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                           ELEMENTARY ZOOLOGY

                                   BY

                        VERNON L. KELLOGG, M.S.

      _Professor of Entomology, Leland Stanford Junior University_

                       _SECOND EDITION, REVISED_

                                NEW YORK

                         HENRY HOLT AND COMPANY

                                  1902

                            Copyright, 1901,

                                   BY

                            HENRY HOLT & CO.

                   ROBERT DRUMMOND, PRINTER, NEW YORK

                                PREFACE

It seems to the author that three kinds of work should be included in
the elementary study of zoology. These three kinds are: (_a_)
observations in the field covering the habits and behavior of animals
and their relations to their physical surroundings, to plants, and to
each other; (_b_) work in the laboratory, consisting of the study of
animal structure by dissection and the observation of live specimens in
cages and aquaria; and (_c_) work in the recitation- or lecture-room,
where the significance and general application of the observed facts are
considered and some of the elementary facts relating to the
classification and distribution of animals are learned.

These three kinds of work are represented in the course of study
outlined in this book. The sequence and extent of the study in
laboratory and recitation-room are definitely set forth, but the
references to field-work consist chiefly of suggestions to teacher and
student regarding the character of the work and the opportunities for
it. Not because the author would give to the field-work the least
important place,--he would not,--but because of the utter
impracticability of attempting to direct the field-work of students
scattered widely over the United States. The differences in season and
natural conditions in various parts of the country with the
corresponding differences in the "seasons" and course of the
life-history of the animals of the various regions make it impossible
to include in a book intended for general use specific directions for
field-work. Further, the amount of time for field-work at the disposal
of teacher and class and the opportunities afforded by the topographic
character of the region in which the schools are located vary much. The
initiation and direction of this must therefore always depend on the
teacher. On the other hand, the work of the other two phases of study
can to a large extent be made pretty uniform throughout the country. For
dissection, specimens properly killed and preserved are about as good as
fresh material, and by modifying the suggested sequence of work a little
to suit special conditions or conveniences, the examination of live
specimens in the laboratory can in most cases be accomplished.

The author believes that elementary zoological study should not be
limited to the examination of the structure of several types. The
student should learn by observation something of the functions of
animals and something of their life-history and habits, and should be
given a glimpse of the significance of his particular observations and
of their general relation to animal life as a whole. The drill of the
laboratory is perhaps the most valuable part of the work, but as a
matter of fact the high school is trying to teach elementary zoology,
an elementary knowledge of animals and their life, and dissection
alone cannot give the pupil this knowledge. On the other hand, without
a personal acquaintance with animals, based on careful actual
observations of their life-history and habits and on the study of the
structural characters of the animal body by personally made
dissections, the pupil can never really appreciate and understand the
life of animals. Reading and recitation alone can never give the
student any real knowledge of it.

The book is divided into three parts, of which Part I should be[1]
first undertaken. This is an introduction to an elementary knowledge of
animal structure, function, and development. It consists of practical
exercises in the laboratory, each followed by a recitation in which the
significance of the facts already observed is pointed out. The general
principles of zoology are thus defined on a basis of observed facts.

Part II is devoted to a consideration of the principal branches of the
animal kingdom; it deals with[2] systematic zoology. In each branch
one or more examples are chosen to serve as types. The most important
structural features of these examples are studied, by dissection, in
the laboratory. The directions for these dissections consist of
technical instructions for dissecting, the calling attention to and
naming of principal parts, together with questions and demands
intended to call for independent work on the part of the student. The
directions follow the actual course of the dissection instead of being
arranged according to systems of organs, and are intended for the
orientation of the student and not to be in themselves expositions of
the anatomy of the types. The condensation of these directions is made
more feasible by the presence of anatomical plates (drawn directly
from dissections). Following the account of the dissection of the type
are brief notes on its life-history and habits. Then follows a
general account of the branch to which the example dissected belongs
and brief accounts of some of the more interesting members of the
branch. In these accounts technical directions are given for brief
comparative examinations and for the study of the life-history and
habits of some of the more accessible of these forms.

It will not be possible, of course, to undertake with any thoroughness
the consideration of all of the branches of animals in a single year.
But all are treated in the book, so that the choice of those to be
studied may rest with the teacher. This choice will of necessity depend
largely on the opportunities afforded by the situation of the school,
as, for example, whether on the seashore or in the interior near a lake
or river, or on the dry plains, and on the relation of the school-terms
to the seasons of the year. The branches are arranged in the book so
that the simplest animals are first considered, the slightly complex
ones next, and lastly the most highly organized forms. But if in order
to obtain examples for study it is necessary to take up branches
irregularly, that need not prove confusing. The author would suggest
that whatever other branches are studied, the insects and birds, which
are readily available in all parts of the country, be certainly
selected, and with this selection in view has given them special
attention. Indeed some teachers may find these two branches to offer
quite sufficient work in classificatory and ecological lines.

Part III is devoted to a necessarily brief consideration of certain of
the more conspicuous and interesting features of animal ecology. It
has in it the suggestion for much interesting field-work. The work of
this part should be taken up in connection with that of Part II, as,
for example, the consideration of social and communal life in
connection with the insects, parasitism in connection with the worms,
and also with the insects, distribution in connection with the birds,
perhaps, and so on.

In appendices there are added some suggestions for the outfitting of
the laboratory, and a list of the equipment each student should have.
Here, also, is appended a list of a few good authoritative reference
books which should be accessible to students and to which specific
references are made in the course of this book. Some practical
directions for the collecting and preserving of specimens are also
given. (Suggestions for the obtaining of material for the various
laboratory exercises outlined in the book are to be found in
"technical notes" included in the directions for each exercise.) The
author believes that the building up of a single school-collection in
which all the pupils have a common interest and to which all
contribute is to be encouraged rather than the making of separate
collections by the pupils. Waste of life is checked by this, and in
time, with the contributions of succeeding classes, a really good and
effective collection may be built up. The "collecting interest" can be
taken advantage of just as well in connection with a school-collection
as with individual collections.

The plates illustrating the dissections have all been drawn originally
for the book from actual dissections. Most of the other figures are
original, either drawn or photographed directly from nature, or from
preserved specimens. Credit is given in each case for figures not
original. The drawings for all of the figures of dissections and for
all original figures not otherwise accredited were made by Miss Mary
H. Wellman, to whom the author expresses his obligations. The thanks
of the author are due to Mr. George Otis Mitchell, San Francisco, who
kindly made the photo-micrographs of insect structure from the
author's slides; to Professor Mark V. Slingerland, Cornell University,
for electros of his photographs of insects; to Dr. L. O. Howard, U.
S. Entomologist, for electros of figs. 45, 52, 56, 68, 81, 82, 83, 84,
87, 90, and 92; to Professor L. L. Dyche, University of Kansas, for
photographs of his mounted groups of mammals; to Mrs. Elizabeth
Grinnell, Pasadena, Calif., for photographs of birds; to Mr. J. O.
Snyder, Stanford University, for photographs of snakes; to Mr. Frank
Chapman, editor of "Bird-lore," for electros of photographs of birds;
to Mr. G. O. Shields, editor of "Recreation," for an electro of the
photograph of a bird; to the American Society of Civil Engineers for
electros of photographs of boring marine worms; to Cassell & Co., for
electros of three photographs from nature; to Geo. A. Clark, secretary
Fur Seal Commission for photographs of seals; and to the Whitaker and
Ray Co., San Francisco, for electros of figs. 46, 59, 60, 61, 64, 65,
93, 94, 97, 98, 99, 100, 102, 119, and 166 to 172, published
originally in Jenkins & Kellogg's "Lessons in Nature Study." The
origin of each of these pictures is specifically indicated in
connection with its use in the book.

The author's sincere thanks are also due to Mrs. David Starr Jordan
and to Mr. J. C. Brown, graduate student in zoology in Stanford
University, for their assistance in the correction of the MS., and in
the preparation of the laboratory exercises respectively. The chapters
of Part II relating to the vertebrates were read in MS. by President
David Starr Jordan, whose aid and courtesy are gratefully
acknowledged. Similar acknowledgments are due Professors Harold Heath
and R. E. Snodgrass for reading the proofs of the directions for the
laboratory exercises.

                             VERNON LYMAN KELLOGG.

  STANFORD UNIVERSITY, May, 1901.

FOOTNOTES:

[1] This is true if a strictly logical treatment of the subject is
held to. As a matter of fact, it is often of advantage to begin with,
or at least to take up from the beginning in connection with the
indoor work, some field-work, such as the collecting and classifying
of insects and the observation of their metamorphosis. As most schools
begin work in the fall, advantage must be taken of the favorable
opportunities for field-work at the beginning of the year. These
opportunities are of course much less favorable in the winter.

[2] The classification of animals used in this book is that adopted in
Parker and Haswell's "Text-book of Zoology" (2 vols., 1897, Macmillan
Co.). Exception is made in the case of the worms, which are considered
as a single branch, Vermes, instead of as several distinct branches.

                                CONTENTS

                                 PART I

                STRUCTURE, FUNCTIONS, AND DEVELOPMENT OF
                                ANIMALS

                I.--THE STUDY OF ANIMALS AND THEIR LIFE.

Our familiar knowledge of animals and their life, 1.--Zoology and its
divisions, 2.--A first course in Zoology, 3.

               II.--THE GARDEN TOAD (BUFO LENTIGINOSUS).

[Laboratory exercise], 5.--External structure, 5.--Internal structure,
7.

          III.--THE STRUCTURE AND FUNCTIONS OF THE ANIMAL BODY.

Organs and functions, 14.--The animal body a machine, 14.--The
essential functions or life-processes, 15.

                   IV.--THE CRAYFISH (CAMBARUS SP.).

[Laboratory exercise], 17.--External structure, 17.--Internal
structure, 21.

             V.--THE MODIFICATION OF ORGANS AND FUNCTIONS.

Difference between crayfish and toad, 26.--Resemblances between
crayfish and toad, 27.--Modification of functions and structure to fit
the animal to the special conditions of its life, 29.--Vertebrate and
invertebrate, 30.

                    VI.--AMŒBA AND PARAMŒCIUM.

[Laboratory exercise], 31.--Amœba, 31.--The slipper-animalcule
(PARAMŒCIUM SP.), 34.

            VII.--THE SINGLE-CELLED ANIMAL BODY; PROTOPLASM
                             AND THE CELL.

The single-celled animal body, 36.--The cell, 37.--Protoplasm, 39.

            VIII.-CELLULAR STRUCTURE OF THE TOAD (OR FROG).

[Laboratory exercise], 40.--The blood, 40.--The skin, 40.--The liver,
41.--The muscles, 41.

           IX.--THE MANY-CELLED ANIMAL BODY; DIFFERENTIATION
                              OF THE CELL.

The many-celled animal body, 43.--Differentiation of the cell, 43.

                               X.--HYDRA.

[Laboratory exercise], 46.

                 XI.--THE SIMPLEST MANY-CELLED ANIMALS.

Cell-differentiation and body-organization in Hydra, 52.--Degrees in
cell-differentiation and body-organization, 54.

                     XII.--DEVELOPMENT OF THE TOAD.

[Field and laboratory exercise], 55.

                 XIII.--MULTIPLICATION AND DEVELOPMENT.

Multiplication, 57.--Spontaneous generation, 58.--Simplest
multiplication and development, 59.--Birth and hatching,
61.--Life-history, 62.

                                PART II

                           SYSTEMATIC ZOOLOGY

                  XIV.--THE CLASSIFICATION OF ANIMALS.

[Laboratory exercise and recitation], 65.--Basis and significance of
classification, 65.--Importance of development in determining
classification, 67.--Scientific names, 68.--An example of
classification, 68.--Species, 69.--Genus, 70.--Family, 72.--Order,
72.--Class and branch, 73.

             XV.--BRANCH PROTOZOA: THE ONE-CELLED ANIMALS.

EXAMPLE: THE BELL ANIMALCULE (VORTICELLA SP.) [Laboratory exercise], 75.

OTHER PROTOZOA.

Form of body, 78.--Marine Protozoa, 80.

                  XVI.--BRANCH PORIFERA: THE SPONGES.

EXAMPLE: THE FRESH-WATER SPONGE (SPONGILLA SP.) [Laboratory exercise],
84.

EXAMPLE: A CALCAREOUS OCEAN-SPONGE (GRANTIA SP.) [Laboratory
exercise], 85.

EXAMPLE: A COMMERCIAL SPONGE [Laboratory exercise], 86.

OTHER SPONGES.

Form and size, 87.--Skeleton, 88.--Structure of body, 88.--Feeding
habits, 88.--Development and life-history, 89.--The sponges of
commerce, 90.--Classification, 91.

        XVII.--BRANCH CŒLENTERATA: THE POLYPS, SEA-ANEMONES,
                        CORALS, AND JELLYFISHES.

POLYPS, SEA-ANEMONES, CORALS, AND JELLYFISHES.

General form and organization of body, 93.--Structure, 94.--Skeleton,
95.--Development and life-history, 95.--Classification, 96.--The
polyps, colonial jellyfishes, etc. (Hydrozoa), 97.--The large
jellyfishes, etc. (Scyphozoa), 101.--The sea-anemones and corals
(Actinozoa), 102.--The Ctenophora, 107.

       XVIII.--BRANCH ECHINODERMATA: THE STARFISHES, SEA-URCHINS,
                          SEA CUCUMBERS, ETC.

EXAMPLE: STARFISH (ASTERIAS SP.) [Laboratory exercise].--External
structure, 108.--Internal structure, 110.--Life-history and habits, 113.

EXAMPLE: SEA-URCHIN (STRONGYLOCENTRUS SP.) [Laboratory
exercise].--External structure, 113.

OTHER STARFISHES, SEA-URCHINS, SEA CUCUMBERS, ETC.

Shape and organization of body, 116.--Structure and organs,
117.--Development and life-history, 119.--Classification,
120.--Starfishes (Asteroidea), 121.--Brittle stars (Ophiuroidea),
122.--Sea-urchins (Echinoidea), 123.--Sea-cucumbers (Holothuroidea),
124.--Feather-stars (Crinoidea), 125.

                    XIX.--BRANCH VERMES: THE WORMS.

EXAMPLE: THE EARTHWORM (LUMBRICUS SP.) [Laboratory
exercise].--External structure, 127.--Internal structure,
129.--Life-history and habits, 133.

OTHER WORMS.

Classification, 135.--Earthworms and leeches (Oligochætæ), 136.--Flat
worms (Platyhelminthes). 137.--Round worms (Nemathelminthes),
140.--Wheel-animalcules (Rotifera), 142.

          XX.--BRANCH ARTHROPODA: THE CRUSTACEANS, CENTIPEDS,
                         INSECTS, AND SPIDERS.

CLASS CRUSTACEA: CRAYFISHES, CRABS, LOBSTERS, ETC.

EXAMPLE: THE CRAYFISH (CAMBARUS SP.). Structure, 146.--Life-history
and habits, 146.

OTHER CRUSTACEANS.

Body form and structure, 147.--Water-fleas (_Cyclops_),
148.--Wood-lice (_Isopoda_), 150.--Lobsters, shrimps, and crabs
(Decapoda), 151.--Barnacles, 155.

                  XXI.--BRANCH ARTHROPODA (CONTINUED).

CLASS INSECTA: THE INSECTS.

EXAMPLE: THE RED-LEGGED LOCUST (MELANOPLUS FEMUR-RUBRUM). [Laboratory
exercise]. External structure, 157.--Life-history and habits, 161.

EXAMPLE: THE WATER-SCAVENGER BEETLE (HYDROPHILUS SP.) [Laboratory
exercise]. External structure, 163.--Internal structure,
166.--Life-history and habits, 169.

EXAMPLE: THE MONARCH BUTTERFLY (ANOSIA PLEXIPPUS) [Laboratory
exercise]. External structure, 171.--Life-history and habits, 175.

EXAMPLE: LARVA OF MONARCH BUTTERFLY [Laboratory exercise]. Structure,
177.

OTHER INSECTS.

Body form and structure, 181.--Development and life-history,
188.--Classification, 191.--Locusts, cockroaches, crickets, etc.
(Orthoptera), 192.--The dragon-flies and May-flies (Odonata and
Ephemerida), 194.--The sucking-bugs (Hemiptera), 197.--The flies
(Diptera), 201.--The butterflies and moths (Lepidoptera), 205.--The
beetles (Coleoptera), 206.--The ichneumon flies, ants, wasps, and bees
(Hymenoptera), 212.

CLASS MYRIAPODA: THE CENTIPEDS AND MILLIPEDS.

CLASS ARACHNIDA: THE SCORPIONS, SPIDERS, MITES, AND TICS.

                 XXII.--BRANCH MOLLUSCA: THE MOLLUSCS.

EXAMPLE: THE FRESH-WATER MUSSEL. (UNIO SP.) [Laboratory exercise].
Structure, 239.--Life-history and habits, 243.

OTHER MOLLUSCS.

Body form and structure, 245.--Development, 246.--Classification,
246.--Clams, scallops, and oysters (Pelecypoda), 246.--Snails, slugs,
nudibranchs, and "sea-shells" (Gastropoda), 252.--Squids,
cuttlefishes, and octopi (Cephalopoda), 255.

          XXIII.--BRANCH CHORDATA: THE ASCIDIANS, VERTEBRATES,
                                  ETC.

Structure of the vertebrates, 259.--Classification of the Chordata,
260.--The ascidians, 261.

                  XXIV.--BRANCH CHORDATA (CONTINUED).

CLASS PISCES: THE FISHES.

EXAMPLE: THE GOLDEN SUNFISH (EUPOMOTIS GIBBOSUS) [Laboratory
exercise]. External structure, 263.--Internal structure,
265.--Life-history and habits, 270.

OTHER FISHES.

Body form and structure, 271.--Development and life-history,
276.--Classification, 277.--The lancelets (Leptocardii), 277.--The
lampreys and hag-fishes (Cyclostomata), 278.--The true fishes
(Pisces), 279.--The sharks, skates, etc. (Elasmobranchii), 279.--The
bony fishes (Teleostomi), 281.--Habits and adaptations,
285.--Food-fishes and fish-hatcheries, 288.

                   XXV.--BRANCH CHORDATA (CONTINUED).

CLASS BATRACHIA: THE BATRACHIANS.

Body form and organization, 292.--Structure, 293.--Life-history and
habits, 295.--Classification, 297.--Mud-puppies, salamanders, etc.
(Urodela), 297.--Frogs and toads (Anura), 299.--Cœcilians
(Gymnophiona), 302.

                  XXVI.--BRANCH CHORDATA (CONTINUED).

CLASS REPTILIA: THE SNAKES, LIZARDS, TURTLES, CROCODILES, ETC.

EXAMPLE: THE GARTER SNAKE (THAMNOPHIS SP.) [Laboratory exercise].
Structure, 303.--Life-history and habits, 308.

OTHER REPTILES.

Body form and organization, 310.--Structure, 311.--Life-history and
habits, 312.--Classification, 313.--Tortoises and turtles (Chelonia),
314.--Snakes and lizards (Squamata), 317.--Crocodiles and alligators
(Crocodilia), 325.

                  XXVII.--BRANCH CHORDATA (CONTINUED).

CLASS AVES: THE BIRDS.

EXAMPLE: THE ENGLISH SPARROW (PASSER DOMESTICUS) [Laboratory
exercise]. External structure, 327.--Internal structure [Laboratory
exercise], 329.--Life history and habits, 335.

OTHER BIRDS.

Body form and structure, 336.--Development and life-history,
339.--Classification, 340.--The ostriches, cassowaries, etc. (Ratitæ),
341.--The loons, grebes, auks, etc. (Pygopodes), 343.--The gulls,
terns, petrels, and albatrosses (Longipennes), 345.--The cormorants,
pelicans, etc. (Steganopodes), 346.--The ducks, geese, and swans
(Anseres), 347.--The ibises, herons, and bitterns (Herodiones),
347.--The cranes, rails, and coots (Paludicolæ), 348.--The snipes,
sand-pipers, plovers, etc. (Limicolæ), 349.--The grouse, quail,
pheasants, turkeys, etc. (Gallinæ), 358.--The doves and pigeons
(Columbæ), 351.--The eagles, hawks, owls, and vultures (Raptores),
351.--The parrots (Psittaci), 353.--The cookoos and kingfishers
(Coccyges), 354.--The woodpeckers (Pici), 354.--The whippoorwills,
chimney-swifts, and humming-birds (Macrochires), 356.--The perchers
(Passeres), 357.--Determining and studying the birds of a locality,
359.--Bills and feet, 362.--Flight and songs, 364.--Nestling and care
of the young, 366.--Local distribution and migration, 367.--Feeding
habits, economics, and protection of birds, 370.

                 XXVIII.--BRANCH CHORDATA (CONTINUED).

CLASS MAMMALIA: THE MAMMALS.

EXAMPLE: THE MOUSE (MUS MUSCULUS) [Laboratory exercise]. Structure,
373.--Life-history and habits, 379.

OTHER MAMMALS.

Body form and structure, 381.--Development and life-history,
387.--Habits, instincts, and reason, 387.--Classification, 388.--The
opossums (Marsupialia), 389.--The rodents or gnawers (Glires),
390.--The shrews and moles (Insectivora), 391.--The bats (Chiroptera),
391.--The dolphins, porpoises, and whales (Cete), 393.--The hoofed
mammals (Ungulata), 394.--The carnivores (Feræ), 396.--The man-like
mammals (Primates), 398.

                                PART III

                             ANIMAL ECOLOGY

             XXIX.--THE STRUGGLE FOR EXISTENCE, ADAPTATION,
                          AND SPECIES-FORMING.

The multiplication and crowding of animals, 404.--The struggle for
existence, 406.--Variation and natural selection, 406.--Adaptation and
adjustment to surroundings, 407.--Species forming, 408.--Artificial
selection, 409.

           XXX.--SOCIAL AND COMMUNAL LIFE, COMMENSALISM, AND
                              PARASITISM.

Social life and gregariousness, 410.--Communal life,
411.--Commensalism, 413.--Parasitism, 415.

               XXXI.--COLOR AND PROTECTIVE RESEMBLANCES.

Use of color, 424.--General, variable, and special protective
resemblance, 426.--Warning colors, terrifying appearances, and
mimicry, 430.--Alluring coloration, 433.

                  XXXII.--THE DISTRIBUTION OF ANIMALS.

Geographical distribution, 435.--Laws of distribution, 437.--Modes of
migration and distribution, 437.--Barriers to distribution,
438.--Faunæ and zoogeographic areas, 440.--Habitat and species,
441.--Species-extinguishing and species-forming, 442.

                               APPENDICES

                         EQUIPMENT AND METHODS

              APPENDIX I.--EQUIPMENT AND NOTES OF PUPILS.

Equipment of pupils, 447.--Laboratory drawings and notes, 447.--Field
observations and notes, 448.

            APPENDIX II.--LABORATORY EQUIPMENT AND METHODS.

Equipment of laboratory, 450.--Collecting and preparing material for
use in the laboratory, 451.--Obtaining marine animals, microscopic
preparations, etc., 453.--Reference-books, 454.

         APPENDIX III.--REARING ANIMALS AND MAKING COLLECTIONS.

Live cages and aquaria, 457.--Making collections, 461.--Collecting and
preserving insects, 463.--Collecting and preserving birds,
466.--Collecting and preserving mammals, 470.--Collecting and
preserving other animals, 472.

                                 PART I

                 STRUCTURE, FUNCTIONS, AND DEVELOPMENT
                               OF ANIMALS

                               CHAPTER I

                  THE STUDY OF ANIMALS AND THEIR LIFE

=Our familiar knowledge of animals and their life.=--We are familiarly
acquainted with dogs and cats; less familiarly probably with toads and
crayfishes, and we have little more than a bare knowledge of the
existence of such animals as seals and starfishes and reindeer. But
what real knowledge of dogs and toads does our familiar
acquaintanceship with them give? Certain habits of the dog are known
to us: it eats, and eats certain kinds of food; it runs about; it
responds to our calls or even to the mere sight of us; it evidently
feels pain when struck, and shows fear when threatened. Another class
of attributes of the dog includes those things that we know of its
bodily make-up: its possession of a head with eyes and ears, nose and
mouth; its four legs with toes and claws; its covering of hair. We
know, too, that it was born alive as a very small helpless puppy which
lived for a while on food furnished by the mother, and that it has
grown and developed from this young state to a fully grown, fully
developed dog. We know also that our dog is a certain kind of dog, a
spaniel, perhaps, while our neighbor's dog is of another kind, a
greyhound, it may be. We know accordingly that there are different
kinds of tame dogs, and we may know that wolves are so much like dogs
that they might indeed be called wild dogs, or dogs called a kind of
tame wolf. But how little we really know about the dog's body and its
life is apparent at a moment's thought. We see only the outside of the
dog, but what an intricate complex of parts really composes this
animal! We see it eat and breathe and run; of what is done with the
food and air inside its body, and of the series of muscle contractions
and mechanical processes which cause its running, we have but the
slightest conception. We see that the pup gets larger, that is, grows;
that it changes gradually in appearance, that is, develops; but of the
real processes and changes that take place in growth and development
how little we know! We know that there are other kinds of dogs; that
wolves and foxes are relatives of the dog; and we have heard that cats
and tigers are relatives also, although more distant ones. We know,
too, that all the backboned animals, some of them very unlike dogs,
are believed to be related to each other, but of the thousands of
these animals and of their relationships our knowledge is scanty.
Finally, of the relations of the dog, and of other animals, to the
outside world, and of the wonderful manner in which the dog's make-up
and behavior fit it to live in its place in the world under the
conditions that surround it, we have probably least knowledge of all.

=Zoology and its divisions.=--What things we do know about the dog,
however, and about its relatives, and what things others know, can be
classified into several groups, namely, things or facts about what the
dog does, or its behavior, things about the make-up of its body,
things about its growth and development, things about the kind of dog
it is and the kinds of relatives it has, and things about its
relations to the outer world, and its special fitness for life.

All that is known of these different kinds of facts about the dog
constitutes our knowledge of the dog and its life. All that is known
by scientific men and others of these different kinds of facts about
all the 500,000 or more kinds of living animals, constitutes our
knowledge of animals and is the science _zoology_.[3] Names have been
given to these different groups of facts about animals. The facts
about the bodily make-up or structure of animals constitute that part
of zoology called animal _anatomy_ or _morphology_; the facts about
the things animals do, or the functions of animals, compose animal
_physiology_; the facts about the development of animals from young to
adult condition are the facts of animal _development_; the knowledge
of the different kinds of animals and their relationships to each
other is called _systematic_ zoology or animal _classification_; and
finally the knowledge of the relations of animals to their external
surroundings, including the inorganic world, plants and other animals,
is called animal _ecology_.

Any study of animals and their life, that is, of zoology, may include
all or any of these parts of zoology. Most zoologists do, indeed,
devote their principal attention to some one group of facts about
animals and are accordingly spoken of as anatomists, or physiologists,
systematists, and so on. But such a specialization of study should be
made only after the zoologist has acquired a knowledge of the
principal or fundamental facts in all the other branches of zoology.

=A first course in zoology.=--The first "course," then, in the study
of animals should include the fundamental facts in all these branches
or parts of zoology. That is what the course outlined in this book
tries to cover. But no text-book of zoology can really give the
student the knowledge he seeks. He must find out most of it for
himself; a text-book, based on the experiences of others, is chiefly
valuable for telling him how to work most effectively to get this
knowledge for himself. And the best students always find out things
which are not in books. Especially can the beginning student find out
things not known before, "new to science," as we say, about the
behavior and habits of animals, and their relations to their
surroundings. The life-history of comparatively few kinds of animals
is exactly known; the instincts and habits of comparatively few have
been studied in any detail. The kinds of food demanded, the feeding
habits, nest-building, care of the young, cunning concealment of nest
and self, time of egg-laying or of producing young, duration of the
immature stages and the habits and behavior of the young animals--a
host, indeed, of observations on the actual life of animals, remain to
be made by the "field naturalist." Any beginning student can be a
"field naturalist" and can find out new things about animals, that is,
can add to the science of zoology.

[Illustration: FIG. 1.--Dissection of the Garden Toad (_Bufo
lentiginosus_).]

FOOTNOTE:

[3] Zoology is formed from two Greek words: _zoon_, meaning animal,
and _logos_, meaning discourse.

                               CHAPTER II

                 THE GARDEN TOAD (_Bufo lentiginosus_)

                          LABORATORY EXERCISE

    TECHNICAL NOTE.--Although this description is written for the toad
    it will fit for the dissection of the frog. It will be found,
    after casting aside a few ungrounded prejudices, that the toad is
    the better for class dissection. Toads are best collected about
    dusk, when they can be picked up in almost any garden in town or
    in the country. During the spring many can be found in the ponds
    where they are breeding. To kill the toad place it in an air-tight
    vessel with a piece of cotton or cloth saturated in chloroform or
    ether. When the toad is dead, wash off the specimen and put in a
    dissecting pan for study. Several specimens should be placed in a
    nitric acid solution for a day or so (for directions for
    preparing, see p. 12) to be used later for the study of the
    nervous system. Also several specimens should be injected for the
    better study of the circulatory system. With an injecting mass
    made as directed on p. 451 introduce through a small canula into
    the ventricle of the heart. This will inject the arterial system,
    and with increased pressure the injecting mass may be forced
    through the valves of the heart, thus passing into the auricles
    and throughout the venous system. After injecting use the specimen
    fresh or after it has been preserved in 4% formalin.

=External structure.=--Note that the body of the toad is divided into
several principal regions or parts, as is the human body, namely, a
_head_, _upper limbs_, _trunk_, and _lower limbs_. As you look at the
toad note the similarity of the parts on one side to those of the other,
as right leg corresponding to left leg, right eye to left eye, etc. This
arrangement of the body in similar halves among animals is known as
_bilateral symmetry_. As a rule animals which show bilateral symmetry
move in a definite direction. The part that moves forward is the
_anterior end_, while the opposite extremity is the _posterior end_. In
most animals we note two other views or aspects; that which is called
the "back" and with most animals is, under ordinary conditions,
uppermost is the _dorsum_ or _dorsal aspect_, while that which lies
below is the _venter_ or _ventral aspect_. When referring to a view from
one side we speak of it as a right or left _lateral aspect_. These terms
hold good for most of the animals that we shall study.

Note at the anterior end of the toad a wide transverse slit, the
_mouth_. What other openings are on the anterior end? Note the two large
_eyes_, the organs of sight. Just back of each eye note an elliptical,
smooth membrane. This is the tympanum of the outer _ear_, and through
this membrane the vibrations produced by sound-waves are transferred to
the inner ear, which receives sensations and transmits them to the
brain. Open the mouth by drawing down the lower jaw. Note just within
the angle of the lower jaw the _tongue_. How is it attached to the wall
of the mouth? On the tongue are a great many fine _papillæ_ in which is
located the sense of taste. It has now been seen that most of the
special senses of the toad have their seat in the head. Pass a straw or
bristle into one of the nostrils. Where does it come out? These internal
openings to the nose are the _inner nares_. Note in the roof of the
mouth just posterior to each of the eyeballs an opening. These are the
internal openings to the wide _Eustachian tubes_, which lead to the
mouth from the chamber of the ear behind the tympanum.

Note far back in the mouth an opening through which food passes. This
is the _œsophagus_ or _gullet_. Note just below this gullet an
elevation in which is a perpendicular slit, the _glottis_. This is the
upper end of the _laryngo-tracheal chamber_, and the flaps within on
either side of the slit are the _vocal cords_.

Note at the posterior end of the body in the median line an opening.
This is the _anal opening_ or _anus_. Note the general make-up of the
toad. How do its arms compare with our own? How do its fore feet
(hands) differ from its hind feet? Note that the body is covered by a
tough enveloping membrane, the _skin_. In the skin are many glands
which by their excretion keep it soft and moist.

    =Internal structure.=--TECHNICAL NOTE.--With a fine pair of
    scissors make a longitudinal median cut through the skin of the
    venter from the anal opening to the angle of the lower jaw. Spread
    the cut edges apart and pin back in the dissecting-pan.

Note the complex system of _muscles_ which govern the movements of the
tongue. Observe a number of pairs of muscles overlying the bones which
support the arms. These are attached to the _pectoral_ or
_shoulder-girdle_. Note the large sheet of muscles covering the
ventral aspect of the toad. These are the _abdominal muscles_, which
consist of two sets, an outer and an inner layer. Note that
posteriorly the abdominal muscles are attached to a bone. This is the
_pubic bone_ of the _pelvic girdle_ which supports the hind legs.

    TECHNICAL NOTE.--With the scissors cut through the muscles of the
    body wall at the pubic bone and pass the points forward to the
    shoulder-girdle. Separate the bones of the shoulder-girdle and pin
    out the flaps of skin and muscle to right and left in the
    dissecting-pan (see fig. 1). Cover the dissection with clear water
    or weak alcohol.

Note two large conspicuous soft brown lobes of tissue. These form the
_liver_, an organ which produces a secretion that assists in the process
of digestion. Note just anterior to the liver and extending between its
two lobes a pear-shaped organ, the _heart_, which may yet be pulsating.
Are these pulsations regular? How many occur in a minute? The lower end
or apex of the heart, _ventricle_, undergoes a contraction, forcing
blood out into the _blood-vessels_. This is followed by a relaxation of
the apex and a contraction of the basal portion, the _auricle_. The
heart is surrounded by a delicate semi-transparent sac, the
_pericardium_. The pericardium is filled with a watery fluid,
_body-lymph_, which bathes the heart. Note between the lobes of the
liver a small bladder-shaped transparent organ of a pinkish color. This
is the _gall-cyst_, or _gall-bladder_, a reservoir for the _bile_, the
secretion from the liver. Separate the lobes of the liver and note,
beneath, the long convoluted tube which fills most of the body-cavity.
This is part of the _alimentary canal_. Is the alimentary canal of
uniform character? The most anterior portion of the canal, the _gullet_
or _œsophagus_, leads to a large U-shaped enlargement, the _stomach_.
From the lower end of the stomach there extends a long, slender, very
much convoluted tube, the _small intestine_, which is followed by a much
larger one, the _large intestine_. This large intestine after one or two
turns passes directly back into the _rectum_, which opens at last to the
exterior through the anus. Note just ventral to the rectum a large
thin-walled membranous sac. This is the _urinary bladder_ which acts as
a reservoir for the secretion from the _kidneys_. Notice a many-branched
yellow structure with a glistening appearance, the _fat-body_ (_corpus
adiposum_). Now push liver and intestine to one side and note the
pinkish sac-like bodies (perhaps filled with air), the _lungs_. The
lungs are paired bodies which open into the laryngo-tracheal chamber.
The toad takes air into its mouth through its nostrils, and then forces
it, by a kind of swallowing action, through the laryngo-tracheal chamber
into the lungs.

Now lift the stomach and note in the loop between its lower end and the
small intestine a thin transparent tissue. This is a part of the
_mesentery_, which will be found to suspend the whole alimentary canal
and its attached organs to the dorsal wall of the body. Note in the
loop of the stomach in the mesentery an irregular pinkish glandular
structure which leads by a small duct into the intestine. This gland is
the _pancreas_, and the duct is the _pancreatic duct_. From it comes a
secretion which aids in the digestion of food. Near the upper end of the
pancreas note a round nodular structure, generally dark red. This is the
_spleen_, a ductless gland, the use of which is not altogether known.

Make a drawing which will show as many of the organs noted as possible.

    TECHNICAL NOTE.--Pass two pieces of thread under the rectum near
    the pubic bone. Tie these threads tightly a short distance apart
    and then cut the rectum in two between the threads. Now carefully
    lift up the alimentary canal with attached organs (liver, etc.),
    and cut it off near the region of the heart.

How is the heart situated with regard to the lungs? The _heart_
consists of a lower chamber with thick muscular walls, the tip, called
the _ventricle_, and two upper thin-walled chambers, the _right_ and
_left auricles_. Can you make out these three chambers? The purified
blood from the lungs flows into the left auricle, while the venous
blood from all over the body laden with its carbon dioxide enters the
right auricle. From these two chambers the blood enters the ventricle.
Here the pure and impure blood are mixed. From the ventricle the blood
enters a large muscular tube on the ventral side of the heart. This is
the _conus arteriosus_, which gives off three branches on each side;
the anterior ones, the _carotid arteries_, supply the head, the next
ones, the _systemic arteries_, or _aortæ_, carry blood to the rest of
the body, while the posterior vessels, the _pulmonary arteries_, go
directly to the lungs and there break up into fine vessels
(_capillaries_) where the carbon dioxide is given off and oxygen is
taken from the air. From the lungs the blood returns through the
_pulmonary vein_ to the left auricle. Meanwhile the blood which has
passed through the systemic arteries and body capillaries is collected
again into other vessels going back to the heart; these are the
_veins_, which empty into a large thin-walled reservoir, the _sinus
venosus_, which in turn connects with the right auricle of the heart.
Three large veins enter the sinus venosus, namely, two _pre-caval
veins_ at the anterior end, and a single _post-caval vein_ at the
posterior end. Trace out the larger arteries and veins from the heart
to their division into or origin from the smaller vessels.

    TECHNICAL NOTE.--Carefully remove the heart together with the lungs.
    The lungs may be inflated by blowing into them through the
    laryngo-tracheal chamber with a quill and tying them tightly, after
    which they should be left for several days to dry. When perfectly
    dry, sections may be cut through them in various places with a sharp
    knife, and by this means a very good idea of the simple lung
    structure of the lower backboned animals can be obtained. With a
    sharp knife cut the heart open, beginning at the tip (ventricle) and
    cutting up through the conus arteriosus and the two auricles. Note
    the valves in the heart which separate the different compartments.

Note on either side of the median line in the dorsal region a pair of
reddish glandular bodies (_the kidneys_). From each kidney trace a
tube (_ureter_) posteriorly toward the region of the anus. The kidneys
are the principal excretory organs of the body. The blood which flows
through the delicate blood-vessels in the kidney gives up there much
of its waste products. These pass out through small tubules of the
kidneys into the ureters, which carry the wastes toward the anus.
Along one side of each kidney may be seen a yellowish glistening mass,
the _adrenal body_.

In some of the specimens studied, the body cavity may be filled with
thousands of little black spherical bodies. These are undeveloped
_eggs_. They are deposited by the mother toad in the water in long
strings of transparent jelly, which are usually wound around sticks or
plant-stems at the bottom of the pond near the shore. From these eggs
the young toads hatch as tadpoles and in their life-history pass
through an interesting metamorphosis. (See Chapter XII.)

    TECHNICAL NOTE.--The teacher should be provided with several
    well-cleaned skeletons of the toad in order that the bones may be
    carefully studied. Boil in a soap solution a toad from which most
    of the muscles and skin have been removed (see p. 452). Leave in
    this solution until the muscles are quite soft and then pick off
    all bits of muscles and tissue from the bones. If this is
    carefully done, the ligaments which bind the bones will be left
    intact and the skeleton will hold together.

Note that the _skeleton_ (fig. 2) consists of a head portion which is
composed of many bones joined together to form a bony box, the
_skull_; of a series of small segments, the _vertebræ_, forming the
_vertebral column_, which with the skull forms the _axial skeleton_;
and of the _appendicular skeleton_, consisting of the bones of the
fore and hind limbs. Note that the skull is composed of many bones
joined together, some by _sutures_, while others are fused. Do the
limbs attach directly to the axial skeleton? The anterior limbs (arms)
articulate with the _pectoral_ or _shoulder-girdle_. The arms will be
seen to be made up of a number of bones placed end to end. Note that
the uppermost, the _humerus_, is attached to the pectoral girdle,
while at its lower end it articulates with the _radio-ulna_. At the
lower end of the radio-ulna is a small series of _carpal_ bones which
afford attachments for the slender finger-bones, the _phalanges_ or
_digital_ bones. The bones of the leg are articulated with a closely
fused set of bones, the _pelvic girdle_. The leg-bones, proceeding
from the pelvic girdle, are named _femur_, _tibio-fibula_, _tarsal_
bones, and _phalanges_ or _digits_. To what bones of the arm do these
correspond? Determine the other principal bones of the skeleton by
reference to figure 2.

[Illustration: FIG. 2.--Skeleton of the garden toad.]

    TECHNICAL NOTE.--In a specimen which has been macerated for some
    time in 20% nitric acid dissect out the nervous system. Place the
    specimen in a pan ventral side uppermost and pin out. Carefully pick
    away the vertebræ and the roof of the mouth-cavity, thereby exposing
    the central nervous system, which will appear light yellow.

Examine the _brain_. In front of the true brain are the _olfactory
lobes_, the nervous centre for the sense of smell. The brain itself is
composed of several parts. The anterior portion consists of two
elongated parts, the _cerebral hemispheres_; just back of these are the
optic lobes or _midbrain_, consisting of two short lobes, which are
followed by the small _cerebellum_, which in turn is followed by a long
part, the _medulla oblongata_, which runs imperceptibly into the long
dorsal nerve, the _spinal cord_. Note the large _optic nerves_ running
out to each eye. How far backward does the spinal cord extend? Note the
many pairs of nerves given off from the brain and spinal cord. These
nerves branch and subdivide until they end in very fine fibres. Some end
in the muscle-fibres, and through them the central nervous system
innervates the muscles. These are _motor endings_. Still others pass to
the surface and receive impressions from the outside. These last are
_sensory endings_. Note that the _spinal nerves_ arise from the spinal
cord by two roots, an _anterior_ or _ventral_, and a _posterior_ or
_dorsal root_. Trace the principal spinal nerves to the body-parts
innervated by them. These nerves are numbered as first, second, etc.,
according to the number of the vertebræ (counting from the head
backward) from behind which they arise.

                   THE STRUCTURE AND FUNCTIONS OF THE
                              ANIMAL BODY

=Organs and functions.=--The body of the toad is composed of various
parts, such as the lungs, the heart, the muscles, the eyes, the
stomach, and others. The life of the toad consists of the performance
by it of various processes, such as breathing, digesting food,
circulating blood, moving, seeing, and others. These various processes
are performed by the various parts of the body. The parts of the body
are called _organs_, and the processes (or work) they perform are
called their _functions_. The lungs are the principal organs for the
function of breathing; the heart, arteries and veins are the organs
which have for their function the circulation of the blood; the
principal organ concerned in the digestion of food is the alimentary
canal, the function of seeing is performed by the organs of sight, the
eyes, and so one might continue the catalogue of all the organs of the
body and of all the functions performed by the animal.

=The animal body a machine.=--The whole body of the toad is a machine
composed of various parts, each part with its special work or business
to do, but all depending on one another and all co-operating to
accomplish the total work of living. The locomotive engine is a
machine similarly composed of various parts, each part with its
special work or function, and all the parts depending on one another
and so working together as to perform satisfactorily the work for
which the locomotive engine is intended. An important difference
between the locomotive engine and the toad's body is that one is a
lifeless machine and the other a living machine. But there is a real
similarity between the two in that both are composed of special parts,
each part performing a special kind of work or function, and all the
parts and functions so fitted together as to form a complex machine
which successfully accomplishes the work for which it is intended. And
this similarity is one which should help make plain the fundamental
fact of animal structure and physiology, namely, the division of the
body into numerous parts or organs, and the division of the total work
of living into various processes which are the special work or
functions of the various organs.

=The essential functions or life-processes.=--The toad has a great
many different special parts in its body. Its body is very complex. It
performs a great many different functions, that is, does a great many
different things in its living. And the structure and life of most of
the other animals with which we are familiar are similarly complex: a
fish, or a rabbit, or a bird has a body composed of many different
parts, and is capable of doing many different things. Are all animals
similarly complex in structure, and capable of doing such a great
variety of things? We shall find that the answer to this question is
No. There are many animals in which the body is composed of but a few
parts, and whose life includes the performance of fewer functions or
processes than in the case of the toad. There are many animals which
have no eyes nor ears nor other organs of special sense. There are
animals without legs or other special organs of locomotion; some
animals have no blood and hence no heart nor arteries and veins. But
in the life of every animal there are certain processes which must be
performed, and the body must be so arranged or composed as to be
capable of performing these necessary life-processes. All animals take
food, digest it, and assimilate it, that is, convert it into new body
substance; all animals take in oxygen and give off carbonic acid gas;
all animals have the power of movement or motion (not necessarily
locomotion); all animals have the power of sensation, that is, can
feel; all animals can reproduce themselves, that is, produce young.
These are the necessary life-processes. It is evident that the toad
could still live if it had no eyes. Seeing is not one of the necessary
functions or processes of life. Nor is hearing, nor is leaping, nor
are many of the things which the toad can do; and animals can exist,
and do exist, without any of those organs which enable the toad to see
and hear and leap. But the body of any animal must be capable of
performing the few essential processes which are necessary to animal
life. How surprisingly simple such a body can be will be later
discovered. But in most animals the body is a complicated object, and
is able to do many things which are accessory to the really essential
life-processes, and which make its life complex and elaborate.

                               CHAPTER IV

                     THE CRAYFISH (_Cambarus_ sp.)

                          LABORATORY EXERCISE

    TECHNICAL NOTE.--The crayfish, or crawfish, is found in most of
    the fresh-water ponds and streams of the United States. (It is not
    found east of the Hoosatonic River, Mass. In this region the
    lobster may be used. On the Pacific coast the crayfishes belong to
    the genus _Astacus_.) Crayfishes may be taken by a net baited with
    dead fish, or they may be caught in a trap made from a box with
    ends which open in, and baited with dead fish or animal refuse of
    any sort. This box should be placed in a pond or stream frequented
    by crayfish. If possible the student should study the living
    animal and observe its habits. Crayfish which are to be kept alive
    should be placed in a moist chamber in a cool place. They will
    keep for a longer time in a moist chamber than in water. Some
    fresh specimens should be injected by the teacher for the study of
    the circulatory system. A watery solution of coloring matter or,
    better, of an injecting mass of gelatine (see p. 451) is injected
    into the heart through the needle of a hypodermic syringe. For the
    purpose of injecting, a small bit of the shell may be removed from
    the cephalothorax above the heart. Specimens which are to be kept
    for some time should be placed in alcohol or 4% formalin.

=External structure= (fig. 3).--Place a specimen in a pan for study.
Note that the body, which of course differs much in shape from that of
the toad, is also unlike that of the toad in being covered by a hard
calcareous _exoskeleton_, which acts as a covering for the soft parts
and also as a place of attachment for the muscles, just as the
internal skeleton does in the case of the toad. The body is composed
of an anterior part, the _cephalothorax_, and a posterior part, the
_abdomen_. The cephalothorax is covered above and on the sides by the
_carapace_, which is divided into parts corresponding to the head and
thorax of the toad by the transverse _cervical suture_. The abdomen
is composed of segments. How many? The flattened terminal segment is
called the _telson_. Is the cephalothorax composed of segments? Where
is the mouth of the crayfish? Where is the anal opening?

[Illustration: FIG. 3.--Ventral aspect of crayfish (_Cambarus_ sp.),
with the appendages of one side disarticulated.]

At the anterior end of the cephalothorax note a sharp projection, the
_rostrum_. Where are the eyes? Remove one of them and examine its
outer surface with a microscope. A bit of the outer wall should be
torn off and mounted on a glass slide. Note that it is made up of a
great many little facets placed side by side. Each of these facets is
the external window of an eye element or _ommatidium_. An eye composed
in this way is called a _compound eye_. In front of the eyes note two
pairs of slender many-segmented appendages. The shorter pair, the
_antennules_, are two-branched. Remove one of them and note at its
base a small slit along the upper surface. This slit opens into a
small bag-like structure which contains fine sand-grains. The bag is
protected by a series of fine bristles along the edge of the slit.
This bag-like structure is believed to be an auditory organ. The
longer pair of appendages are the _antennæ_, and in the fine hair-like
projections upon the joints is believed to be located the sense of
smell. Thus it will be seen that the sense-organs of the crayfish,
like those of the toad, are located on the head. Beneath the basal
portion of each antenna there is a flat plate-like projection, at the
base of which on the upper edge will be noted a small opening, the
exit of the kidney, or _green gland_.

Make a drawing of the surface of part of an eye; also of an antennule;
and of an antenna.

    TECHNICAL NOTE.--Stick one point of the scissors under the
    posterior end of the carapace on the right side, and cut forward,
    thus exposing a large cavity, the gill-chamber. Remove all of the
    mouth-parts, legs and abdominal appendages from the right side,
    being careful to leave the fringe-like parts, the gills, attached
    to their respective legs. Place all of the appendages in order on
    a piece of cardboard.

Examine the abdominal appendages, called _pleopods_, or swimming feet.
How many pairs are there? Each is composed of a basal part, the
_protopodite_, and two terminal segments, an inner one, the
_endopodite_, and an outer, the _exopodite_. In the males the first
and second pleopods of the abdomen are larger and less flexible than
the others. In the female the pleopods serve to carry the eggs and the
first two pairs are very small or absent. Note the last set of
abdominal appendages. These are the _uropods_, which together with the
telson form the tail.

Make a drawing of the pleopods of one side.

Examine the appendages of the cephalothorax. Like the appendages of
the abdomen the typical composition of each includes a protopodite, an
exopodite and an endopodite, but some of these appendages are much
modified, and show a loss of one of these parts, or the addition of an
extra part. The cephalothoracic appendages may be divided into three
groups, an anterior group of three pairs of mouth-parts (belonging to
the head) of which the first pair is the _mandibles_ and the others
are the _maxillæ_; a second group of three pairs of foot-jaws or
_maxillipeds_, belonging to the thorax, and a third group of five
pairs of _walking-legs_. The mandibles, lying next to the
mouth-opening, are hard and jaw-like and lack the exopodite; the first
maxillæ are small and also lack the exopodite; the second maxillæ have
a large paddle-like structure which extends back over the gills on
each side within the space, the _branchial chamber_, above the gills.
It is by means of this paddle-like structure (the _scaphognathite_)
that currents of water are kept up through the gill-chambers. The
maxillipeds increase in size from first to third pair. Each pair of
walking-legs except the last bears _gills_. These gills are the organs
by which the blood is purified. The blood of the crayfish flows into
the large vessels on the outer sides of the gill and thence into the
fine vessels in the little leaf-like lamellæ. At the same time the air
which is mixed with the water bathing the gills passes freely through
the thin membranous walls of these lamellæ and blood-vessels, and the
blood gives off its carbonic acid gas to the water and takes up oxygen
from the air in the water. Thus it will be seen that the office of the
gill is like that of the lung in the toad, namely, to act as an organ
for the elimination of carbonic acid gas and the taking up of oxygen.

Note the pincer-like appendages of the first pair of legs. These pincers
are the _chelæ_, with which food is torn into bits and placed in the
mouth. In the basal segment of each of the last pair of legs of the male
note the _genital pore_. In the female the _genital pores_ are in the
basal segments of the next to last pair of legs. Is the crayfish
bilaterally symmetrical? Note the repetition of parts in the crayfish,
that is, the recurrence of similar parts in successive segments. This
serial repetition of parts among animals is called _metemerism_.

    =Internal structure= (fig. 4).--TECHNICAL NOTE.--With a pair of
    scissors cut through the dorsal wall of the cephalothorax into the
    body-cavity. Cut the body-wall away from both sides and remove the
    middle portion.

[Illustration: FIG. 4.--Diagrammatic median longitudinal section of
crayfish (_Cambarus_ sp.).]

At the anterior end of the cephalothorax note the large membranous
sac, the _stomach_. Attached to each end of this are sets of muscles
which control its movements. To the right and left of the stomach
notice attached to the shell large muscles which connect by stout
ligaments at their lower ends with the mandibles. Note a yellow
fringe-like structure, the _digestive gland_, which fills most of the
region about the stomach. It connects by a pair of small tubes, the
_bile-ducts_, with the alimentary canal. Within the posterior portion
of the cephalothorax note a pentagonal sac, the _heart_, contained
within a delicate membrane, the _pericardium_. Remove the pericardium
and note a pair of dorsal openings into the heart, called _ostia_.
(There are also two lateral pairs and a ventral pair of ostia.) Note
passing anteriorly from the heart along the median line to the eyes a
blood-vessel, the _ophthalmic artery_. Arising from the anterior
portion of the heart are the _antennary arteries_, running to the
antennæ. Yet another pair running anteriorly from the heart to the
stomach and digestive glands are called the _hepatic arteries_. From
the posterior end of the heart arises the _dorsal abdominal artery_,
running back to the telson. Below this arises the _sternal_ artery,
which will be seen later.

In the region below the heart are located the reproductive organs.
They are whitish glandular masses from each of which runs a tube which
opens at the base of the last pair of walking-legs in the male, and at
the base of the third pair of walking-legs in the female.

    TECHNICAL NOTE.--Cut longitudinally through the dorsal wall of the
    abdomen on either side of the median line and remove the piece of
    shell.

Note the powerful _muscles_ within which flex and extend the abdomen. By
a rapid contraction of these muscles the tail is brought beneath the
body, propelling the animal strongly backwards. When the crayfish crawls
it generally goes forward, but in swimming it reverses this direction.

Make a drawing showing, in their natural position, the internal organs
which have been studied.

Examine the alimentary canal for its whole length. Note that the large
bladder-shaped stomach is attached to the mouth-opening by a short
tube. What part of the canal is this? From the posterior end of the
stomach is a short thick-walled part, the _small intestine_, followed
by a long straight tube, the _large intestine_, which opens to the
exterior through the _anus_.

    TECHNICAL NOTE.--Remove the alimentary canal, detaching it from
    the anal end first, and working forward.

Cut the stomach open. Note an anterior portion, the _cardiac chamber_,
and a smaller posterior portion, the _pyloric chamber_. Examine its
inner surface. What do you find here? This structure is called the
_gastric mill_. Food, which for the most part consists of any dead
organic matter, is chewed by the "stomach-teeth" into fine bits, and
is then passed into the pyloric chamber. It is here that the digestive
glands empty their secretion into the food. These glands have the same
office as have the liver and pancreas combined in the toad, and so
they are often called the _hepato-pancreas_. When the stomach has been
removed there will be noted in the anterior portion of the body
paired, flattened bodies, already mentioned, which connect with
openings at the base of each of the antennæ by means of wide
thin-walled sacs, the _ureters_. These organs are the _kidneys_, or
_green glands_. Their office is similar to that of the kidneys in the
toad, namely, the elimination of waste from the body.

    TECHNICAL NOTE.--Carefully remove all of the alimentary canal,
    digestive glands, and reproductive organs. This process will
    expose the floor of the cephalothorax. Now cut away from either
    side the horny floor or bridge at the bottom of the cephalothorax.
    If the specimen has not already been immersed, place it in clear
    water for further dissection.

The foregoing dissection will expose the _central nervous system_. It
extends as a series of paired _ganglia_ connected by a double nerve-cord
along the ventral median line from the œsophagus to the last segment of
the abdomen. From what points do the lateral nerves arise? Anteriorly
the double nerve-cord divides, the two parts passing upward on each
side of the œsophagus, where they again meet to form the
_supra-œsophageal ganglion_ or _brain_. Where do the nerves run which
rise from the brain? What is the difference between the position of the
central nervous system in the crayfish and in the toad?

Make a drawing of the nervous system.

Just beneath the nerve-cord note a blood-vessel extending the length
of the body. This is the _sternal artery_, which arises from the
posterior end of the heart and passes ventrally at one side of the
alimentary canal and between the nerve-cords. Here the sternal artery
divides into an anterior and a posterior branch, from which lesser
branches are given off to each one of the appendages. The various
arteries running to all parts of the body finally pour out the blood
into the body-cavity, where it flows freely in the spaces among the
various tissues and organs. After the blood has bathed the body
tissues it flows to the gills on either side, passing up the outer
side of the gill through delicate thin-walled vessels, where it is
oxygenated as has already been described. From the gills the purified
blood flows back on the inner side through a large chamber, _sinus_,
into the pericardium, through the ostia of the heart, whence it is
driven into the arteries once more. This sort of a circulatory system
in which the blood in places is not enclosed in a definite vessel is
known as an _open system_. In the toad we find the blood in a _closed
system_, i.e., arteries leading into capillaries which in turn lead
into veins, in no case allowing the blood to pass freely through the
spaces of the body.

                               CHAPTER V

                THE MODIFICATION OF ORGANS AND FUNCTIONS

=Differences between crayfish and toad.=--In the dissection of the
crayfish one of the most important things in the study of zoology has
been learned. It is plain that the crayfish has a body composed, like
the toad's, of parts or organs, and that most of these organs,
although differing much in appearance and actual structure from those
of the toad, correspond to similarly named organs of the toad, and
perform the same functions or processes, although with many striking
differences, essentially in the same way as in the toad. But the
structure of the body is very different in the two animals. The toad
has an internal body skeleton to which the muscles are attached, and a
soft, yielding, outer body-covering or skin; the crayfish has no
internal skeleton, but has its body covered by a horny, firm body-wall
to which the muscles are attached. The toad has its main nervous chain
lying just beneath the dorsal wall of the body; the crayfish has its
main nervous chain lying just above the ventral wall of the body. The
toad has lungs and takes up oxygen from the air of the atmosphere; the
crayfish has gills and takes up oxygen from the air which is mixed
with the water. The toad has a single pair of jaws; the crayfish has
several pairs of mouth-parts. The toad has four legs fitted for
leaping; the crayfish has numerous legs fitted for crawling or
swimming. The crayfish's body is composed of a series of body-rings
or segments; the toad's body is a compact apparently unsegmented mass.
The toad has eyes each with a single large lens and capable of moving
in the head and of changing their shape and hence their focus; the
crayfish's eyes are immovable and have a fixed focus, and are composed
of hundreds of tiny eyes each with lens and special retina of its own.
And so a long list of differences might be gone through with.

=Resemblances between toad and crayfish.=--But on the other hand there
are many resemblances--resemblances both in structure and life-processes
or physiology. Both toad and crayfish have organs for the prehension of
food, its digestion and its assimilation. And these organs, the organs
of the digestive system, while differing in details are alike in being
composed principally of a long tube, the alimentary canal, running
through the body, open anteriorly for the taking in of food, and open
posteriorly for the discharge of indigestible useless matter. Both
alimentary canals are divided into various special regions for the
performance of the various special processes connected with the
digestion and assimilation of food. Each is adapted for the special kind
of food which it is the habit of the particular animal to take. The two
sets of organs are essentially alike and have the same essential
function or life-process to perform. But this process differs in the
details of its performance, and the organs which perform this function
and which constitute the digestive system of each are modified to suit
the special habits or kind of life of the animal.

Both toad and crayfish have a heart with blood-vessels leading from
it. In the case of the toad the heart is more complex than in the
crayfish, and the system of blood-vessels is far more extensive and
elaborate. But the heart and blood-vessels in both animals subserve
the same purpose; their function is the circulation of the blood,
this being the means by which oxygen and food are carried to all
growing or working parts of the body, and by which carbonic acid gas
and other poisonous waste products are brought away from these parts.
But this function differs somewhat in its performance in the two
animals, and the organs which perform the function are correspondingly
modified in structural condition.

Both toad and crayfish have organs for respiration, that is, for
breathing in oxygen and breathing out carbonic acid gas. But the toad
takes its oxygen from the atmosphere about it; its respiratory organs
are the lungs, the sac-like tube leading to the mouth, and the
external openings for the ingress and exit of the gases. The crayfish,
living mostly in the water, takes its oxygen from the air which is
mechanically mixed with the water. Its respiratory organs are its
gills. There is a great difference, apparently, in the structural
conditions of the organs of respiration in the two animals. As a
matter of fact the difference is less great than, at first sight,
appears to be the case. The lungs of the toad are composed primarily
of a thin membrane, in the form of a sac, richly supplied with
blood-vessels. Air is brought to this thin respiratory membrane and by
osmosis the oxygen passes through the membrane and through the thin
walls of the fine blood-vessels, and is taken up by the blood. At the
same time the carbonic acid gas brought by the blood to the lungs from
all parts of the body is given up by it and passes through the
membranes in order to leave the body. The air comes in contact with
the respiratory membrane (which is situated inside the body) by means
of a system of external openings and a conducting chamber, and by
these same openings and chamber the carbonic acid gas leaves the body.
In the crayfish the gills are nothing else than a large number of
small flattened sacs each composed of a thin membrane richly supplied
with blood-vessels. This respiratory membrane is not, in the crayfish,
situated inside the body, but on the outer surface, although protected
by being in a sort of pocket with a covering flap, and it comes into
immediate contact with the air held in the water which freely bathes
the gills. By osmosis the oxygen of this air passes in through the
gill-membranes, while the carbonic acid gas brought by the blood
passes out through them. Exactly the same exchange of gases is
accomplished as in the toad. But because of the great difference in
the conditions of life of the toad and crayfish, one living in water,
the other living out of water, the character of the performance of the
function of respiration, and correspondingly the structural condition
of the organs performing this function, are strikingly different.

=Modification of functions and structure to fit the animal to special
conditions of its life.=--As has been done with the organs of
digestion, circulation, and respiration, so we might compare the other
organs of the crayfish and the toad. There would be found not only
many very marked differences between organs which have the same
general function in the two animals, but we should find also numerous
organs in the toad which are not present at all in the crayfish, and
conversely; and this means, of course, that the toad can do numerous
things, perform numerous functions, which the crayfish cannot, and,
conversely, that the crayfish does some things which the toad cannot.
But both of these animals agree in possessing in common the capability
of performing those processes such as taking food, breathing,
reproducing, etc., to which attention has been called as being
indispensable to all animal life. These processes, however, are
performed by the two animals in different ways and the organs for the
performance of these processes, although at very bottom essentially
alike, are in outer and superficial details of position, appearance
and general structure markedly different. Animals are fitted to live
in different places amid different surroundings by having their bodies
modified and the performance of their life-processes modified to suit
the special conditions of their life.

=Vertebrate and invertebrate.=--In selecting the toad and the crayfish
as the first animals to study and to compare with each other, we have
chosen representatives of the two great groups into which the complexly
organized animals are divided, viz., the group of backboned or
vertebrate animals, and the group of backboneless or invertebrate
animals. To the vertebrates belong all those which have an internal bony
skeleton (and a few without such a skeleton) and which have also an
arrangement of body-organs on the general plan of the toad's body. A
conspicuous feature of this arrangement is the situation of the spinal
cord or main great nerve-trunk along the back or dorsal wall of the
animal, and inside of a backbone. All the fishes, batrachians (frogs,
toads, salamanders, etc.), reptiles (snakes, lizards, alligators, etc.),
birds, and mammals (quadrupeds, whales, seals, etc.) belong to the
vertebrates. The backboneless or invertebrate animals have no internal
bony skeleton and have their main nerve-trunk usually along the ventral
wall of the body, sometimes in a circle around the mouth, but never in a
backbone. To the invertebrates belong all insects, lobsters, crabs,
clams, squids, snails, worms, starfishes and sea-urchins, corals and
sponges, altogether a great host of animals, mostly small.

                               CHAPTER VI

                       AMŒBA AND PARAMŒCIUM

                          LABORATORY EXERCISE

    =Amœba.=--TECHNICAL NOTE.--_Amœbæ_ are found in stagnant pools of
    water on the dead leaves, sticks and slime at the bottom. To
    obtain them, collect slime and water from various puddles in
    separate bottles and take them to the laboratory. Place a small
    drop of slime on a slide under a cover-glass. Examine under the
    low power first and note any small transparent or opalescent
    objects in the field. Examine these objects with the higher power
    and note that some are mere granular jelly-like specks, which
    slowly (but constantly) change their form. These are _Amœbæ_.

    A teacher of zoology recommends the following method of obtaining
    a large supply of _Amœbæ_: "For rearing _Amœbæ_ place two or
    three inches of sand in a common tub, which is then filled with
    water and placed some feet from a north window; three or four
    opened mussels, with merest trace of the mud from the stream in
    which they are taken, are partially buried in the sand and a
    handful of _Nitella_ and a couple of crayfish cut in two are
    added; as decomposition goes on a very gentle stream is allowed
    to flow into the tub, and after from two to four weeks abundant
    _Amœbæ_ are to be found on the surface of the sand and in the
    scum on the sides of the tub; small _Amœbæ_ appear at first, and
    later the large ones."

Having found an _Amœba_ (fig. 5) note its irregular shape, and if it
moves actively observe its method of moving. How is this accomplished?
The viscous, jelly-like substance which composes the whole body of an
_Amœba_ is called _protoplasm_. The little processes which stick out
in various directions are the "false feet" (_pseudopodia_). Note that
the outer portion, the _ectosarc_, of the protoplasmic body is clear,
while the inner, the _endosarc_, is more or less granular in
structure. Has _Amœba_ a definite body-wall? Do the pseudopodia
protrude only from certain parts of the body? Within the endosarc
note a clear globular spot which contracts and expands, or pulsates,
more or less regularly. This is the _contractile vacuole_. Note the
small granules which move about within the endosarc. These are
food-particles which have been taken in through the body-wall. Note
how pseudopodia flow about food-particles in the water and how these
are digested by the protoplasm. If an _Amœba_ comes into contact with
a particle of sand, note how it at once retreats. Note within the
endosarc an oval transparent body which shows no pulsations. This is
the _nucleus_, a very complex little structure of great importance in
the make-up of _Amœba_.

[Illustration: FIG. 5.--_Amœba_ sp.; showing the forms assumed by a
single individual in four successive changes. (From life.)]

Note that _Amœba_ has no mouth or alimentary canal; no nostrils or
lungs, no heart or blood-vessels, no muscles, no glands. It is an
animal body not made up of distinct organs and diverse tissues. Its
whole body is a simple minute speck of protoplasm, a single animal
cell. But it takes in food, it moves, it excretes waste matter from
the body, is sensitive to the touch of surrounding objects, and, as we
may be able to see, it can reproduce itself, i.e., produce new
_Amœbæ_. _Amœba_ is the simplest living animal.

It is only rarely that we can find an _Amœba_ actually reproducing.
The process, in its gross features, is very simple. First the _Amœba_
draws in all of its pseudopodia and remains dormant for a time. Next,
certain changes take place in the nucleus, which divides into equal
portions, one part withdrawing to one end of the protoplasmic body,
the other to the opposite end. Soon the body protoplasm itself begins
to divide into two parts, each part collecting about its own half of
the nucleus. Finally the two halves pull entirely away from each other
and form two new _Amœbæ_, each like the original, but only half as
large. This is the simplest kind of reproduction found among animals.

_Amœbæ_ continue to live and multiply as long as the conditions
surrounding them are favorable. But when the pond dries up the _Amœbæ_
in it would be exterminated were it not for a careful provision of
nature. When the pond begins to dry up each _Amœba_ contracts its
pseudopodia and the protoplasm secretes a horny capsule about itself.
It is now protected from dry weather and can be blown by the winds
from place to place until the rains begin, when it expands, throws off
the capsule and commences active life again in some new pond.

    =The Slipper Animalcule= (_Paramœcium_ sp.)--_Technical
    Note_.--_Paramœcia_ can be secured in most pond water where leaves
    or other vegetation are decaying. However, if specimens are not
    readily secured place some hay or finely cut dry clover in a glass
    dish, cover with water and leave in the sun for several days. In
    this mixture specimens will develop by thousands. Place a drop of
    water containing _Paramœcia_ on a slide with cover-glass over it.
    Using a low power, note the many small animals darting hither and
    thither in the field. Run a thin mixture of cherry gum in water
    under the cover-glass. In this mixture they can be kept more quiet
    and be better studied.

How does _Paramœcium_ (fig. 6) differ from _Amœba_ in form and
movement? Has the body an anterior and a posterior end? The delicate,
short, thread-like processes, on the surface of the body, which beat
about very rapidly in the water are called _cilia_, and they are
simply fine prolongations of the body protoplasm. What is their
function? Note a fine _cuticle_ covering the body. Note also many
minute oval sacs lying side by side in the ectosarc. These are called
_trichocysts_ and from each a fine thread can be thrust out.

Note on one side, beginning at the anterior end, the _buccal groove_
leading into the interior through the _gullet_. Observe also that by the
action of the cilia in the buccal groove food-particles are swept into
the gullet. Rejected or waste particles are ejected from the body
occasionally. Where? Note about midway of the _Paramœcium_ an ovoid body
with a smaller oval one attached to its side, the former being the
_macronucleus_, the latter the _micronucleus_. Note that there are two
contractile vacuoles in the _Paramœcium;_ also that the food-vacuoles
have a definite course in their movement inside the endosarc.

Make a drawing of a _Paramœcium_.

In comparing _Paramœcium_ with _Amœba_ it is apparent that the body of
the first is less simple than that of the second. The definite opening
for the ingress of food, the two nuclei, the fixed cilia, and the
definite cell-wall giving a fixed shape to the body, are all
specializations which make _Paramœcium_ more complex than _Amœba_. But
the whole body is still composed of a single cell, and there is, as in
_Amœba_, no differentiation of the body-substance into different
tissues, and no arrangement of body-parts as systems of organs.

_Paramœcium_ may occasionally be found reproducing. This process takes
place very much as in _Amœba_. The animal remains dormant for a while,
the micronucleus then divides, the macronucleus elongates and finally
divides in two, the protoplasm of the body becomes constricted into two
parts, each part massing itself about the withdrawn halves of the macro-
and micro-nuclei, and lastly the whole breaks into two smaller organisms
which grow to be like the original. After multiplication or reproduction
has gone on in this way for numerous generations (about one hundred), a
fusion of two _Paramœcia_ seems necessary before further divisions take
place. This process of fusion, called _conjugation_, may be noted at
some seasons. Two _Paramœcia_ unite with their buccal grooves together,
part of the macronucleus and micronucleus of each passes over to the
other, and the mixed elements fuse together to form a new macro- and
micronucleus in each half. The conjugating _Paramœcia_ now separate, and
each divides to form two new individuals.

[Illustration: FIG. 6.--_Paramœcium_ sp.; buccal groove at right.
(From life.)]

                              CHAPTER VII

               THE SINGLE-CELLED ANIMAL BODY.--PROTOPLASM
                              AND THE CELL

=The single-celled body.=--The study of _Amœba_ and _Paramœcium_ has
made us acquainted with an animal body very different from that of the
toad or the crayfish. These extraordinarily minute animals have a body
so simple in its composition, compared with the toad's, that if the
toad's body be taken for the type of the animal body, _Amœba_ might
readily be thought not to be an animal at all. The body of _Amœba_ is
not composed of organs, each with a particular function or work to
perform. Whatever an _Amœba_ does is done, we may say, with its whole
body. But as we learn the things that this formless viscid speck of
matter does, we see that it is truly an animal; that it really does
those things which we have learned are the necessary life-processes of
an animal. _Amœba_ takes up and digests food composed of organic
particles; it has the power of motion; it knows when its body comes in
contact with some external object, that is, it can feel or has the
power of sensation. _Amœba_ takes in oxygen and gives out carbonic
acid gas, and it can produce new individuals like itself, that is, it
has the power of reproduction. But for the performance of these
various life-processes or functions it has no special parts or organs,
no mouth or alimentary canal, no lungs or gills, no legs, no special
reproductive organs. We have here to do with one of the "simplest
animals." With a minute, organless, soft speck of viscous matter
called protoplasm for a body, the simplest structural condition to be
found among living beings, _Amœba_ nevertheless is capable of
performing, in the simplest way in which they may be performed, those
processes which are essential to animal life.

_Paramœcium_ has a body a little less simple than _Amœba_. The
food-particles are taken into the body always at a certain spot; this
might be spoken of as a mouth. And the body has some special
locomotory organs, if they may be so called, in the presence of the
cilia. The body, too, has a definite shape or form. But, as in _Amœba_
there is no alimentary canal, nor nervous system, nor respiratory
system, nor reproductive system. The whole body feels and breathes and
takes part in reproduction.

A long jump has been made from the toad and crayfish to _Amœba_ and
_Paramœcium_; from the complex to the simplest animals. But, as will
later be seen, the great difference between the bodies of these
simplest animals and those of the highly complex ones is only a
difference of degree; there are animals of all grades and stages of
structural condition connecting the simplest with the most complex.
When animals are studied systematically, as it is called, we begin
with the simplest and proceed from them to the slightly complex, from
these to the more complex, and finally to the most complex. There are
hundreds of thousands of different kinds of animals, and they
represent all the degrees of complexity which lie between the extremes
we have so far studied.

=The cell.=--The characteristic thing about the body of _Amœba_ and
_Paramœcium_ and the other "simplest animals"--for there are many
members of the group of "simplest animals," or Protozoa--is that it is
composed, for the animal's whole lifetime, of a single cell. A cell is
the structural unit of the animal body. As will be learned in the
next exercise, the bodies of all other animals except the Protozoa,
the simplest animals, are composed of many cells. These cells are of
many kinds, but the simplest kind of animal cell is that shown by the
body of an _Amœba_, a tiny speck of viscous, nearly colorless
protoplasm without fixed form. The protoplasm composing the cell is
differentiated to form two parts or regions of the cell, an inner
denser part, called the nucleus, and an outer clearer part, called the
cytoplasm. Sometimes, as in the _Paramœcium_, the cell is enclosed by
a cell-wall which may be simply a denser outer layer of the cytoplasm,
or may be a thin membrane secreted by the protoplasm. Thus the cell is
not what its name might lead us to expect, typically cellular in
character; that is, it is not (or only rarely is) a tiny sac or box of
symmetrical shape. While the cell is composed essentially of
protoplasm, yet it may contain certain so-called cell-products, small
quantities of various substances produced by the life-processes of the
protoplasm. These cell-products are held in the protoplasmic body-mass
of the cell, and may consist of droplets of water or oil or resin, or
tiny particles of starch or pigment, etc. The cell cannot be said to
be composed of organs, because the word organ, as it is commonly used
in the study of an animal, is understood to mean a part of the animal
body which is composed of many cells. But the single cell can be
somewhat differentiated into parts or special regions, each part or
special region being especially associated with some one of the
life-processes. In _Paramœcium_, for example, the food is always taken
in through the so-called mouth-opening; the fine protoplasmic cilia
enable the cell to swim freely in the water, the waste products of the
body are always cast out through a certain part, and so on. But this
is a very simple sort of differentiation, and the whole body is only
one of those structural units, the cells, of which so many are
included in the body of any one of the complex animals.

=Protoplasm.=--The protoplasm, which is the essential substance of the
typical animal cell and hence of the whole animal body, is a substance
of very complex chemical and physical make-up. No chemist has yet been
able to determine its exact chemical constitution, and the microscope
has so far been unable to reveal certainly its physical characters.
The most important thing known about the chemical constitution of
protoplasm is that there are always present in it certain complex
albuminous substances which are never found in inorganic bodies. And
it is certain that it is on the presence of these substances that the
power possessed by protoplasm of performing the fundamental
life-processes depends. Protoplasm is the primitive physical basis of
life, but it is the presence of the complex albuminous substances in
it that makes it so.

The physical constitution of protoplasm seems to be that of a viscous
liquid containing many fine globules of a liquid of different density
and numerous larger globules of a liquid of still other density. Some
naturalists believe the fine globules to be solid grains, while still
others believe that numerous fine threads of dense protoplasm lie
coiled and tangled in the clearer, viscous protoplasm. But the little
we know of the physical structure of protoplasm throws almost no light
on the remarkable properties of this fundamental life-substance.

                              CHAPTER VIII

                   CELLULAR STRUCTURE OF THE TOAD (OR
                                 FROG)

                          LABORATORY EXERCISE

    =The blood.=--TECHNICAL NOTE.--The blood of a frog can be studied
    as it flows through the small vessels in the membranes between the
    toes while the animal is alive. Place a frog on a small flat board
    which has had a hole cut near one end, and with a piece of cloth
    bind it to the board. Spread the web between two toes over the
    hole in the board and keep it in place with pins. This done,
    examine the distended web under the compound microscope first with
    low then with higher power, and observe the blood-vessels and the
    blood circulating in them. For a further study of the blood kill a
    toad or frog and place a drop of the blood on a slide with a
    cover-glass over it.

Put the prepared slide under the microscope and note that the blood,
which as seen with the unaided eye appears to be a red fluid, is made
up of a great many yellowish elliptical disks or _cells_, the
_blood-corpuscles_, floating in a liquid, the _blood-plasma_. Here and
there you may notice _amœboid blood-corpuscles_. These are
irregular-shaped cells which move about by thrusting out pseudopodia.
They look like some of the unicellular animals, as the _Amœba_. Can
you distinguish a nucleus and cell-wall in the blood-cells?

Make drawings of these blood-cells.

    =The skin.=--TECHNICAL NOTE.--Keep a live toad or frog in water
    for some time and note if its skin becomes loose or begins to slip
    away. If the outer skin, epidermis, comes off, take some of the
    shed skin and wash it in water, then stain for three or four
    minutes in a solution of methyl-green and acetic acid (see p.
    451). Cut the pieces of stained skin into small bits and examine
    one of these under the microscope.

With the low power of the microscope you will note that the skin is
made up of a great many flat _cells_ placed edge to edge. Each one has
its cell-wall and a central darkly stained nucleus.

Make a drawing of a portion of the toad's skin.

    =The liver.=--TECHNICAL NOTE.--Cut through the fresh liver of a
    toad, and with a knife-blade scrape from the cut surface some of
    the liver-cells and place them on a slide with cover-glass.

Examine under the microscope and observe many polygonal _cells_. Place
some of the methyl-green acetic stain under the cover-glass and note,
after the cells are stained, that they have definite boundaries and a
central nucleus.

Draw some of these scattered liver-cells.

    =The muscles.=--TECHNICAL NOTE.--Take a piece of intestine from a
    freshly killed toad, wash it thoroughly and place it in a
    concentrated solution of salicylic acid in 70% alcohol for 24 hours,
    then gradually heat until about the boiling-point, when the muscles
    will fall to pieces. Transfer the preparation to a watch-crystal and
    tease small bits of isolated muscle with dissecting-needles. Place
    some of the teased muscle-fibres on a slide, cover with cover-glass,
    and add a drop of the methyl-green acetic acid.

Note the small spindle-shaped _muscle-fibres_. Each one of these
fibres is a _cell_ possessing all of the structures common to cells,
namely, cell-wall, nucleus, etc.

Make a drawing of a few isolated fibres of muscle.

From this study of some of the tissues in a toad it will be noted that
in the first case we had in the blood separate cells which moved about
freely in the plasma. In the second case, that of the epidermis, the
cells are fixed edge to edge, thus forming a thin tissue; while in the
third and fourth cases, that of the liver and muscle, the cells are not
only placed edge to edge, but aggregated into vast masses or bundles,
in one case to form the liver and in the other case a muscle. The entire
body of the toad is built up of a colony of simple units (cells)
combined in various forms to make all the various tissues and organs.

                               CHAPTER IX

             THE MANY-CELLED ANIMAL BODY.--DIFFERENTIATION
                              OF THE CELL

=The many-celled animal body.=--In the study of certain of the tissues
and organs of the toad we have learned that the body of this animal is
composed of many cells, thousands and thousands of these microscopic
structural units being combined to form the whole toad. This
many-celled or multicellular condition of the body is true of all the
animals except the simplest, the unicellular Protozoa. Corals,
starfishes, worms, clams, crabs, insects, fishes, frogs, reptiles,
birds, and mammals, all the various kinds of animals in which the body
is composed of organs and tissues, agree in the multicellular
character of the body, and may be grouped together and called the
many-celled animals in contrast to the one-celled animals. This
division is one which is recognized by many systematic zoologists as
being more truly primary or fundamental than the division of animals
into Vertebrates and Invertebrates. The one-celled animals are called
Protozoa, and the many-celled animals Metazoa.

=Differentiation of the cell.=--It is apparent at first glance that
the cells which compose the body of a many-celled animal are not like
the simple primitive cell which makes up the body of the _Amœba_, nor
are they like the more complexly arranged cell of the _Paramœcium_.
Nor are they all like each other. The cells in the toad's blood are of
two kinds, the white blood-cells, which are very like the body of
_Amœba_, and the elliptical disk-like red blood-cells. The cells
composing the muscles are, moreover, like neither kind of blood-cells,
and the cells of which the liver is composed are not like the cells of
the muscles. That is, there are many different kinds of cells in the
body of a many-celled animal. While the single cell which composes the
whole body of the _Amœba_ is able to do all the things necessary to
maintain life, the various cells in the body of a complex animal are
differentiated or specialized, certain cells devoting themselves to a
certain function or special work, and others to other special
functions. For example, the cells which compose the organs of the
nervous system, the brain, ganglia, and nerves, devote themselves
almost exclusively to the function of sensation, and they are
especially modified for this purpose. The highly specialized
nerve-cells resemble very little the primitive generalized body-cell
of _Amœba_. The muscle-cells of the complex animal body have developed
to a high degree that power of contraction which is possessed, though
in but slight degree, by _Amœba_. These muscle-cells have for their
special function this one of contraction, and massed together in great
numbers they form the strongly contractile muscular tissue and muscles
of the body on which the animal's power of motion depends. The cells
which line certain parts of the alimentary canal are the ones on which
the function of digestion chiefly rests. And so we might continue our
survey of the whole complex body. The point of it all is that the
thousands of cells which compose the many-celled animal body are
differentiated and specialized; that is, have become changed or
modified from the generalized primitive amœboid condition, so that
each kind of cell is devoted to some special work or function and has
a special structural character fitting it for its special function. In
the Protozoan body the single cell can perform and does perform all
the functions or processes necessary to the life of the animal. In the
Metazoan body each cell performs, in co-operation with many other
similar cells, some one special function or process. The total work of
all the cells is the living of the animal.

                               CHAPTER X

                                 HYDRA

                          LABORATORY EXERCISE

    TECHNICAL NOTE.--_Hydra_ lives in fresh water, attached to stones,
    sticks, or decayed leaves. It can be found in most open
    fresh-water ponds not too stagnant, often attached to _Chara_.
    There are two species occurring commonly, _H. viridis_, the green
    _Hydra_, and _H. fuscus_, the brown or flesh-colored _Hydra_. Both
    are very small forms and have to be looked for carefully.
    Specimens should be brought to the laboratory, put into a large
    dish of water and left in the light. _Hydra_ is best studied
    alive. Place a living specimen attached to a bit of weed in a
    watch-crystal filled with water or on a slide with plenty of water
    and examine with the low power of the microscope.

Note the cylindrical body (fig. 7, _A_, _B_) with its flat basal
attachment and _radial tentacles_ (varying in number) which crown the
upper end and surround the centrally located _mouth_. Note the movements
of _Hydra_, its powers of contraction, and method of taking in food.

    TECHNICAL NOTE.--To feed _Hydra_, place very small "water-fleas"
    (_Daphnia_ sp.) in the water with it.

Observe the method by which "water-fleas" are taken into the mouth.
Food is caught on stinging cells (to be studied later) and conveyed to
the mouth by the tentacles. Note that the cylindrical body encloses a
cavity, the _digestive cavity_. How is this connected with the
exterior? If _Hydra_ captures prey too large or is no longer hungry,
the prey is released.

[Illustration: FIG. 7.--A, _Hydra fusca_, with expanded body and a
budding individual; B, _H. fusca_, contracted; C, _H. fusca_, part of
outer surface of a tentacle, greatly magnified. (A and B drawn from live
specimens, C, from a preparation) D, _Grantia_ sp. (a sponge), three
individuals; E, _Grantia_ sp., longitudinal section; F, _Grantia_ sp.,
spicules. (D, E, and F drawn from preserved specimens.)]

    TECHNICAL NOTE.--Place small slips of paper on the slide near the
    _Hydra_, put cover-glass over the whole, and examine with the low
    power of the microscope.

Note that the whole animal is made up of cells closely joined. Are the
cells in the tentacles all alike? Note nodule-like projections above
some of the cells; these are _stinging cells_, or _cnidoblasts_. In
some cases a small hair-like process, the trigger hair or _cnidocil_,
may be seen projecting above the surface of the cell. Note in some of
the tentacles dark-colored particles. These are food-particles which
have been taken through the mouth into the digestive cavity and have
passed thence into the tentacles. The central digestive cavity
communicates freely with the cavities in the tentacles, for the
tentacles are merely evaginations of the body-wall.

Make drawings of the _Hydra_ expanded and of the same individual
contracted.

    TECHNICAL NOTE.--From the preparation which you have under the
    microscope pull out the slips of paper, thus letting the cover-glass
    drop down on the specimen. With a small pipette put a drop of
    anilin-acetic stain (see p. 451) on the slide at one side of the
    cover-glass and with a piece of filter-paper draw the water through
    from the other side of the cover-glass. When the stain is diffused
    press down the cover-glass gently and examine the tentacles first
    under a low power of the microscope, then under a high one.

Note the distortion that the animal has undergone through the action
of the reagent. Observe the cnidoblasts of the tentacles and note that
many of them have thrown out long whip-like processes (fig. 7, _C_).
On what parts of the body do the cnidoblasts occur? Carefully examine
one of the cnidoblasts which has been discharged and note a clear
transparent bag-like structure within, the _nematocyst_, to which is
attached the long whip-like process. In another cnidoblast cell which
has not been discharged note that the whip-like process is coiled
about inside of the bag-like structure. The whole apparatus is like
the inturned finger of a glove which can be blown out by pressure from
the inside. The mechanism is simple. The cnidocil or trigger-hair is
touched by some animal, an impulse is conveyed to the delicate fibres
interspersed among the cells (nerve-cells) which stimulate the
cnidoblast cell, whereupon there is a contraction of the contents and,
the cnidoblast being compressed, the inverted whip-like process turns
wrong side out and impales the animal on its points or barbs.

    TECHNICAL NOTE.--The teacher should be provided with microscopical
    sections, both transverse and longitudinal, of the _Hydra_ stained
    in some good general stain (hæmatoxylin or borax carmine). If the
    teacher has no means of making such preparations, they may be
    procured from dispensers of microscopical supplies.

From the cross-section of the _Hydra_ make out the general structure
of the body. Note that it is a hollow cylinder consisting of two
well-defined layers of cells, an outside _ectoderm_ layer and an inner
_endoderm_ layer. Between these two is yet another thin non-cellular
layer called the _mesoglœa_.

Thus it will be seen that _Hydra_ is made up of two layers of cells,
the outer ectoderm or skin, which is specialized to perform the office
of capturing prey as well as that of protection, and the inner
endoderm, which surrounds the digestive cavity and performs the
function of digestion. The endoderm lines the body-cavity, particles
taken in as food being digested by certain digestive cells which
thrust out amœboid processes and ingest particles of food. Other cells
in the endoderm have long flagellate processes which vibrate back and
forth in the digestive cavity, thereby creating currents in the water
containing food-particles.

Note, in a cross-section, that there are small ovoid or cuboid cells
at the bases of the large ectoderm cells. These are the _interstitial
cells_. Some of the interstitial cells become modified and pushed up
between the ectoderm cells to form cnidoblast cells. Many of the
endoderm as well as ectoderm cells have muscle-processes which spread
out from the base of the cell and which serve to contract and expand
the body.

    TECHNICAL NOTE.--In the specimens which have been collected
    perhaps two methods of reproduction will be observed. Place
    healthy _Hydræ_ in a wide-mouthed jar in the sunlight with plenty
    of water and food. In a few days active budding will take place.

Observe the method of reproduction in _Hydra_. Commonly the parent
produces small buds, which at first are only evaginations of the
body-wall, but which later develop tentacles and a mouth of their own.
Subsequently the bud becomes constricted at the base, separates from
the parent, and the young _Hydra_ begins a distinct existence.

Another mode of reproduction takes place which, in distinction from the
asexual method just mentioned, is called sexual reproduction. This last
is the method common to most of the higher organisms. You may note that
in some _Hydræ_ there is a swelling or bulging of the ectoderm of the
body-wall in the region just below the tentacles. These are the
_sperm-glands_. Within these are produced sperm-cells which break away
in great clusters to fertilize the ova, or eggs. Note a larger bulging
of the body-wall nearer the lower end of the body which, under high
power, has a granular appearance. This is the _egg-gland_, in which
develops a single _ovum_ or _egg_. The ovum breaks from its covering and
is fertilized by sperm-cells from another individual. In forms like
_Hydra_, where both sexes are represented in a single individual, the
organism is termed _monœcious_ or _hermaphroditic_. In connection with
reproduction Chapter XIII should be studied.

An instructive experiment can be performed by cutting a _Hydra_ into
two or more parts, when (usually) each of the various parts will
develop into a complete _Hydræ_. This process may be called
reproduction by fission, but it rarely occurs naturally.

                               CHAPTER XI

                    THE SIMPLEST MANY-CELLED ANIMALS

=Cell differentiation and body organization in Hydra.=--From the
examination of _Hydra_ we have learned that there are true many-celled
animals which are much less complex in structure than the toad and
crayfish. The body of _Hydra_, like the body of the toad, is composed of
many cells, but these cells are of only a few different kinds; that is,
show but little differentiation. There is relatively little division of
the body into distinct organs. Still, certain parts of the body devote
themselves principally to certain particular functions. Thus all the
food is taken in through the single "mouth-opening" at the apical free
end of the cylindrical body, and there are certain organs, the
tentacles, whose special business or function it is to find and seize
food and to convey it to the mouth. After the food is taken into the
cylindrical body-cavity it is digested by special cells which line the
cavity. Some of these cells are unusually large, and each contains one
or more contractile vacuoles. From the free ends of these cells, the
ends which are next to the body-cavity, project pseudopods or flagella.
These protoplasmic processes are constantly changing their form and
number. In addition to these large sub-amœboid cells there are, in this
inner layer of cells lining the body-cavity, and especially abundant
near the base or bottom of the cavity, many long, narrow, granular
cells. These are gland-cells which secrete a digestive fluid. The food
captured by the tentacles and taken in through the mouth-opening
disintegrates in the body-cavity, or digestive cavity as it may be
called. The digestive fluid secreted by the gland-cells acts upon it so
that it becomes broken into small parts. These particles are seized by
the projecting pseudopods of the sub-amœboid cells and taken into the
body-protoplasm of these cells. The cells of the outer layer of the body
do not take food directly, but receive nourishment only by means of and
through the cells of the inner layer. The body-cavity of _Hydra_ is a
very simple special organ of digestion.

In the outer layer of cells there are some specially large cells whose
inner ends are extended as narrow pointed prolongations directed at
right angles with the rest of the cell. These processes are very
contractile and are called muscle-processes. Each one is simply a
specially contractile continuation of the protoplasm of the cell-body.
There are also in this layer some small cells very irregular in shape
and provided with unusually large nuclei. These cells are more
irritable or sensitive than the others and are called nerve-cells. We
have thus in _Hydra_ the beginnings of muscular organs and of
nerve-organs. But how simple and unformed compared with the muscular
and nervous systems of the toad and crayfish! There is no circulatory
system, nor are there any special organs of respiration.

But _Hydra_ is far in advance of _Amœba_ or _Paramœcium_. Its body is
composed of thousands of distinct cells. Some of these cells devote
themselves especially to food-taking, some especially to the digestion
of food; some are specially contractile, and on them the movements of
the body depend, while others are specially irritable or sensitive,
and on them the body depends for knowledge of the contact of prey or
enemies. In the cnidoblast cells, those with the stinging threads,
there is a very wide departure from the simple primitive type of
cells. There is in _Hydra_ a manifest differentiation of the cells
into various kinds of cells. The beginnings of distinct tissues and
organs are indicated.

=Degrees in cell differentiation and body organization.=--In the study
of the cellular constitution of the tissues and organs of the toad, we
found to what a high degree the differentiation of the cells may
attain, and in the study of the anatomy of the toad we found how
thoroughly these differentiated cells may be combined and organized
into body-parts or organs. The body of the toad is made up of distinct
organs, each composed of highly differentiated or specialized cells.
The body of _Hydra_ is composed of cells for the most part only
slightly differentiated and hardly recognizably grouped or combined
into organs. These two conditions are the extremes in the
body-structure of the many-celled animals. Between them is a host of
intermediate conditions of cell differentiation and body organization.
When we come to the study of other members of the great branch of
simple many-celled animals to which _Hydra_ belongs (see Chapter
XVII), it will be found that some of them show a slight advance in
complexity beyond _Hydra_. Higher in the scale of animal life the
forms will be found still more and more complex, with ever-increasing
differentiation of the cells, with the combination of the
differentiated cells into distinct organs, and the co-ordination of
organs into systems of organs up to the extreme shown by the birds and
mammals. And hand in hand with this increasing complexity of structure
goes ever-increasing complexity or specialization of function.
Breathing is a simple function or process with _Hydra_, where each
body-cell takes up oxygen for itself, but it is a complex business
with the toad, or with a bird or mammal, where certain complex
structures, the lungs and accessory parts, and the heart,
blood-vessels and blood all work together to distribute oxygen to all
parts of the body.

                              CHAPTER XII

                        DEVELOPMENT OF THE TOAD

                     FIELD AND LABORATORY EXERCISE

    TECHNICAL NOTE.--As the work of this chapter, or some similar work
    in getting acquainted with the postembryonic development of a
    many-celled animal, should be done early in the course, and as
    most schools open in the fall, it will perhaps be impossible to
    make this first study of development from live specimens in the
    field. In such case the examination of a series of prepared
    specimens, previously obtained by the teacher, must be resorted
    to. In the spring the development of several kinds of animals,
    including the toad, can be studied from live specimens in the
    field or in breeding-cages and aquaria in the laboratory. The eggs
    of the toad may be found in April and May (the toads are heard
    trilling at egg-laying time) in ponds. The eggs look like the
    heads of black pins, and are in single rows in long strings of
    transparent jelly, which are usually wound around sticks or
    plant-stems at the bottom of the pond near the shore. Bring some
    of these strings into the schoolroom and keep them in water in
    shallow dishes. Keep them in the light, but not in direct
    sunlight. In the dishes put some small stones and mud from the
    pond, arranging them in a slope, thus making different depths of
    water. Stones with green algæ on should be selected, for algæ are
    the food of the tadpoles. The eggs will hatch in two or three
    days, and if too many tadpoles are not kept in the dish, and the
    little aquarium be well cared for, the whole postembryonic
    development of the toad can be well observed. For the study of the
    development from prepared specimens the teacher should have a
    complete series of stages from egg to adult toad in alcohol. The
    specimens may be examined by the students in connection with a
    talk from the teacher on the life-history of the toad.

If the study is made from prepared specimens, make drawings of
egg-strings, and of a single egg magnified and shaded to indicate its
color. Draw each specimen of the series of tadpoles, noting in the
youngest the presence of gills and tail and absence of legs and eyes;
in the older the appearance of eyes, the shrivelling of the gills,
shrinking of the tail and development of legs; in the still older the
characteristic shape, in miniature, of the adult toad.

In observing the course of development of the living specimens there
should be made, in addition to the drawings, notes showing the
duration of the egg stage, and the time elapsing between all important
changes (as seen externally) in the body of the young. Observations
and notes on the general behavior of tadpoles should also be made;
note the swimming, the feeding, the gradual leaving of the water, etc.

In addition to the easily seen external changes in the body, very
important ones in the internal organs take place during development.
Perhaps the most important of these concerns the lungs. The young
gilled toad breathes as a fish does, but gradually its gills are lost,
while at the same time lungs develop and the tadpole comes to the
surface to breathe air like any lunged aquatic animal. The toad on
leaving the water changes its diet from vegetable to animal food; a
tadpole feeds on aquatic algæ; a toad preys on insects. Correlated
with the change in habit, the intestine during development undergoes
some marked changes, becoming relatively diminished in length.

For an account of the development of the toad see Gage's "Life-history
of a Toad" or Hodge's "The Common Toad."

                              CHAPTER XIII

            MULTIPLICATION AND DEVELOPMENT.--MULTIPLICATION
                         OF ONE-CELLED ANIMALS

=Multiplication.=--We know that any living animal has parents; that
is, has been produced by other animals which may still be living or be
now dead or, as with _Amœba_, may have changed, by division, into new
individuals. Individuals die, but before death, they produce other
individuals like themselves. If they did not, their kind or species
would die with them. This production of new animals constantly going
on is called the reproduction or multiplication of animals. The
process is well called multiplication, because each female animal
normally produces more than one new individual. She may produce only
one at a time, one a year, as many of the sea-birds do or as the
elephant does, but she lives many years. Or she may produce hundreds,
or thousands, or even millions of young in a very short time. A
lobster lays 10,000 eggs at a time. Nearly nine millions of eggs have
been taken from the body of a thirty-pound female codfish. As a matter
of fact but very, very few of these eggs produce new animals which
reach maturity. From the 10,000 eggs produced by the lobster each year
an average of but two new mature lobsters is produced. There is always
a struggle for food and for place going on among animals, for many
more are produced than there are food and room for, and so of all the
new or young animals which are born the great majority are killed
before they reach maturity. In a later chapter more attention will be
given to this great struggle for life.

In the preceding paragraph it has been stated that "we know that any
living animal has parents; that is, has been produced by other animals
which may still be living or be now dead." This is a statement,
however, which has found complete acceptance only in modern times. It
is a familiar fact that a new kitten comes into the world only through
being born; that it is the offspring of parents of its kind. But we
may not be personally familiar with the fact that a new starfish comes
into the world only as the production of parent starfish, or that a
new earthworm can be produced only by other earthworms. But
naturalists have proved these statements. All life comes from life;
all organisms are produced by other organisms. And new individuals are
produced by other individuals of the same kind. That these statements
are true all modern observations and investigations of the origin of
new individuals prove. But in the days of the earlier naturalists the
life of the microscopic organisms like _Amœba_ and _Paramœcium_, and
even that of many of the larger but unfamiliar animals, was shrouded
in mystery. And various and strange beliefs were held regarding the
origin of new individuals.

=Spontaneous generation.=--The ancients believed that many animals
were spontaneously generated. The early naturalists thought that flies
arose by spontaneous generation from the decaying matter of dead
animals. Frogs and many insects were thought to be generated
spontaneously from mud, and horse-hairs in water were thought to
change into water-snakes. But such beliefs were easily shown to be
based on error, and have been long discarded by zoologists. But the
belief that the microscopic organisms, such as bacteria and infusoria,
were spontaneously generated in stagnant water or decaying organic
liquids was held by some naturalists until very recent times. And it
was not so easy to disprove the assertions of such believers. If some
water in which there are apparently no living organisms, however
minute, be allowed to stand for a few days, it will come to swarm with
microscopic plants and animals. Any organic liquid, as a broth or a
vegetable infusion, exposed to the air for a short time becomes foul
through the presence of innumerable microscopic organisms. But it has
been certainly proved that these organisms are not spontaneously
produced in the water or organic fluid. A few of them enter the water
from the air, in which there are always greater or less numbers of
spores of microscopic organisms. These spores germinate quickly when
they fall into water or some organic liquid, and the rapid succession
of generations soon gives rise to the hosts of bacteria and one-celled
animals which infest all standing water. If all the active organisms
and inactive spores in a glass of water are killed by boiling the
water, and this sterilized water be put into a sterilized glass, and
this glass be so well closed that germs or spores cannot pass from the
air without into the sterilized liquid, no living animals will ever
appear in it. We know of no instance of the spontaneous generation of
animals, and all the animals whose life-history we know are produced
by other animals of the same kind.

=Simplest multiplication and development.=--The simplest method of
multiplication and the simplest kind of development shown among animals
are exhibited by such simple animals as _Amœba_ and _Paramœcium_. The
production of new individuals is accomplished in _Amœba_ by a simple
division or fission of its body (a single cell) into two practically
equivalent parts. An _Amœba_ which has grown for some time contracts all
of its finger-like processes, the pseudopodia, and its body becomes
constricted. This constriction or fissure increases inwards so that the
body is soon divided fairly in two. There are now two _Amœbæ_, each half
the size of the original one; each, indeed, actually one-half of the
original one. The original _Amœba_ was the parent; the two halves of it
are the young. Each of the young possesses all of the characteristics
and powers of the parent; each can move, eat, feel, grow, and reproduce
by fission. The only change necessary for the young or new _Amœba_ to
become like its parent, is that of simple growth to a size about twice
its present size. The development here is reduced to a minimum. Just as
the simplest animals perform the other life-processes, such as taking
and digesting food, breathing and feeling, in an extremely primitive
simple way, so do they perform the necessary life-process of
reproduction or multiplication in the simplest way shown among animals.

In the case of _Paramœcium_ the process of multiplication is slightly
more complex than that of _Amœba_ in the fact that sometimes before
the simple fission of the body takes place the interesting phenomenon
of conjugation occurs. _Paramœcium_ may reproduce itself for many
generations by simple fission, but a generation finally appears in
which conjugation takes place. Two individuals come together and each
exchanges with the other a part of its nucleus. Then the two
individuals separate and each divides into two. The result of the
conjugation, or the coming together, of two individuals with mutual
interchange of nuclear substance is to give to the new _Paramœcia_
produced by the conjugating individuals a body which contains part of
the body-substance of two distinct individuals. If the two conjugating
individuals differ at all--and they always do differ, because no two
individual animals, although belonging to the same species, are
exactly alike--the new individual, made up of parts of each of them,
will differ slightly from both. Nature seems intent on making every
new individual differ slightly from the individual which precedes it.
And the method of multiplication which Nature has adopted to produce
the result is the method which we have seen exhibited in its simplest
form in the case of _Paramœcium_--the method of having two individuals
take part in the production of a new one.

The development of the new _Paramœcia_ is a little more complex than
that of _Amœba_. Not only must the new _Paramœcium_ grow to the size
of the original one, but it must develop those slight, but apparent,
modifications of the parts of its body which we can recognize in the
full-grown, fully developed _Paramœcium_ individual. A new
mouth-opening must develop on the new individual formed of the hinder
half of the original _Paramœcium_ and new cilia must be developed.
Thus there is a slight advance in complexity of development, just as
there is in complexity of structure in _Paramœcium_ as compared with
_Amœba_. In the many-celled animals this complexity of development is
carried to an extreme.

=Birth and hatching.=--When a young animal is born alive, it usually
resembles in appearance and structure the parent, although of course
it is much smaller, and requires always a certain time to complete its
development and become mature. A young kangaroo or opossum is carried
for some time after its birth in an external pouch on the mother's
body and is a very helpless animal. A young kitten is born with eyes
not yet opened and must be fed by the mother for several weeks. On the
other hand young Rocky Mountain sheep are able to run about swiftly
within a few hours after birth.

Most animals appear first as eggs laid by the mother. This is true of
the birds, the reptiles, the fishes, the insects, and most of the
hosts of invertebrate animals. This egg may be cared for by the parent
as with the birds, or simply deposited in a safe place as with most
insects, or perhaps dropped without care into the water as with most
marine invertebrates. The young animal which issues from the egg may
at the time of its hatching resemble the parent in appearance and
structural character (although always much smaller) as with the birds,
some of the insects, and many of the other animals. Or it may issue in
a so-called _larval_ condition, in which it resembles the parent but
slightly or not at all, as is the case with the gill-bearing, legless,
tailed tadpole of the frog or the crawling, wingless, wormlike
caterpillar of the butterfly, or the maggot of the house-fly.

=Life-history.=--Any animal which hatches from an egg has undergone a
longer or shorter period of development within the egg-shell before
hatching. The development of an animal from first germ-cell to the time
it leaves the egg, for example, the development of the embryo chick from
the first cell to time of hatching, is called its _embryonic_
development; and the development from then on, for example, that of the
chick to adult hen or rooster, or that of tadpole to frog, is called the
_post-embryonic_ development. Beginning students of animals cannot study
the embryonic development (_embryology_) of animals readily, but they
can in many cases easily follow the course of the post-embryonic
development, and this study will always be interesting and valuable.
When the "life-history" of an animal is spoken of in this book, or other
elementary text-book of zoology, it is the history of the life of the
animal from the time of its birth or hatching to and through adult
condition that is meant, not the complete life-history from beginning
single egg-cell to the end. In all of the study of the different kinds
of animals to which the rest of this book is devoted, attention will be
paid to their life-history.

                                PART II

                           SYSTEMATIC ZOOLOGY

                              CHAPTER XIV

                     THE CLASSIFICATION OF ANIMALS

=Basis and significance of classification.=--It is the common
knowledge of all of us that animals are classified: that is, that the
different kinds are arranged in the mind of the zoologist and in the
books of natural history, in various groups, and that these various
groups are of different rank or degree of comprehensiveness. A group
of high rank or great comprehensiveness includes groups of lower rank,
and each of these includes groups of still lower rank, and so on, for
several degrees. For example, we have already learned that the toad
belongs to the great group of back-boned animals, the Vertebrates, as
the group is called. So do the fishes and the birds, the reptiles and
the mammals or quadrupeds. But each of these constitutes a lesser
group, and each may in turn be subdivided into still lesser groups.

In the early days of the study of animals and plants their
classification or division into groups was based on the resemblances
and the differences which the early naturalists found among the
organisms they knew. At first all of the classifying was done by
paying attention to external resemblances and differences, but later
when naturalists began to dissect animals and to get acquainted with
the structure of the whole body, the differences and likenesses of
inner parts, such as the skeleton and the organs of circulation and
respiration, were taken into account. At the present time and ever
since the theory of descent began to be accepted by naturalists (and
there is practically no one who does not now accept it), the
classification of animals, while still largely based on resemblances
and differences among them, tells more than the simple fact that
animals of the same group resemble each other in certain structural
characters. It means that the members of a group are related to each
other by descent, that is, genealogically. They are all the
descendants of a common ancestor; they are all sprung from a common
stock. And this added meaning of classification explains the older
meaning; it explains why the animals are alike. The members of a group
resemble each other in structure because they are actually blood
relations. But as their common ancestor lived ages ago, we can learn
the history of this descent, and find out these blood relationships
among animals only by the study of forms existing now, or through the
fragmentary remains of extinct animals preserved in the rocks as
fossils. As a matter of fact we usually learn of the existence of this
actual blood relationship, or the fact of common ancestry among
animals, by studying their structure and finding out the resemblances
and differences among them. If much alike we believe them closely
related; if less alike we believe them less closely related, and so
on. So after all, though the present-day classification means