in the 
later portion of the Ice Age, and quite probably earlier. His 
implements of stone are found along with the remains of such 
animals as the cave-bear and mastodon and man-like apes, 
either extinct now or found only in tropical regions. The oldest 
of these human remains are found in southern Europe and in 
northern Africa. In a limestone cave in the Neanderthal in 
Germany fossil remains have been found of a man believed to be 
much more primitive than any other known. In other words 

MAN 473 

this is regarded as an extinct species; one of several now extinct 
types of men that have been discovered. We have signs of the 
gradual improvement of his stone implements from rough to 
smooth; the introduction of other material as bone and, later, 
copper, bronze, and iron. It is only after the thousands of years 
of this primitive, unrecorded history that we come to the his- 
tory of such well-separated nations as those of the Euphrates 
and Nile valleys, whose monuments and inscriptions are believed 
to take us back 6,000 years or more. 

476. The Principal Types of Men and Their Distribution. 

It is not perfectly certain that all the men of to-day should really 
be placed in one species, although if we follow them back far 
enough they may have had a common origin. The pigmies of 
Africa or the Negritos of the Philippines are more removed from 
the Caucasian than is true of many separate species of the wild 
animals, and the differences are both characteristic and constant. 
The varieties of men are innumerable, but there is a tendency 
to group them all under three main heads which we may call: 
(i) The white or Caucasian; (2) the yellow or Mongolian, and 
(3) the black or African. To these are often added the red or 
native American, and certain island and peninsular types that 
do not agree very well with any of the others. The most striking 
external differences are : The color of the skin, the structure and 
appearance of the hair, the form of the nose (which is also 
used in distinguishing the apes), the form of the jaw and skull, 
and the character of the language. Under the Caucasian race 
are included the wavy-haired peoples, as the chief European 
peoples, the Egyptians, the Jews, the Arabs, the East Indians, 
and the peoples of the Caucasus who give the name to the race. 
The Mongolians would include the straight-haired, yellowish 
and brownish varieties, as the Chinese, Mongols, Manchus, 
Tartars, Japanese, Turks, Finns, and possibly the native 
American tribes, as Esquimo, Indians, and the South and 
Central American peoples. The black or negro race includes 
many tropical forms dark in color and with wavy or kinky hair. 
Such are, the dwarf Negrillos of central Africa; the Hottentots 
and Bushmen; the Negroes of the Nile, of the Senegambia, and 


of Guinea; the Caffres, Zulus, and other tribes of the east coast; 
and many others. 

The great number of these human races, or varieties, is an 
evidence of the long time man has been on the earth and of his 
adaptability. They have arisen and have been increased by the 
natural tendency to vary, by the effects of climate, by migration 
and isolation, by interbreeding, by conscious selection of mates 
in accordance with local standards of attractiveness, and the like. 
This phase of zoology is known as Ethnography. 

477. Topics for the Library. i. Study the effects of tropical 
conditions on the human race, as judged by the races of men now 
found there: Effects on skin; on physical and mental states; on 
industrial and social life. 

2. Study similarly the effects of life in the temperate 
zones. What is the value to man of the alternation of summer 
and winter, in encouraging thrift and foresight; in the perma- 
nency and character of homes, etc. The effects of these things 
on stability of government and of social institutions generally. 

3. Similarly study the conditions of extreme cold, as seen 
in the life of the Esquimos, Finns, Laplanders, and the like. 

4. Where were the highest examples of aboriginal civilization 
on the American continent when it was discovered ? Describe. 

5. Describe the steps whereby the clan, the tribe, and 
the nation may be built up with the home as the starting-point. 
What new personal qualities are called for and cultivated by the 
social life of the tribe ? 

6. Illustrate the effects of the physical geography of the 
earth on man's life and history. Examples: Egypt; Greece; 
Palestine ; the Mediterranean Sea ; the depression formed by the 
Hudson River and Lake Champlain; the Allegheny mountains; 
effects of continents on races. 

7. On the whole, does it seem that man has been sub- 
ject to the same influence by external conditions which we have 
found in the lower animals? Illustrate. 

8. Human migrations. Cite some of those that belong to 
recorded history and look at their causes and results. 

MAN 475 

9. What is the zoological character of mixed or hybrid races 
of men? 

10. What is the human population of the earth estimated to 
be? How is it believed to be distributed among the leading 
races? Which is growing with greatest rapidity? What rea- 
sons can you assign ? 



478. In the preceding pages the words evolution and devel- 
opment have been used frequently, but no effort has been made 
to define or justify them. In all that has been said it has been 
assumed that the animals now on the earth have come to their 
present condition of variety and complexity by natural growth 
and development, rather than by outright creation as we find 
them now. This has been assumed, not because there can be 
any complete demonstration of the view, but because it is more 
in accordance with the facts as we find them than any other 
theory which has been offered. 

The student is now in a position to study the question of 
evolution more broadly, and to appreciate a more full statement 
of it and some of the discoveries and inferences most closely 
related to it. 

479. The Meaning of Evolution. There is a good deal of 
haziness in the thought of people generally as to just what the 
zoologist means by evolution. Evolution is not in any sense a 
cause; it is a term for a process, for the way in which present 
conditions have come about. Briefly, the most important ele- 
ments in the thought of evolution are gradualness and natural- 
ness. In more detail, the following features may be said to 
belong to the idea of evolution : 

1 . All life, so far as we can know, has come from pre-existing 
life. This year's animals are descended from those of last year; 
they from those of the year before, and so on back. 

2. All animals are subject to change. Offspring are never 
just like the parents. If given time enough animals may thus 
change in any degree. New species may come from old by 

3. The animals of the present time are descended from 
simpler, more generalized ones; and these from still earlier types. 



So even the most complex animals of the present have arisen 
ultimately from ancestors as simple as the simplest. 

4. All animals have fundamental likenesses. Some are 
more alike; some are less so. The fundamental likenesses mean 

5. The process of life is gradual rather than sudden, although 
the rapidity of it may differ at different times ; it is natural rather 
than supernatural; it is subject to the same laws of cause and 
effect which operate in chemistry and physics, and is not lawless 
and arbitrary. 

6. On the whole, the life-processes result in a closer and more 
perfect adjustment of organisms to one another and to the inor- 
ganic environment. 

480. Evidences for the Development Theory. Biologists 
generally are agreed as to the fact of evolution, and there is no 
longer any direct search for proofs of the belief. Any disagree- 
ment among them is in respect to the manner in which evolution 
has come about, and the present search is for the causes and the 
factors which produce it. Many people, however, look with 
some suspicion on the theory. For this reason the student 
should have before his mind some of the classes of facts that 
have convinced biologists of its truth. 

481. Variability as an Evidence. The changeableness of 
organisms is the fact that makes it impossible for the biologist 
to deny evolution. Every day we see differences in organisms 
of the same species, which differences have been brought about 
by differences in the surroundings, by the behavior of the organ- 
isms .themselves, by cultivation by man, or by something 
inherited from their parents. We know that man can take ad- 
vantage of these differences and can select certain types; can 
cultivate and select again in such a way as to get, in a few gen- 
erations of cultivation and breeding, animals strikingly different 
from those with which he started. After a certain time these 
new forms seem to breed reasonably true, and a new race is said 
to be established. In this way the different breeds or varieties 
of dogs, pigeons, chickens, and many other domestic animals 
have apparently arisen. This is not merely a proof of evolution; 


it is evolution. Any one who believes in this is an evolutionist 
by just that much. 

There can be no reasonable doubt that just this kind of 
thing is happening in nature, without the help of man. It 
cannot take place, however, so rapidly as when man deliberately 
aids the process, by artificial selecting and breeding according 
to his preference, and then eliminates those that he does not want. 
It sometimes happens, both in nature and in cultivation, that 
large variations (''sports" or "mutations") appear suddenly, 
and breed true in succeeding generations. These marked varia- 
tions are not so frequent as the slighter ones, but seem to be 
more persistent when they once appear. 

482. Evidences from Geographical Distribution. In the 
wild state, the changes in animals are too slow for us to detect, 
within a few human generations, that there has been any change 
in a species; yet we do have some evidence on a broader scale 
which is of a nature quite similar to that in the last section. 
In the way in which animals occur on the face of the earth a 
great many interesting factors enter, and it sometimes happens 
that we get some good indication of the long-time effects of 
variation. Sometimes we find two forms of plant or animal 
flourishing in two regions that are separated from, each other 
by some kind of a barrier which does not allow them to pass 
back and forth and thus to mingle. These forms are in general 
similar, and yet are constantly and recognizedly different. 
Sometimes we can find that these two regions (b and c) have 
been stocked from some third region (a), and that both varieties 
are apparently descendents of the same ancestors. Indexed, we 
may be able to find forms of all grades connecting "a" with "b" 
on one side, and other intermediate types connecting "a" with 
' ' c " on the other side. In cases like this, which are by no means 
infrequent, it is believed that "b" and "c" have migrated into 
their respective regions and in becoming adapted to their new 
condition of life have so evolved as to become different from their 
parent stock and from each other. 

In a similar way, but on a much larger scale and in a form 
too complex to discuss here, we find evidences of change and 


development in the animals of the great continental and other 
natural divisions of the earth. 

483. Evidences from Geological History. In the rock 
strata of the crust of the earth we find abundant plant and ani- 
mal remains in the form of fossils. In the most recent strata 
we find remains similar to the species of the present time,, 
whether of mollusks, of fishes, or of mammals. The further 
back we go in the earth's history the less similarity we find be- 
tween the fossils and the present-day life. The earlier strata 
show only invertebrate remains ; later the fishes appear, although 
much simpler and more primitive than the fishes of to-day.. 
Later still appear amphibians, reptiles, mammals, and birds; 
and last of all man's remains. 

The conditions are just what we should expect if life ap- 
peared on the earth in its simpler forms and gradually, by 
evolution through the ages, became complex and modern. 

These things are not only true in a general way, but have 
been found to be true of the special types of animals. For 
example, the fossil remains of modern horses have been found 
in recent geological strata. The modern horse has only one toe 
on each foot and walks on the end of that toe. He has, however, 
some splints on either side of this digit which point us to the 
history of his toes. In the geological age preceding the present 
we find the remains of an animal clearly like the horse, in which 
these splints are larger and show more nearly the structure of 
normal toes. By tracing the conditions backward in geological 
times, links have been found which connect the skeleton of the 
horse of the present day with an animal of the Eocene period, 
which had four toes on the forefeet and three toes on the hind 
feet and was little larger than a fox. These steps are so com- 
plete that expert students of fossils do not hesitate to regard 
that we have a fair knowledge of the ancestry of the horse for 
perhaps millions of years. Similar series of gradual changes 
are shown among many species of fossil animals, as mollusks, 
Crustacea, fishes, birds, and mammals. 

All this is entirely without explanation if animals have not 
been subject to gradual evolution through the ages. 


484. Evidences from General Similarities of Structure. 

In studying animals we need to keep in mind not merely the 
varieties and dissimilarities which we see arising, but the under- 
lying likenesses as well. This underlying likeness of structure 
is really an evidence of relationship; which is another way of 
saying that animals, in spite of their differences, are apparently 
descended from a common stock. If, for example, we examine 
the offspring of a given pair of parents, as a litter of kittens or of 
puppies, we expect these individuals to be more alike in behavior, 
in disposition, and structure than will be the same number of 
offspring of totally different parents. This likeness is a sign 
and measure of their kinship. Two spaniels or two newfound- 
lands are more like each other than either is like to any other 
breed of dogs. The greater similarity is again the sign of their 
greater kinship. But the spaniel is more like the newfoundland 
than either is like the wolf, for the spaniel and the newfoundland 
probably belong to one species (Canis jamilaris) and the wolf 
to another (Canis lupus). 

The wolf and dog are more alike, and hence closer akin, 
than either is to the members of the cat family. The dogs and 
wolf belong to the genus Canis and the cats to the genus Felis. 
This is the way we express their degrees of difference. But in 
turn the dogs and cats are much more similar to each other than 
they are to the horse and cow. Because of this similarity in 
structure and behavior, the dogs and cats are classed together as 
carnivores, and the horse and cow together as ungulates. 

With all their differences the carnivores and the ungulates 
have many fundamental points in common. They are all warm- 
blooded, covered with hair, have mammary glands, carry the 
young in the uterus attached by a placenta. Hence they belong 
to the subclass placentals and to the class of mammals. They 
have similar parts to their skeletons, similar arrangements of 
the principal muscles, similar structure of the brain and central 
nervous system. Thus it might be shown that the newfound- 
land and the spaniel are similar to all the vertebrates, and 
finally to all the animals. 

If the similarities of structure in a litter of kittens is a sign 
of kinship, we may equally believe that the similarities between 


the dog and the cow are also evidences of kinship ; and that their 
differences mark a gradual evolution from a common stock in 
special directions and in adjustment to special modes of life 
through long ages of time. 

485. Evidences from Rudimentary Organs. It often hap- 
pens that some animals possess organs in only a slight or rudi- 
mentary way, which in other animals are well developed and 
useful. In the rabbit, for example, there is a pouch called the 
caecum at the junction of the small and the large intestine. It 
is several inches in length, well supplied with glands, and proba- 
bly of considerable value in digestion. In many mammals this 
structure is much reduced, and in man it is only found as a 
"vermiform appendix," which certainly has no such value as it 
has in the rabbit, and is thought by many physiologists to be a 
positive menace. 

Similarly, in many mammals there are certain muscles by 
which the external ear is moved and directed so as to catch the 
sound waves. In man these muscles, though present, are so 
reduced as to be of no value. Most animals have many rudi- 
mentary remnants of organs which are useful in other apparently 
related animals. It is said that man alone has several hundred 
such rudimentary structures. The rudimentary eyes of fishes 
and Crustacea in caves, and the almost or entirely reduced organs 
of many parasites are mute testimony of the loss of organs once 
useful, through changed life conditions. In other words, 
vestigial organs are also evidence of evolution of animals into 
adjustment with the surroundings. 

486. Evidences from Embryology. The study of em- 
bryology the course of life in the individual has probably 
furnished us the most suggestive evidence of the evolution of 
animals. The main facts, and the use that has been made of 
them in reaching our conclusions, may be expressed in a few brief 
statements. The student must go to more extended texts 
for the complete handling of this complex but most interesting 

Some important facts of individual development are : 
i. Each individual animal (with certain exceptions in 



non-sexual reproduction), no matter how high in the scale 
of life, starts its life as a single cell, similar in many respects 
to the permanent single-celled protozoans, except that it has 
the power of developing rapidly into its own peculiar species. 
(See Fig. 250, 7.) 

FIG. 250. 

<5. XfotochorA 

3. Sey mentation* 

. Gtzstrulet 


FIG. 250. A diagram to suggest the way in which individuals of the different phyla have 
parallel development for a while, and later diverge in their own particular way. For example, an 
Amphibian begins life as a single cell, similar to the simple protozoa (i); later it forms a mass of 
cells similar to some of the colonial protozoa (2) ; in forming a gastrula (3) it is going through a 
stage similar to the Hydra, which is a kind of permanent gastrula. Amphibians run parallel to the 
fishes in development, in the formation of a body cavity (4), in forming segments (5), in develop- 
ment of notochord (6), in the presence of gills in early life (7); but part company with the fishes in 
the fact that they lose the gills and develop lungs (8). 

Questions on the figure. Trace out other instances of parallelism until the 
purpose and meaning of the diagram become clear. Then find other facts with 
respect to development of animals and show how the diagram cannot do justice 
to the whole truth. Compare diagram with text, sections 486 to 487. 

2. All higher forms agree likewise in the next step of their 
development, which consists in the division of this single cell 


into a simple mass of cells. This is known as the morula, which 
is not unlike the adult, and highest stage, of such types as Eudor- 
ina and Volvooc (colonial protozoa, Fig. 250, 2). 

3. Practically all the higher animals pass next through 
a stage in which the cells become differentiated into two layers, 
more or less well defined and arranged in a kind of double sac 
ectoderm on the outside and entoderm on the inside (see Fig. 
13 , A^) . This is known as the gastrula. Animals like the adult 
Hydra (Fig. 81) are really a kind of permanent gastrula some- 
what modified in form to be sure, but a gastrula nevertheless. 

The facts thus far stated may be taken as suggesting the 
following conclusions: 

1. All organisms, even the highest, begin life at essen- 
tially the same point; that is, as a single cell. This similarity of 
individual origin indicates their fundamental kinship and simi- 
larity of racial origin. 

2. The development of all the forms above the protozoa 
is parallel for at least a brief period; that is, through the morula 
and the blastula stages (Fig. 13, 3 and 4). This parallelism of 
development added to the similarity of origin points even more 
strongly to their kinship. 

3. There is tendency for some forms to drop out of the race 
and to become permanently fixed about certain of these stages: 
as most protozoa at the single-celled stage, Volvox at the morula 
stage, and sponges and hydra at the gastrula stage. Others go 
on and introduce new steps of differentiation before reaching their 
adult development (Fig. 250: 1-9). 

487. Careful study of the later stages of development of 
the higher animals gives us further illustrations of these truths 
and enables us to state even more broadly the principles deduced 
above. For example, we find that insects have a parallel course 
of embryonic development in which the great body of insects 
agree (Fig. 250). The same is true of the vertebrates. One in- 
sect agrees with another insect in the mode of its development 
more nearly up to the complete adult character than an insect 
will agree with an echinoderm, or either with a vertebrate. 
In a similar way, all vertebrates have a course of develop- 


ment parallel for a longer time than they parallel any other 
group (Fig. 250, 1-7). 

Furthermore, the same principle applies to the subgroups 
of the vertebrates, or of any other branch. The vertebrate 
development begins to differ from the insect development lower 
down than the divergence of the reptiles from the birds, or the 
mammals from the fishes (Fig. 2 50, 8) . The parallelism between 
birds and reptiles ends earlier than that between one type of 
birds and another, or between man and the other mammals. 
Finally within a species, as of grasshoppers, of reptiles, or of 
rabbits, the course of development of different individuals con- 
tinues identical through to the mature form. It is just those 
forms that seem most similar in structure (section 484) in which 
the embryonic development is most nearly identical. The two 
forms of similarity, structure and embryology, strengthen the 
idea of real kinship. Hence we have reached the conclusion that 
all parallelisms in the fundamentals of individual history mark 
some degree of relationship. The longer and more profound the 
parallelism the closer the relationship. A careful study of the 
diagram (Fig. 250) will serve to make this point more clear. 

488. The Biogenetic Law. Out of such considerations as 
these has come one of the most important and far-reaching laws 
that the biologists have ever stated. It is as follows: "Each 
individual animal, in passing from the egg to the adult, repeats 
in an abbreviated way, in a few days or years, many of the steps 
taken by the race to which it belongs, in its evolution from its 
single-celled ancestors to its present condition." Put briefly, it 
reads : Individual history is a brief recapitulation of race history. 

Why can the individual egg cell of a frog, in a few months, 
pass through a morula, a blastula, a water-breathing legless fish- 
like stage, and into an air-breathing, four-legged adult? Be- 
cause its ancestors were first single-celled animals, and through 
millions of years and countless generations gradually developed 
first into a mass of cells, later into a gastrula-like stage, and later 
still for long ages breathed by gills, like the fishes, and last 
of all became what they now are water-breathing tadpoles 
with the power to breathe air in the adult life. 



FIG. 251. 






























PIG. 251. A diagram designed to suggest something of the possible relationships of the various 
groups of animals. The vertical position indicates roughly the relative specialization or progress 
of'the groups. 

Questions on the figure. Which of the groups seem to be centers from which 
several types may have sprung? What does this mean? Which phyla do not 
seem to have others springing from them? What does this mean? What three 
theories^as to the origin of the Chorda ta are suggested in the figure? 


Because the history is so much shortened, only some of the 
more profound stages are recapitulated, and the advances of the 
race in thousands of years are compressed into moments in the 
individual history. Furthermore, each species has introduced 
idiosyncrasies that belong in no way to the race. For these 
reasons the general truth of recapitulation must not be applied 
too literally or widely. 

489. The Principal Factors Entering into Evolution. As was 

indicated earlier, biologists no longer question the idea of the 
evolution of the animal kingdom. They are now seeking to 
find the principal elements that are bringing about this result 
and how each contributes to the process. We doubtless have not 
found all the factors. However, those mentioned below cer- 
tainly enter into the explanation. 

Evolution could not take place without variation, nor with- 
out some device to preserve (i.e., repeat) and to accumulate these 
variations from generation to generation after they arise. Fur- 
thermore, evolution seems to follow rather definite courses. 
These courses are not haphazard. They have led toward adap- 
tation of the organisms to their most pressing surroundings. 
In other words there seems to have been selection and elimination 
of variations, and thus a guidance of evolution. 

The following outline may aid the student in grasping some 
of the important elements of the case. 

i . Variation makes evolution possible, but only this'. We have 
learned (60) that body cells and germ cells lead a parallel 
life in every organism. They are very different in their proc- 
esses and their fate. Theoretically variations may arise in 
either of two ways: 

A. The body of a given organism may change by the direct 
effect of the environment upon the body or through the use 
or disuse of its organs. 

B. Changes may occur in germ cells. 

a. Germ cells may be modified by their environment, 
which is chiefly the body in which they are being housed. 

b. Germ cells may be modified by the mingling of the two 
different germ cells, ovum and sperm, that united to 


form them. This new compound is different from any 
ever formed before. 

2. Preservation, repetition, and accumulation of variations from 
generation to generation. Only this can insure evolution. There 
are two problems: 

a. Preservation of fluctuations that come to the body. If 
there is an early separation of germ cells and body cells and they 
lead a parallel course of development, it becomes a problem 
whether a special quality that is gained by the body cells in 
their life can be imparted to their cousins among the germ cells. 
Unless they can do this it is clear that these changes cannot be 
transmitted to the next generation, since the body cells do not 
themselves pass over. A body may have acquired a very strik- 
ing quality without any part of this quality being found in the 
germ cells. 

b. Preservation of changes that come to the substance of the 
germ cells. Clearly, since the germ cells make the next genera- 
tion, changes in these cells may very well influence both the germ 
and body cells of the next generation. Biologists are coming 
more and more to feel that this is the really great field of heredi- 
tary influence, and that evolutionary studies must concern 
themselves increasingly with the history of these cells and the 
kinds of influences that can change them. 

3 . The Guidance of Evolution . This might take place in either, 
or both, of two ways. 

a. Variation might itself be guided from within, so that 
chiefly suitable or appropriate variations shall occur. 

b. Variations may have any range, and the environment 
through its life and death pressure on the organisms may elimi- 
nate some and select others on the basis of fitness. The result 
would be to make evolution follow the demand of the environ- 
ment independently of the range of the variations. 

490. Variability and Variation. This furnishes the materials 
of evolution. All organisms vary. Variation suggests two or 
three things: (i) An animal may differ from day to day as the 
result either of its own activities or of the action of the environ- 
ment upon it; (2) it may differ, at any stage or condition of its 


life, from what its parents were at the corresponding stage; and 
(3) it may differ also from its brothers and sisters even at the 
same stage of life and under similar conditions. Some of these 
changes are evidently caused by conditions outside the organ- 
ism; others by internal conditions. These changes make evolu- 
tion possible. We do not believe that all these classes of changes 
enter equally into evolution. One of the most important ques- 
tions of the modern biologist is this: "What produces the varia- 
tion actually found in individuals of a species, and to what extent 
are these variations due to internal or to external causes?" 

When we actually study variations in nature we find them 
of two principal kinds. First, and chiefly, we find what are 
called fluctuations or continuous variations. They are so called 
because they tend to cluster about a mean, with no sharp break 
in the series from the lowest to the highest. We should find this 
if we were to take at random a thousand men and arrange them 
on the basis of weight or height or intelligence. A curve drawn 
to show their distribution would show the greatest number near 
the average and the extremes would be gradually less numerous. 
The curve, however, would be a gradual one, and we would find 
cases illustrating all degrees within the normal range. 

In the second case, however, while most members of a 
species will arrange themselves as above, we may find an occa- 
sional individual so different from all others that there are no 
' ' connecting links. ' ' These are known as ' ' sports "or " mutants ' ' 
and illustrate discontinuous variation. Experiment shows that 
the mutations are much less frequent, but much more liable to 
breed true, than the fluctuations. In the case of fluctuations 
even the infrequent, extreme forms produce offspring whose 
average tends to be not about the extreme parent, but rather 
about the mean of the group. The offspring of the mutants, on 
the other hand, have their mean about the parent, and are quite 
independent of general mean. 

491. Heredity, the Organic Continuity between Genera- 
tions. We have already seen (131) that the germ cells, ovum 
and sperm, are the organic bridge which connects the body of one 
generation with that of the next. Just as new qualities cannot 


influence evolution until they are represented in the protoplasm 
of the germ cells (germ plasm) , so all the old qualities of the race, 
both deep and superficial, in order to continue, must be repre- 
sented in this germ plasm. When the germ cells have united, 
inheritance has been determined. After this the parent may 
influence the development of the new individuals, as the 
mother mammal doubtless does while the young is carried in the 
uterus or nourished by milk; but neither of these is a matter of 
heredity in any sense. 

In those organisms in which there is no union, where a single 
germ cell may develop into the adult, only one line of germ plasm 
is involved and heredity seems a somewhat simpler thing. How- 
ever, when two different strains of germ cells unite to start the 
new generation, every cell of the new individual receives chromo- 
somes, and probably other material, that came by way of both 
ovum and sperm. It is believed that the hereditary qualities 
are carried in some way by these chromosomes. Chromosomes 
from the sperm, carrying definite characteristics of the father's 
line, are closely mixed in the same nucleus with chromosomes 
of the egg carrying characteristics of the mother's line. In 
what way will these diverse complex chemical bodies influence 
one another in the development of the body ? Will they blend 
or will one swamp or exclude the other? If they blend, will 
they add their tendencies; will they neutralize each other; or 
will their union give rise to compounds so new as to carry 
qualities that neither parent had ; or may all of these happen with 
the different qualities? These are the questions of heredity. 
Only recently have we begun to get real statistical and breeding 
data that will help us to answer them. 

492. Mendel's Experiments. Gregor Mendel, an Austrian 
monk, published in 1866 the results of breeding experiments 
conducted by him, together with his interpretation of these. 
His work was unknown until 1900. Since that time his facts 
have been verified and enlarged, and his conclusions tested and 
elaborated until we now include under " Mendelism " a large 
body of exceedingly important facts and suggestive theories 
and principles. 


Mendel hybridized varieties of garden peas. One true- 
breeding variety is dwarf and another tall. When these are 
artificially crossed, no matter which variety furnishes the pollen, 
all the offspring are like the tall parent in height. The dwarf 
quality, which evidently must be represented in the offspring, 
does not show in the body at all. He therefore concluded that 
tallness was dominant over dwarfness. Dwarfness is said to be 

FIG. 252. 
TALL (pure) X DWARF 




^ s 

TALL ( 3 ) DWARF ( i ) . . . . F 2 

/ i I 

TALL (i) TALL (2) DWARF F 3 

(pure) (hybrid) (all) 

FlG. 252. Diagram illustrating Mendel's results in breeding tall and dwarf peas. Pi, Fz, F, 
firtt, second, and third filial generations. The first generation are all tall; but we know they are 
impure (hybrid) both because we know the parents, and by their offspring when bred among them- 
selves. The dwarfs (recessive) when they once appear are necessarily pure, and breed true. The 
tails of Fj generation are of two kinds as shown by their offspring, i pure to 2 impure. The figures 
in parenthesis indicate relative numbers. All generations after F, are self-bred. 

When these tall hybrids were self -fertilized, three-fourths of 
their offspring were tall like the parents, and one-fourth were 
dwarf like the dwarf grandparent. In the case of the dwarf 
offspring the parents clearly transmit something which the 
parent body does not possess. There were no intermediate 
forms. Further breeding showed that these dwarfs were pure 
dwarfs with no taint of tallness on them ; for when self -fertilized 



they bred true dwarf indefinitely. Close-breeding the tails 
showed them to be of two kinds: (i) one- third of them (that is, 
one-fourth of the whole offspring) were just as pure as the dwarfs 
were, and continued to produce tall forms indefinitely. Two- 
thirds of the tails (one-half the total offspring) when inbred 
had three tall offspring to one dwarf, just as had been true of the 
first hybrid generation. Further inbreeding of the hybrid off- 
spring continues to repeat these proportions indefinitely:-! 12:1, 
that is one pure tall, two hybrid tails, and one pure dwarf. 

FIG. 253. 

C X c Parental Generation 

C(c) ist Filial Generation 

"C C(c) ' 



: c 

. \ 

c c 



c. . 

\ \ 
'C CC C(c) C(c) 

cc C 

1 \ 
cc C 

C C( 

c) C(c) a 

11 " 

r cc 

C c 

FIG. 253. A general formula for Mendelian crosses. C represents any dominant quality; 
c, the absence of the quality, or the recessive. The first filial generation are all hybrids that look 
like the dominant parent. If these are self-bred, the offspring of the second filial generation, F, 
will be one-fourth pure dominant (C), one-fourth pure recessive (c}, and one-half hybrids which look 
and breed like their hybrid parents. CC and C(c) look alike, but breed differently. Compare 
with Fig. 252. 

Mendel also found that yellowness of seed coat was dominant 
over greenness; that smoothness of seed was dominant and 
wrinkledness was recessive. He also found that these different 
qualities were inherited absolutely separately; that is to say 
that the yellow or green and the smooth or wrinkled quality 
of the seeds might be transmitted equally in connection with 
either the dwarf or the tall. 

The results stated in general terms might be expressed as 
follows : When pure bred parents differ by the presence or ab- 
sence of a certain heritable quality, all the offspring of the first 
generation will be like one of the parents in bodily appearance. 



This parent and its quality are said to be dominant. When the 
individuals of this first hybrid generation are self-fertilized, or 
crossed with each other, 25 per cent, will be pure and like the 
recessive grandparent; 25 per cent, will be pure and like the 
dominant grandparent; and 50 per cent, will be hybrids like the 
immediate parents, and having the bodily appearance of the 
dominant grandparent. 

The diagram (Fig. 253) will aid the student in following the 
general results. 

493 . Mendel's Laws. Three laws or principles were deduced 
by Mendel from his experiments, and have been perfected by 
later workers. 

PIG. 254. 
Possible Kinds of Male Gametes 



Pure tall 


Hybrid tall 



1 Hybrid tall Pure dwarf 

FlG. 254. Diagram showing the possible kinds of crosses of the hybrids of the first filial genera- 
tion in Mendel's peas. Each hybrid parent may produce eggs and sperm carrying, when they 
segregate, either tallness or dwarfness, but not both. These in the long run will be equal in num- 
bers. Each kind of sperm will have equal chances of uniting with either kind of egg. 

Questions on the figure. Why are these the only possible gametes? Follow 
out the details of the diagram and see just why the lettering in the squares is as it 
is, and determine the quality of the resulting offspring in each case. Is it clear 
why the resulting proportions are as they are? 

a. The principle of unit characters. This suggests that 
organisms do not inherit the whole parental nature as a unit, 
but that each parental quality acts as a unit in inheritance. 
Each individual, from the point of view of inheritance, is made 
up of many independent unit characters which may be inherited 
in any combination. 


Later researches support this principle in the main, but show 
that these unit characters do influence one another. Some- 
times they are linked or coupled in such a way that one cannot 
be inherited without another; sometimes they repel one another 
so that they cannot be inherited together; and sometimes a 
quality that appears to us as one in the body is known to be 
made up of two or more unit characters on the germ cells. 

.Fie. 255. 






T (D) DO 

FIG. 255. A diagram to show graphically the formation of gametes, dominance, possible 
matinga, segregation of qualities in the gametes, and the proportion of resulting offspring, in the 
Pi and Pj generations of the tall and dwarf peas. Black indicates tallness; white, dwarfnesi. 
Black outside the white indicates its dominance. TT, a plant in which tallness comes from each 
parent; T(D), in which tallness comes from one parent and dwarf ness from the other; DD, dwarf- 
ness from each parent. 

Questions on the figure. Why are the gametes of each original parent figured 
alike? Why are those of the FI parents unlike? Why are the gametes always 
either white or black, not mixed as the parents may be? Carry for yourself the 
figure one step further, that is to the P 3 generation. What kind and proportion of 
offspring would you get by crossing the possible gametes of TT X TT? TT X 
T(D)? T(D) X T(D)? T(D) X DD? DD X DD? Make a diagram of each. 

b. The principle of dominance. There are determiners in the 
germ plasm which stand for these unit characters in the body. 
When these are mingled by the union of two pure germ cells, 
one of these (e.g., that for tallness in the peas) will dominate the 
other so that the latter though present in all the cells, both body 


cells and primordial germ cells (see 49, 60), has no part in 
developing the body. The recessive determiner cannot develop 
in the presence of the dominant. Whenever the dominant deter- 
miner is absent, the alternative recessive qualities will appear. 
This explains why the inbred recessives always breed true. 
They would not show at all if there were any taint of the 
dominant quality. 

c. The principle of segregation, or purity of the gametes. 
This principle asserts that, while both dominant and recessive 
determiners are to be found in every primordial germ cell, both 
determiners cannot go into one sperm or one ovum. In the 
process of maturing ova and sperm by nuclear division the two 
kinds of determiners separate, and thus an equal number of 
sperm and ova carry the dominant and the recessive characters. 
In other words, while the body of an organism may be a hybrid, 
the eggs and sperm are always pure. 

In the peas, for example, if T represents the determiner for 
tallness, and t that for dwarf ness, one-half the egg cells of the 
first hybrid generation will carry T and one-half will carry the 
recessive t. The same will be true of the male nuclei of the pollen 
tube. In mating the chances are even that each kind of male 
cell will fertilize each kind of female. The "checker-board" 
diagram (Fig. 254) will show the probable number of offspring of 
each possible type. 

494. The Relation of MendePs Conclusions to the Process of 
Maturation of Ova and Sperm. The student will realize that 
these views of Mendel fit in remarkably with the facts that were 
brought out (49 and 50) relative to the behavior of chromo- 
somes in nuclear division, and especially in the divisions in the 
forming of sperm and ova. It is not possible to develop the sub- 
ject at length here. These two lines of study, experiments in 
breeding, and studies of the structure and behavior of the minute 
parts of the germ cells, supplement one another; and both are 
throwing increasing light upon the problems of inheritance. It 
only adds to the appreciation of Mendel's results to recall that 
nothing was known in his time of these remarkable facts of 
cell behavior. 


495. Isolation. When new variations occur it is clear that 
they will have a better chance to accumulate and result in a 
new and permanent stock if for any reason these new characters 
cannot be "swamped" or buried by crossing with the more 
numerous and conservative members of the parent stock. 

The effects of geographic or physical isolation is seen on is- 
lands which have been populated from the mainlands, or in 
neighboring lakes, or in valleys between which there are barrier 
mountains. In such conditions the animals and plants are dis- 
tinctly different from the species from which they are believed 
to have sprung, and those on different islands are different from 
one another. It is believed by many observers that a larger 
number of related species are found under such conditions than 
where there is free interbreeding and migration. 

It is possible also that there are internal hindrances to mating 
which would operate in much the same way as geographic iso- 
lation. There may arise some instinctive feeling which would 
prevent mating between the average members of a species and 
individuals that had varied considerably from that average. 
We see this in some degree in human races. Or changes in the 
copulating organs or in the reactions of the germ cells might 
render such free intercrossing impossible. 

Finally, we have seen in Mendel's experiments that certain 
unit characters, even when the sperm and ova unite freely, do 
not permanently coalesce and blend; but sooner or later may 
separate out pure again. In a sense this is a case of physiological 
isolation of the structures that carry the varying characters 
even in the union of germ cells. 

We may say then that the varying animals may be kept 
apart by geographic barriers; or the germ cells may be kept 
apart even when the animals live freely together; or the char- 
acter-bearing determiners may be kept apart even when the 
germ cells come together. 

496. The Environment. Assuming that variations occur 
and make evolution possible, and that heredity enables some of 
these variations to pass from one generation to another and thus 
to accumulate, and that isolation keeps the various individuals 


from evening up and thus destroying their variations by inter- 
crossing, it still remains to find the factors that have determined 
and guided the actual course of evolution in any given instance. 
Why has development taken the course it has ? 

Theoretically, as we have seen, there might be an inherent 
tendency in living matter at the beginning to vary or evolve in 
a certain direction. Biologists are not agreed upon the exist- 
ence of such a tendency. On the other hand, the environ- 
ment meaning the total external conditions of the life of 
organisms may guide or direct the course of development. 
Any guidance must come from the one or the other of these 
sources, internal or external. 

The environment may act in either of two ways to mold 
evolution. We have seen in the first place that the environment 
does act directly to produce specific changes in organisms. 
If these changes can be inherited, this will be a most important 
means of guiding evolution. It is not certain, however, that 
these direct effects of the environment on the body of an organ- 
ism can be transmitted to the germ cells. External conditions 
may likewise act directly upon the germ plasm so as to produce 
changes in its nature. Doubtless such changes would perma- 
nently affect future generations. 

Whether this is true or not, there is another effect of the 
environment which has certainly had a great influence on the 
course of evolution. The principle was discovered by Darwin 
and by him called ' ' natural selection. ' ' It refers to the fact that 
the struggle for existence among organisms is so severe that 
some will inevitably be eliminated. We have seen that this is 
true among all species. A hundred are born of two parents: 
only two of these, on an average, will be preserved. Which 
will be successful? In the long run, those will be preserved 
which are best adjusted to the conditions of the life which they 
meet. This fact is independent of how they came to be ad- 
justed. This is known as the "survival of the fittest." "Fit- 
test" merely means those best adapted to live and reproduce 
in the environment encountered. This process of natural selec- 
tion does not cause variations, but it may make use of any 
variations that arise, no matter what their source, provided they 


can be inherited. Furthermore, it may use variations of small 
amount, or the more striking variations known as mutations. 
In so far as it acts, its result is to guide evolution into suitable 
adaptation to the conditions that are able in any degree to in- 
fluence life. Nature undoubtedly can exercise a veto power on 
the direction of evolution. 

497. Evolution and Man. Many people are deterred from 
accepting the general theory of evolution because of unwilling- 
ness to believe that man and his high mental and moral qualities 
could have come about in this fashion. It is of course impossible 
absolutely to prove that men or any other animals have thus 
evolved from the lower orders. All that can ever be done is to 
make it the most reasonable explanation. In reality it cannot 
make any material difference, one way or the other, whether 
man developed or was created outright. Just as much power 
and intelligence are necessary in the evolution of man as in out- 
right creation, and no quality of man is any less valuable or 
dignified under the assumption that it has grown than under 
the assumption that it was made outright. Man's moral place 
in the universe, like his place in American society, is properly 
determined by what he really is and not by the mere incident of 
the method of becoming. 

498. Topics for Library. i. Find definitions of evolution. Write one of 
your own. 

2. What are some of the arguments that have been offered to support the view 
that "acquired" bodily characters are transmissible? 

3. State some arguments that have been advanced against the view? 

4. State briefly Galton's law of heredity. 

5. What does the gradual development of the physical, mental, and moral 
nature of the human individual suggest? 

6. State DeVries' doctrine of "mutation" in a brief way. 

7. State Johanssen's "pure line" theory. 



499. Reference has already been made in the appropriate 
places in the text to some of the most important ways in which 
the various branches of animals are related to human welfare. 
We are now in a position to look back over the economic bearings 
of animals and to get a broader view of the part they play in 
the life of man. This is a point at which Zoology touches the 
interest of every citizen of the world, whether he has any other 
interest in it or not. The economic aspect should be one of 
the features of the study of Zoology in our schools. 

500. An Analysis of the Ways in which Animals Affect the 
Welfare of Man. i. Helpfully: (a) as food; (b) for clothing; 
(c) in the saving of labor; (d) in scientific experiment and 
medicine; (e) for pleasure, as companions and pets; (f) in minor 
miscellaneous ways. 

2. Hurtfully: (a) directly hurtful or dangerous; (b) as causes 
or conveyors of disease in man; (c) as enemies of our friends 
among the animals; (d) as destructive to vegetation; (e) as 
injurious to various manufactured products. 

These headings are by no means exhaustive nor of equal 
merit, but they will give the student some conception of the 
closeness of the bond between man and the rest of the animal 

501. Animals as a Food Supply. The difficulties of human 
life have been such at various places and times that man has 
experimented with almost all kinds of food that have held out 
any sort of promise of pleasure or nutrition. Notwithstanding 
this, very few of the invertebrates have been given a wide place 
in his bill-of-fare. The lowest forms eaten are the "trepangs" 
(holothurians) which are used in great numbers by the Chinese. 
In the branch of mollusks the oyster is at the head in importance. 



There are also a number of marine and fresh-water clams and 
mussels which are used in smaller numbers. Some species of 
gasteropods (snails) are prized in certain parts of the world. 
It is believed that the mussels were an important article of food 
to some tribes of North American Indians, and to some of the 
peoples of the old world when they were in a corresponding stage 
of development. Among the arthropods the lobster comes first 
in importance, although it has been fished to such an extent that 
the output is rapidly diminishing and the size of specimens is 
steadily decreasing. Crayfish, shrimps, prawns, and crabs are 
also used to some extent. The great class of insects furnishes 
only the honey of the honey-bee. 

Every class of vertebrates furnishes food species. The 
amphibians and the reptiles stand lowest in this respect. The 
frogs and turtles and a few lizards are their only edible repre- 
sentatives. The fishes, the birds, and the mammals furnish 
the staple meat foods. A very large per cent, of the fishes are 
recognized as edible. Many species of the sharks, even, are 
prized although they are actively carnivorous, and the car- 
nivorous animals have usually not been regarded as good to eat. 
Many species of birds have been eaten, but those that rank as of 
real importance belong chiefly to the Gallinae, the Columbae, and 
the Anseres. These three orders of birds supply to man hun- 
dreds of millions of dollars worth of his choicest food annually. 

Among the mammals many species have been eaten, but 
the great division of ruminants and the swine furnish the 
bulk of the meat food now used by the human race. The 
horse is increasing in importance in this respect. Among 
uncivilized people, before the domestication and improvement 
of the ox, the sheep, and the hog, this division (ruminants) 
still furnished the chief wild game animals. Besides meat they 
furnish milk, butter, and cheese. In the far north the walrus, 
the bear, and the reindeer take the place of these well-known 

502. Animals as a Source of Clothing for Man. In the 

case of primitive man the skins and furs of animals were the 
sole source of clothing and one of the means of making tents 


and dwelling places comfortable as well. As the art of weaving 
was developed and the discovery and perfecting of textiles 
derived from plants made headway, we became less dependent 
on the animals. But even now, in one form or another, the 
skins and the fur, hair, and wool of animals are among our 
choicest clothing materials. The mammals are of course the 
main animal source of clothing, but the warmth of feathers has 
long been recognized, and the skins and feathers of birds furnish 
articles of clothing and decoration. The important mammals 
supplying the kind of hair suitable for clothing and for carpets 
and other coarser fabrics are: the various species of sheep and 
goats, the camels, the alpacas, and their relatives. Our leathers 
are made from the skins of these and related animals, and 
from some of the carnivora. Practically all the ruminants 
produce valuable leathers. Horse hides are also used for this 
purpose. The skins of many of the soft, thick-haired animals 
are dressed with the hair on, and are known as furs. Most of 
these, as the seals, the sable, mink, ermine, weasel, raccoon, 
foxes, skunk, members of the cat family, and some others, be- 
long to the carnivora. A few, as the squirrel, the hare, and the 
beavers (now almost extinct) are from the rodents. 

One of the most marvelously delicate and beautiful of 
our fabrics, silk, is a secretion sfc>un out by the larvae of the silk- 
moth in making the cocoon in which it pupates, or changes to 
the adult stage. It is killed by being put into hot water, and 
then the silk thread is unwound. The silk industry is of much 
importance in China, Japan, Italy, and France. 

503. Animals Used in Saving Human Labor. In the 

earlier stages of civilization this help consisted of aid in hunting 
and capture of food-animals. The dog was probably one of 
the first animals domesticated. Later others came to be used 
for riding, for drawing loads in vehicles or otherwise, for plow- 
ing the soil, and the like. The camel, the ox, the horse, and 
the elephant rank among the most valuable in these respects. 
In the earlier civilizations of the orient, the' camel has been of 
the utmost value. His adjustment to the tropical and semi- 
arid conditions is striking. The "ship of the desert" is an older 


means of comrru'ive than the navigation of the sea. His 
relatives in South America, the llamas, have long been used as 
the chief animal of burden in the Andes. The reindeer is at 
once camel, horse, and ox of the frigid zone. The dog is also 
an efficient beast of burden over the snows. 

In modern times the horse has r-eme to be one of the most 
valuable of animals used by man, .ranking in money value next 
to the cattle in the United States. The mule, which is a hybrid 
between the female horse and the male ass is a hardy, strong, 
infertile animal and one of our most valuable beasts of burden 
in agricultural communities. The student will not fail to 
realize that more powerful groups of human beings have used 
other men as animals in very much the manner we have been 
describing. While slavery is not legalized over much of the 
earth it is still known in other forms. 

Steam and electricity are replacing animal power at many 
points, but there is little likelihood that it will entirely displace 
them until population becomes so numerous that the animals 
are more valuable as food than they can be as labor-saving 

504. Animals in Science and Medicine. A most inter- 
esting way in which animals have been of value to the human 
race grows out of the fact of the general likeness between man 
and the lower animals. It is safe to say that the great ad- 
vances in surgery which have accomplished so much in the 
saving of human life have been made by early experimentation 
on animals in the laboratory, even more than by the actual 
direct study of the human body and its conditions. Experi- 
ments quite impossible of being made on human beings have 
first been tried on animals and have been found to accomplish 
what was expected of them. But it is not merely in surgery 
that they have shown themselves most valuable to human life. 
In all the work on antitoxins and serums with which to combat 
the germ diseases, the work must first be done on animals. 
The antitoxin of diphtheria comes from the horse; the virus by 
which we vaccinate for smallpox is obtained by giving the 
disease to the ox, which is only mildly affected by it; and so on 


through a considerable list of such diseases. The most hopeful 
work that is now being done to give us control of the terrible 
disease of cancer is being done on mice and rats, otherwise an 
enemy to human interests. 

It has been discovered of late years that many organs of 
the body secrete into the blood that passes through them, 
certain substances that have a most important bearing on life. 
Sometimes these organs become diseased and refuse to do their 
work. This may be true of the digestive glands, of the thyroid 
glands in the neck, and others. In such cases the gastric juice 
of the pig, or the thyro-iodin obtained from the thyroid of the 
sheep, or pancreatin obtained from other animals may be ad- 
ministered in such a way as to carry on the function or even to 
overcome the disease. 

Frogs, rabbits, guinea-pigs, cats, dogs, sheep, cows, horses, 
and monkeys are among the animals that have been most 
useful in these respects. 

505. Miscellaneous Uses of Animals. A long list of minor 
and exceptional uses of animals could be mentioned. The dog 
and cats have some protective value about the homes of men, 
where they were long before recorded history began. They have 
kept homes free' of vermin, as rats and mice and the smaller 
animals, and the dog has doubtless been of protection from 
other enemies. They have been companions for childhood 
through all history. 

The skins and tendons and bone of animals figured largely 
in all the early home industries, as sewing and tying and the 
like. In modern times the skins are used in many ways, as rugs 
and home ornaments, in the making of bags and other receptacles. 
Horn, bone, and ivory from the teeth and tusks of animals are 
largely used in knife handles, piano keys, billiard balls, and 
numerous other objects. Whale oil (sperm oil) has long been 
used in making candles, though it has now been supplanted 
by the cheaper mineral oils. Bristles, brushes, perfumes, oils, 
glue, buttons, and fertilizers are only a portion of the numerous 
by-products of the modern meat-packing houses. Ornaments 
are made of teeth, of corals, and of pearls. Many animals are 


scavengers, attacking and destroying carrion and other decaying 
organic matter. 

506. Animals Directly Injurious to Man. When man first 
appeared on the earth many mammals now extinct, much larger 
and fiercer than those of the present time, were abundant. Un- 
questionably these were much more of a menace to him then 
than the predaceous animals are now. These animals, except 
in a few poorly inhabited parts of the world, are now practically 
negligible. A few poisonous snakes, some sharks and crocodiles, 
a few members of the cat and dog families and a few species of 
bear, almost extinct, about exhaust the list of animals really 
dangerous because of size or ferocity. But in place of these there 
are now numerous species that are no less dangerous to him 
because of diseases which they bring to man directly or indirectly. 
Reference has been made to Protozoa which produce malaria 
and yellow fever, and to the intestinal and other parasites that 
belong to the unsegmented worms and produce all sorts of dis- 
comfort, inefficiency, and disease among men. The mosquitoes 
and flies and other insects that spread these diseases are just as 
important to us as the germs themselves. The bubonic plague 
is believed to be a disease of rats and other rodents carried to 
man from the rat by fleas. All these temporary external para- 
sites that go from animal to animal, as lice, fleas, bedbugs, 
mosquitoes, etc., are especially favorably situated to be the 
carrier of disease. Cattle are subject to tuberculosis; and many 
investigators believe that this is the same as the human disease, 
or at least that it is intercommunicable in man and cattle. 

507. Animals Hurtful to our Animal Friends. Our domestic 
animals are apparently almost as open to diseases as man 
himself. Hence it is that there are many animal diseases, caused 
by the various parasites of blood and organs. The Texas fever 
of cattle; the haemoglobinuria of cattle and sheep; pebrine, a 
disease of the silkworm; the sleeping sickness of Africa; etc., 
are due to protozoa, and are carried by ticks, flies, and the like. 
The liver-rot and staggers of sheep, and the various tape- worm 
and hook-worm diseases of hogs, dogs, cattle, and man are 
caused bv the unsegmented worms. The "bot" diseases of 


horses, sheep, and cattle are produced by the larval stages of 
flies. Mange, itch, etc., as found in various domestic animals, 
is due to the action of mites. These also attack poultry. 

508. Animals Hurtful to Plants and Plant Products. At 

this point animals, more particularly the insects, do great 
damage to man's interests. The story is too long to tell here, 
but insects attack plants at every stage of their history from 
the time they germinate until the time they are stored. They 
devour foliage, fruits, timbers, seeds, stored grain, and manu- 
factured products. It is said that "the elm has eighty species 
of insects that are more or less supported by it ; birches and 
maples, over one hundred; corn is attacked by about two 
hundred species, of which fifty do notable injury and some 
twenty are pests; apple insects number some four hundred 

It is estimated that the farmers and fruit growers of America 
alone lose 500,000,000 dollars annually from the ravages of 
insects on crops and forests. Man has done much to control 
most animals except the insects, and something to control them. 
The most effective means thus far found is by spraying plants, 
to keep away the females when about to lay, or to kill the young; 
the rotation of the crops in such a way that the insects may hatch 
without finding the kind of food on which they depend ; and the 
introduction of animals, either predatory or parasitic, that attack 
the hurtful species. 

Occasionally through a period of years such animals will 
multiply to such an extent as to become a plague, when in 
ordinary years they merely reduce the returns of the agricultur- 
alist. Such are the outbreaks of the Rocky Mountain locust, 
the Hessian fly, the San Jose scale, the chinch bug, and the like. 
Less frequently similar things happen in respect to other animals, 
as the plague of rabbits in Australia, and of the rats in the 

509. Domestication of Animals. Occasional reference has 
been made in the preceding paragraphs to the domestic animals. 
This means something quite different from the merely useful 
animals. This term refers to the control by man of the life 


and habits of animals to such degree that he can work his will 
with them almost irrespective of the natural conditions. This 
domestication began before man emerged from the savage 
state before recorded history began. The dog and cat were 
doubtless among the earliest forms, and the camel, the horse, 
the sheep, the donkey, the hog, the pigeon, the chicken, all 
have been very long in domestication. 

The qualities that would make an animal suitable for 
domestication, and which doubtless helped to determine what 
forms should be domesticated, are: some gentleness of temper 
and lack of extreme nervousness, capability of taking training, 
ability to become adapted to new surroundings, some love of 
locality, and usefulness, for food or for some other of man's 
real or fancied needs. The effect of domestication on animals 
has been to soften and dull their original wild instincts, to 
render them less active and alert, to render them dependent on 
the care which man gives them, and to produce a tendency to- 
take on fat easily when food is plenty. 

510. The Animal Industries. The animal industries clus- 
ter chiefly, but not wholly, about the domestic animals. These 
domestic industries relate to horses, cattle, mules, sheep, hogs, 
poultry (including chickens, turkeys, pigeons, geese, and ducks), 
and constitute a large part of agriculture in the United States 
to-day. The successful pursuit of these industries involves a 
scientific knowledge of the animals reared and of their possi- 
bilities and demands. Much of this in the past has been done 
in a haphazard sort of way, somewhat incidental to the raising 
of crops. In the last half-century, however, there has been a 
better application of the known principles of stock-breeding 
and of selection than before. The modern stock-breeder crosses 
and recrosses various strains of cattle or hogs in order to get 
variety of result. Then he selects what pleases him most and 
breeds that in such a way as to fix and increase the features 
which he regards as most profitable, whether it be swiftness, 
size, milk-giving, cream-producing, finer wool, fertility, or the 
power to take on fat. This is the reason we have so many 
varieties of all the animals that have been long in domes- 


ti cation. Different breeders have been, more or less consciously, 
selecting different qualities in animals. Men are now pushing 
this process even further, and are purposely and systematically 
producing and selecting new and better breeds. The evolution 
will be correspondingly more rapid. 

All the animal culture mentioned above is done in con- 
nection with plant culture and often on the best land the 
country has. There are other industries which have great 
possibilities in regions not suited to agriculture. Man has used 
some of the natural resources of these less used regions in a 
very prodigal and reckless way. He has not taken pains to 
look ahead and to act scientifically, simply because he has not 
been compelled to do so. The time, however, has come when 
the farmer uses systematically the less productive parts of his 
farm as well as the more fertile. If not fit for cultivation he 
uses it for sheep or goats or something which is best adapted 
to it. So it must be in the future with the whole earth. The 
oceans, the rivers, the swamps, the ponds and lakes, the arid 
regions, and the mountains must be stocked with animals that 
will contribute to man's food. These animals must be helped 
to take the place of those which are of no use. This means that 
the various fishing industries, as mackerel, cod, herring; and 
even more particularly the fresh-water fisheries, as the salmon, 
whitefish, and other lake and river fish, shall be more than the 
mere catching and canning of such fish as have been able to 
fight their own way through to successful maturity; they will 
include the bending of all possible agencies to the building up, 
improving, and maintaining the numbers and quality of these 
fish. Oyster, lobster, pearl-mussel, and sponge fishing will be 
more than merely to put on the market the biggest possible 
amount regardless of the future; it will rather be the stocking of 
all suitable places of the ocean margin that are not more profit- 
ably used for something else with the young of the types wanted, 
keeping away the enemies of the species as well as possible, and 
doing artificially all that can be done to insure their steady and 
profitable growth. The scientific treatment of these industries 
means the putting on the market only those that have got their 


best growth and with the least injury to the partly grown in- 
dividuals, just as it does on the farm or in the forest. 

The conservation of our natural resources also means that the 
swamp lands, that cannot be drained and used more profitably, 
and the shallow streams shall be made to produce edible frogs 
or turtles instead of the other inedible amphibians and reptiles. 
Wherever edible fish can grow pains will be taken to place them 
and see that the conditions of their best life are given them. 

Various bureaus of the United States Government, as that 
of Animal Industry, of Fish and Fisheries, of Entomology, and 
the like; the Agricultural Experiment Stations of the various 
States; the scientific men in the universities and colleges, and 
many practical workers who are in these fields in a commercial 
way, are studying the problems suggested in this chapter and 
are trying to add to the vital resources of the human race and 
its increasing population. It is the duty of every American to 
get into a sympathetic attitude to all this work. 

511. Topics for the Library. i. Give a report of the United States Bureau of 
Fish and Fisheries. How is it organized? What are its principal stations? 
What work does it undertake to do? What are its chief publications? 

2. Discuss similarly the Bureau of Animal Industry; the Bureau of Entomology; 
the Biological Survey. 

3. Give an account of one of the inland fish hatcheries and its work. 

4. Why are the vegetable feeding animals preferred for food above the 
carnivora ? 

5. Why are woolen fabrics warmer than cotton or linen? 

6. Will the vegetable food or the animal food that can be produced and sup- 
ported on ten acres of ground go further in supporting human life ? Discuss 

7. Do you think the tendency will be to increase or to decrease animal culture 
as population begins to press upon the resources of the soil? Give the biological 
reasons fully. 

8. What are the chief arguments against vivisection and experiments on living 
animals by investigators? What the chief arguments in favor of it? 

9. Discuss the various products of the hog, utilized in the packing houses. 

10. Discuss the animals that frequent human habitations, becoming pests 
in some degree. Give the habits of life of each so far as you can by study and 

1 1 . Discuss with some care what is known of the history, the different varieties, 
the use, etc., of some of the domestic animals, as the dog, the horse, the fowl, the 
ox, and others. 

12. Discuss human slavery as a zoological phenomenon. To what is due the 
tendency to abolish it? 



512. Introduction. Long before man discovered a way to 
record his knowledge he must have learned much about plants 
and animals. His well-being depended on it. Here he got his 
food. His most dreaded enemies -were the great mammals that 
were on the earth when he appeared. His success in evading 
his enemies and in capturing his food made it necessary that he 
know something of their haunts and habits. As he learned to 
domesticate some of these, both for protection and for food 
supply, his knowledge was extended. As the animals were 
slaughtered for food and clothing and for his primitive religious 
rites, he learned something of their internal structure. This 
early knowledge probably had little pure scientific interest back 
of it. Most of it was incidental to the uses he made of the 
animals, doubtless reinforced somewhat by the feeling of wonder 
and the esthetic sense. In a similar way human accidents, dis- 
eases, and deaths doubtless led to some study of the human body 
and its care and cure, as soon as human intelligence was able to 
grasp the situation. 

It is not easy for us to forget the discoveries of the last 400 
years and realize how fragmentary and unscientific were the 
ideas of the early times. 

513. The Greek and Roman Periods. So far as we need con- 
sider here, the foundations of modern zoology were laid by Aris- 
totle (384-322 B.C.). It must not be imagined however that 
his was the first effort to bring together the knowledge already 
gained. For example, ancient Egyptian documents twelve 
centuries older than Aristotle give elaborate discussions of 
medical subjects. 

Aristotle is chiefly known as a philosopher and logician; but 
he was a most prolific student in many realms. He made two 
great contributions to natural history: (i) he brought together 



in three works "Historic, animalium," il De partibus, " and "De 
generatione" what was known of the nature and classification 
of animals, their anatomy, and their development; and (2) in 
his own great discoveries he insisted on the scientific method of 
depending on observed facts rather than on tradition. 

He did the first task so well that his successors paid little 
attention to his scientific point of view for nearly 2,000 years, 
but merely asked what Aristotle had said on the subject. 

Another Greek, Galen, did for medicine very much what 
Aristotle did for zoology. He made anatomy and physiology 
the foundation of medicine. Because it was forbidden to 
dissect the human body, he studied the bodies of monkeys 
and other mammals. For centuries his successors followed his 
conclusions without using his methods. 

The name of the Roman, Pliny (23-79 A - D is usually men- 
tioned here because of a great collection of writings on natural 
phenomena. He fell away, however, from the spirit of Aristotle 
and Galen, and brought together without much discrimina- 
tion facts and nature-anecdotes of the most fanciful kind. The 
Romans did very little for science. 

514. The Middle Ages. During this remarkable period of 
human history (400-1500 A.D.) practically no progress was 
made in the natural sciences. It was a period of waning of 
the old human civilizations and the incubation of the new. 
Wars, difficulty of travel, the other-world attitude of the 
Christian church, and many other things discouraged observa- 
tion of nature. It was a time of mysticism, of metaphysical 
speculation, and of complete dependence on the authorities. 
A large portion of it has been called, not inappropriately, the 
Dark Ages. 

515. The Modern Period (1500-1900) and its Specializations. 

In every department of human interest the revival following 
the "dark ages" is one of the most remarkable movements of 
history. This is no less true in our science than elsewhere. It 
was more than a revival of interest. The authority of the 
church and of the past had so intrenched itself that the greatest 
difficulty lay in getting back freedom of investigation and 


utterance. The first task was to throw off tradition and 
authority. The names of Gallileo, Descartes, and Vesalius be- 
long to this period of revival. Italy had a large part in the 
renewal of scientific work, as in other fields. 

Starting with the general interest in natural history and 
classification and in medicine and anatomy, which was seen in 
the work of Aristotle and Galen, the modern period has de- 
veloped a great many special departments, as the various 
workers have followed particular studies. We may profitably 
consider the progress in Biology under the following heads: 
(i) Natural history and classification; (2) Anatomy and its 
departments; (3) Physiology; (4) Embryology; (5) Philosophy 
of Biology; and (6) Applications of Biology. Of course these 
divisions of interest took shape only gradually, and a worker 
often contributed to several departments, especially at the 

516. Natural History and Systematic Zoology. Naturally 
enough until the more important animals had been recorded 
and compared and classified, and their more striking habits and 
modes of living studied this would present a most fruitful and 
popular field of investigation. Gesner (1516-1565), a Swiss 
physician, studied animals widely and wrote a "history of 
animals" which was the best general zoology since Aristotle. 
His work, which was profusely illustrated, largely influenced 
later studies. 

Ray (1629-1705), an Englishman, was a student of both 
plants and animals. In addition to his own original studies and 
discoveries he introduced the idea of the species as a definite 
group of organisms arising from similar parents. He was thus 
the real founder of systematic biology, and the forerunner of 
the great Swedish naturalist Linnaeus (1707-1778), who pub- 
lished the Sy sterna Natures in an effort to describe all the known 
species of plants and animals. Linnaeus combined Ray's species 
with the genus, and first used the generic and specific names as 
the name of the organism (binominal nomenclature). He in- 
vented also the brief descriptions of genera and species. His 
work greatly stimulated systematic studies and laid the founda- 
tion for work in other fields of zoology. 


Buff on (1707-1788), in his Natural History, Cuvier (1769- 
1832) in his "Animal Kingdom," Lamarck (1774-1829), von 
Siebold (1804-1885) and many later naturalists have worked 
to perfect our system of classification of animals. 

517. Anatomy and its Divisions. We have seen that Galen 
studied the anatomy of lower mammals to throw light on 
medicine. His accounts became the accepted rule for centuries. 
Vesalius (1514-1564) a physician, born in Brussels, made elabo- 
rate studies of the anatomy of the human body as well as of 
many other animals. He published a great work on the "Struc- 
ture of the Human Body," in which he denied many of the views 
held previously. His discoveries were illustrated by many 
most effective figures and plates. Both because of the scope of 
his work and the investigative spirit Vesalius laid the foundation 
of modern scientific Biology. It was not so much that all his 
conclusions were correct. He did what was much better; he 
made it possible for his successors to criticise and add to his 
own work. This makes progress. 

From the study of the habits and exterior of animals, and 
the study of human anatomy, students would naturally pass to 
a study of internal structure. As time passed and details of 
structure accumulated, two lines of study would be developed: 
a comparison of the structure of different organisms, and, second, 
the study of smaller and smaller structures. The former is 
called Comparative Anatomy, and the latter Histology and 

As in all other branches there