Elementary Zoology, Second Edition
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Title: Elementary Zoology, Second Edition
Author: Vernon L. Kellogg
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*** START OF THE PROJECT GUTENBERG EBOOK ELEMENTARY ZOOLOGY, SECOND EDITION ***
ELEMENTARY ZOOLOGY
BY
VERNON L. KELLOGG, M.S.
_Professor of Entomology, Leland Stanford Junior University_
_SECOND EDITION, REVISED_
[Illustration]
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.
CHAPTER III
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
something more, means a great deal more, in fact, than the
classification of the earlier naturalists means, it is largely based
on and determined by resemblances and differences just as was the old
classification. Sometimes the fossil remains of ancient animals tell
us much about the ancestry and descent of existing forms. For example,
the present-day one-toed horse has been clearly shown by series of
fossils to be descended from a small five-toed horse-like animal which
lived in the Tertiary age.
=Importance of development in determining classification.=--A very
important means of determining the relationships among animals is by
studying their development. If two kinds of animals undergo very
similar development, that is, if in their development and growth from
egg-cell to adult they pass through similar stages, they are nearly
related. And by the correspondence or lack of correspondence, by the
similarity or dissimilarity of the course of development of different
animals much regarding their relationship to each other is revealed.
Sometimes two kinds of animals which are really nearly related come to
differ very much in appearance in their fully developed adult
condition because of the widely different life-habits the two may
have. But if they are nearly related their developmental stages will
be closely similar until the animals are almost fully developed. For
example, certain animals belonging to the group which includes the
crabs, lobsters, and crayfishes, have adopted a parasitic habit of
life, and in their adult condition live attached to the bodies of
certain kinds of true crabs. As these parasites have no need of moving
about, being carried by their hosts, they have lost their legs by
degeneration, and the body has come to be a mere sac-like pulsating
mass, attached to the host by slender root-like processes, and not
resembling at all the bodies of their relatives the crabs and
crayfishes. If we had to trust, in making out our classification,
solely to structural resemblances and differences, we should never
classify the _Sacculina_ (the parasite) in the group Crustacea, which
is the group including the crabs and lobsters and crayfishes. But the
young _Sacculina_ is an active free-swimming creature resembling the
young crabs and young shrimps. By a study of the development of
_Sacculina_ we find that it is more closely related to the crabs and
crayfishes and the other Crustaceans than to any other animals,
although in adult condition it does not at all, at least in external
appearance, resemble a crab or lobster.
=Scientific names.=--To classify animals then, is to determine their
true relationships and to express these relationships by a scheme of
groups. To these groups proper names are given for convenience in
referring to them. These proper names are all Latin or Greek, simply
because these classic languages are taught in the schools and colleges
of almost all the countries in the world, and are thus intelligible to
naturalists of all nationalities. In the older days, indeed, all the
scientific books, the descriptions and accounts of animals and plants,
were written in Latin, and now most of the technical words used in
naming the parts of animals and plants are Latin. So that Latin may be
called the language of science. For most of the groups of animals we
have English names as well as Greek or Latin ones and when talking
with an English-speaking person we can use these names. But when
scientific men write of animals they use the names which have been
agreed on by naturalists of all nationalities and which are understood
by all of these naturalists. These Latin and Greek names of animals
laughed at by non-scientific persons as "jaw-breakers," are really a
great convenience, and save much circumlocution and misunderstanding.
AN EXAMPLE OF CLASSIFICATION.
TECHNICAL NOTE.--There should be provided a small set of bird-skins
which will serve just as well as freshly killed birds, and which may
be used for successive classes, thus doing away with the necessity
of shooting birds. The birds suggested for use are among the
commonest and most easily recognizable and obtainable. They may be
found in any locality at any time of the year. The skins can be
made by some boy interested in birds and acquainted with making
skins, or by the teacher, or can be purchased from a naturalists'
supply store, or dealer in bird skins. The skins will cost about 25
cents each. This example or lesson in classification can be given
just as well of course with other species of birds, or with a set of
some other kinds of animals, if the teacher prefers. Insects are
especially available, butterflies perhaps offering the most readily
appreciated resemblances and differences.
=Species.=--Examine specimens of two male downy woodpeckers (the males
have a scarlet band on the back of the head). (In the western States
use Gardiner's downy woodpecker.) Note that the two birds are of the
same size, have the same colors and markings, and are in all respects
alike. They are of the same kind; simply two individuals of the same
kind of animal. There are hosts of other individuals of this kind of
bird, all alike. This one kind of animal is called a _species_. The
species is the smallest[4] group recognized among animals. No attempt
is made to distinguish among the different individuals of one kind or
species of animal as we do in our own case.
Examine a specimen of the female downy woodpecker. It is like the male
except that it does not have the scarlet neck-band. But despite this
difference we know that it belongs to the same species as the male
downy because they mate together and produce young woodpeckers, male
and female, like themselves. There are thus two sorts of
individuals,[5] male and female, comprised in each species of animal.
A _species_ is a group of animals comprising similar individuals which
produce new individuals of the same kind usually after the mating
together of individuals of two sexes which may differ somewhat in
appearance and structure.
Examine a male hairy woodpecker and a female; (in western States
substitute a Harris's hairy woodpecker). Note the similarity in markings
and structure to the downy. Note the marked difference in size. Make
notes of measurements, colors and markings, and drawings of bill and
feet, showing the resemblances and the differences between the downy
woodpecker and the hairy woodpecker. These two kinds of woodpeckers are
very much alike, but the hairy woodpeckers are always much larger
(nearly a half) than the downy woodpeckers and the two kinds never mate
together. The hairy woodpeckers constitute another species of bird.
=Genus.=--Examine now a flicker (the yellow-shafted or golden-winged
flicker in the East, the red-shafted flicker in the West). Compare it
with the downy woodpecker and the hairy woodpecker. Make notes
referring to the differences, also the resemblances. The flicker is
very differently marked and colored and is also much larger than the
downy woodpecker, but its bill and feet and general make-up are
similar and it is obviously a "woodpecker." It is, however, evidently
another species of woodpecker, and a species which differs from either
the downy or the hairy woodpecker much more than these two species
differ from each other. There are two other species of flickers in
North America which, although different from the yellow-shafted
flicker, yet resemble it much more than they do the downy and hairy
woodpeckers or any other woodpeckers. We can obviously make two groups
of our woodpeckers so far studied, putting the downy and hairy
woodpeckers (together with half a dozen other species very much like
them) into one group and the three flickers together into another
group. Each of these groups is called a _genus_, and genus is thus the
name of the next group above the species. A genus usually includes
several, or if there be such, many, similar species. Sometimes it
includes but a single known species. That is, a species may not have
any other species resembling it sufficiently to group with it, and so
it constitutes a genus by itself. If later naturalists should find
other species resembling it they would put these new species into the
genus with the solitary species. Each genus of animals is given a
Greek or Latin name, of a single word. Thus the genus including the
hairy and downy woodpeckers is called _Dryobates_; and the genus
including the flickers is called _Colaptes_. But it is necessary to
distinguish the various species which compose the genus _Colaptes_,
and so each species is given a name which is composed of two words,
first the word which is the name of the genus to which it belongs,
and, second, a word which may be called the species word. The species
word of the Yellow-shafted Flicker is _auratus_ (the Latin word for
golden), so that its scientific name is _Colaptes auratus_. The
natural question, Why not have a single word for the name of each
species? may be answered thus: There are already known more than
500,000 distinct species of living animals; it is certain that there
are no less than several millions of species of living animals; new
species are being found, described and named constantly; with all the
possible ingenuity of the word-makers it would be an extremely
difficult task to find or to build up enough words to give each of
these species a separate name. This is not attempted. The same species
word is often used for several different species of animals, but never
for more than one species belonging to a given genus. And the names of
the genera are never duplicated. (There are, of course, much fewer
genera than species, and the difficulty of finding words for them is
not so serious.) Thus the genus word in the two-word name of a species
indicates at once to just what particular genus in the whole animal
kingdom the species belongs, while the second or species word
distinguishes it from the few or many other species which are included
in the same genus. This manner of naming species of animals and plants
(for plants are given their scientific names according to the same
plan) was devised by the great Swedish naturalist Linnæus in the
middle of the eighteenth century and has been in use ever since.
=Family.=--Examine a red-headed woodpecker (_Melanerpes
erythrocephalus_) and a sapsucker (_Sphyrapicus varius_) and any other
kinds of woodpeckers which can be got. Find out in what ways the hairy
and downy woodpeckers (genus _Dryobates_), the flickers (genus
_Colaptes_) and the other woodpeckers resemble each other. Examine
especially the bill, feet, wings and tail. These birds differ in size,
color and markings, but they are obviously all alike in certain
important structural respects. We recognize them all as woodpeckers.
We can group all the woodpeckers together, including several different
genera, to form a group which is called a _family_. A family is a
group of genera which have a considerable number of common structural
features. Each family is given a proper name consisting of a single
word. The family of woodpeckers is named _Picidæ_.
We have already learned that resemblances between animals indicate
(usually) relationship, and that classifying animals is simply
expressing or indicating these relationships. When we group several
species together to form a genus we indicate that these species are
closely related. And similarly a family is a group of related genera.
=Order.=--There are other groups[6] higher or more comprehensive than
families, but the principle on which they are constituted is exactly
the same as that already explained. Thus a number of related families
are grouped together to form an _order_. All the fowl-like birds,
including the families of pheasants, turkeys, grouse and quail, all
obviously related, constitute the order of gallinaceous birds called
_Gallinæ_. The families of vultures, hawks and owls constitute the
order of birds of prey, the _Raptores_, and the families of the
thrushes, wrens, warblers, sparrows, black-birds, and many others
constitute the great order of perching birds (including all the
singing birds) called the _Passeres_.
=Class and branch.=--But it is evident that all of these orders,
together with the other bird orders, ought to be combined into a great
group, which shall include all the birds, as distinguished from all
other animals, as the fishes, insects, etc. Such a group of related
orders is called a _class_. The class of birds is named _Aves_. There
is a class of fishes, _Pisces_, and one of frogs and salamanders,
_Batrachia_, one of snakes and lizards called _Reptilia_, and one of
the quadrupeds which give milk to their young called _Mammalia_. Each
of these classes is composed of several orders, each of which includes
several families and so on down. But these five classes of Pisces,
Batrachia, Reptilia, Aves and Mammals agree in being composed of
animals which have a backbone or a backbone-like structure, while
there are many other animals which do not have a backbone, such as the
insects, the starfishes, etc. Hence these five backboned classes may
be brought together into a higher group called a _branch_ or _phylum_.
They compose the branch of backboned animals, the branch _Vertebrata_;
all the animals like the starfishes, sea-urchins and sea-lilies which
have the parts of their body arranged in a radiate manner compose the
branch _Echinodermata_; all the animals like the insects and spiders
and centipedes and crabs and crayfishes which have the body composed
of a series of segments or rings and have legs or appendages each
composed of a series of joints or segments make up the branch
_Arthropoda_. And so might be enumerated all the great branches or
principal groups into which the animal kingdom is divided.
In the remainder of this book the classification of animals is always
kept in sight, and the student will see the terms species, genus,
family, order, etc., practically used. In it all should be kept
constantly in mind the significance of classification, that is, the
existence of actual relationships among animals through descent.
FOOTNOTES:
[4] The lesser group called _variety_, or subspecies, we may leave out
of consideration for the present.
[5] Some species of animals are not represented by male individuals:
and in some all the individuals are hermaphrodites, as explained in
chapter XIV.
[6] Each of these higher groups has a proper name composed of a single
word. In the case of no group except the species is a name-word ever
duplicated. Each genus, family, order, or higher group has a name-word
peculiar to it, and belonging to it alone.
CHAPTER XV
BRANCH PROTOZOA: THE ONE-CELLED
ANIMALS
Of this group the structure and life-history of the Amœba (_Amœba_
sp.) and the Slipper Animalcule (_Paramœcium_ sp.) have already been
treated in Chapter VI. Another example is the
BELL ANIMALCULE (_Vorticella_ sp.)
TECHNICAL NOTE.--Specimens of _Vorticella_ may usually be found in
the same water with _Amœba_ and _Paramœcium_. The individuals live
together in colonies, a single colony appearing to the naked eye
as a tiny whitish mould-like tuft or spot on the surface of some
leaf or stem or root in the water. Touch such a spot with a
needle, and if it is a Vorticellid colony it will contract
instantly. Bring bits of leaves, stems, etc., bearing Vorticellid
colonies into the laboratory and keep in a small stagnant-water
aquarium (a battery-jar of pond-water will do).
Examine a colony of _Vorticella_ in a watch-glass of water or in a
drop of water on a glass slide under the microscope. Note the stemmed
bell-shaped bodies which compose the colony. Each bell and stem
together form an individual _Vorticella_ (fig. 8.) How are the members
of the colony fastened together? Tap the slide and note the sudden
contraction of the animals; also the details of contraction in the
case of an individual. Watch the colony expand; note the details of
this movement in the case of an individual.
Make drawings showing the colony expanded and contracted.
With higher power examine a single individual. Note the thickened,
bent-out, upper margin of the bell. This margin is called the
_peristome_. With what is it fringed? The free end of the bell is
nearly filled by a central disk, the _epistome_, with arched upper
surface and a circlet of _cilia_. Between the epistome and peristome
is a groove, the _mouth_ or _vestibule_, which leads into the body.
Study the internal structure of the transparent, bell-shaped body.
Note the differentiation of the protoplasm comprising the body into an
inner transparent colorless _endosarc_ containing various dark-colored
granules, vacuoles, oil-drops, etc., and an outer uniformly granular
_ectosarc_ not containing vacuoles. Is the stalk formed of ectosarc or
endosarc or of both? Note the curved _nucleus_ lying in the endosarc.
(This may be difficult to distinguish in some specimens.) Note the
numerous large circular granules, the _food vacuoles_. Note the
_contractile vesicle_, larger and clearer than the food vacuoles. Note
the thin _cuticle_ lining the whole body externally. A high
magnification will show fine transverse ridges or rows of dots on the
cuticle.
[Illustration: FIG. 8.--_Vorticella_ sp.; one individual with stalk
coiled, and one with stalk extended. (From life.)]
Make a drawing showing the internal structure.
Observe a living specimen carefully for some time to determine all of
its movements. Note the contraction and extension of the stalk, the
movements of the cilia of peristome and epistome, the flowing or
streaming of the fluid endosarc (indicated by the movements of the
food vacuoles), the behavior of the contractile vesicle.
Make notes and drawings explaining these motions.
Specimens of _Vorticella_ may perhaps be found dividing, or two
bell-shaped bodies may be found on a single stem, one of the bodies
being sometimes smaller than the other. These two bodies have been
produced by the longitudinal division or fission of a single body. In
this process a cleft first appears at the distal end of the
bell-shaped body, and gradually deepens until the original body is
divided quite in two. The stalk divides for a very short distance. One
of the new bell-shaped bodies develops a circlet of cilia near the
stalked end. After a while it breaks away and swims about by means of
this basal circlet of cilia. Later it settles down, becomes attached
by its basal end, loses its basal cilia and develops a stalk.
"Conjugation occurs sometimes, but it is unlike the conjugation of
_Paramœcium_ in two important points: Firstly, the conjugation is
between two dissimilar forms; an ordinary large-stalked form, and a much
smaller free-swimming form which has originated by repeated division of
a large form. Secondly, the union of the two is a complete and permanent
fusion, the smaller being absorbed into the larger. This permanent
fusion of a small active cell with a relatively large fixed cell,
followed by division of the fused mass, presents a striking analogy to
the process of sexual reproduction occurring in higher animals."
OTHER PROTOZOA
Besides the _Amœba_, _Paramœcium_, and _Vorticella_ there are
thousands of other Protozoa. Most of them live in water, but a few
live in damp sand or moss, and some live inside the bodies of other
animals as parasites. Of those which live in water some are marine,
while others are found only in fresh-water streams and lakes.
[Illustration: FIG. 9.--Sun animalcule, a fresh-water protozoan with a
siliceous skeleton, and long thread-like protoplasmic prolongations.
(From life.)]
=Form of body.=--The Protozoa all agree in having the body composed
for its whole lifetime of a single cell,[7] but they differ much in
shape and appearance. Some of them are of the general shape and
character of _Amœba_, sending out and retracting blunt, finger-like
pseudopodia, the body-mass itself having no fixed form or outline but
constantly changing. Others have the body of definite form, spherical,
elliptical, or flattened, enclosed by a thin cuticle, and having a
definite number of fine thread-like or hair-like protoplasmic
prolongations called flagella or cilia. Many of the familiar Protozoa
of the fresh-water ponds always have two whiplash-like flagella
projecting from one end of the body. By means of the lashing of these
flagella in the water the tiny creature swims about. Others have many
hundreds of fine short cilia scattered, sometimes in regular rows,
over the body-surface. The Protozoan swims by the vibration of these
cilia in the water.
[Illustration: FIG. 10.--_Stentor_ sp.; a protozoan which may be
fixed, like _Vorticella_, or free-swimming, at will, and which has the
nucleus in the shape of a string or chain of bead-like bodies. The
figure shows a single individual as it appeared when fixed, with
elongate, stalked body, and as it appeared when swimming about with
contracted body. (From life.)]
There is no stagnant pool, no water standing exposed in watering-trough
or barrel which does not contain thousands of individuals of the
one-celled animals. And in any such stagnant water there may always be
found several or many different kinds or species. A drop of this water
examined with the compound microscope will prove to be a tiny world (all
an ocean) with most of its animals and plants one-celled in structure. A
few many-celled animals will be found in it preying on the one-celled
ones. There are sudden and violent deaths here, and births (by fission
of the parent) and active locomotion and food-getting and growth and all
of the businesses and functions of life which we are accustomed to see
in the more familiar world of larger animals.
=Marine Protozoa.=--One usually thinks of the ocean as the home of the
whales and the seals and the sea-lions, and of the countless fishes,
the cod, and the herring, and the mackerel. Those who have been on the
seashore will recall the sea-urchins and starfishes and the
sea-anemones which live in the tide-pools. On the beach there are the
innumerable shells, too, each representing an animal which has lived
in the ocean. But more abundant than all of these, and in one way more
important than all, are the myriads of the marine Protozoa.
Although the water at the surface of the ocean appears clear and on
superficial examination seems to contain no animals, yet in certain
parts of the ocean (especially in the southern seas) a microscopical
examination of this water shows it to be swarming with Protozoa. And
not only is the water just at the surface inhabited by one-celled
animals, but they can be found in all the water from the surface to a
great depth below it. In a pint of this ocean-water there may be
millions of these minute animals. In the oceans of the world the
number of them is inconceivable. And it is necessary that these
Protozoa exist in such great numbers, for they and the marine
one-celled plants (Protophyta) supply directly or indirectly the food
for all the other animals of the ocean.
Among all these ocean Protozoa none are more interesting than those
belonging to the two orders Foraminifera (fig. 11) and Radiolaria. The
many kinds belonging to these orders secrete a tiny shell (of lime in
the Foraminifera, of silica in the Radiolaria) which encloses most of
the one-celled body. These minute shells present a great variety of
shape and pattern, many being of the most exquisite symmetry and
beauty. The shells are perforated by many small holes through which
project long, delicate, protoplasmic pseudopodia. These fine
pseudopodia often interlace and fuse when they touch each other, thus
forming a sort of protoplasmic network outside of the shell. In some
cases there is a complete layer of protoplasm--part of the body
protoplasm of the Protozoan--surrounding the cell externally.
[Illustration: FIG. 11.--_Rosalina varians_, a marine protozoan
(Foraminifera) with calcareous shell. (After Schultze.)]
When these tiny animals die their hard shells sink to the bottom of
the ocean, and accumulate slowly, in inconceivable numbers, until they
form a thick bed on the ocean floor. Large areas of the bottom of the
Atlantic Ocean are covered with this slimy ooze, called Foraminifera
ooze or Radiolaria ooze, depending on the kinds of animals which have
formed it. Nor is it only in present times that there has been a
forming of such beds by the marine Protozoa. All over the world there
are thick rock strata composed almost exclusively of the fossil shells
of these simplest animals. The chalk-beds and cliffs of England, and
of France, Greece, Spain, and America, were made by Foraminifera.
Where now is land were once oceans the bottoms of which have been
gradually lifted above the water's surface. Similarly the rock called
Tripoli found in Sicily and the Barbadoes earth from the island of
Barbadoes are composed of the shells of ancient Radiolaria.
It is thus evident that the Protozoa is an ancient group of animals.
As a matter of fact zoologists are certain that it is the most ancient
of all animal groups. All of the animals of the ocean depend upon the
marine Protozoa and the marine Protophyta, one-celled plants, for
food. Either they feed on them directly, or prey on animals which in
turn prey on these simplest organisms. A well-known zoologist has
said: "The food-supply of marine animals consists of a few species of
microscopic organisms which are inexhaustible and the only source of
food for all the inhabitants of the ocean. The supply is primeval as
well as inexhaustible, and all the life of the ocean has gradually
taken shape in direct dependence on it." The marine Protozoa are the
only animals which live independently; they alone can live or could
have lived in earlier ages without depending on other animals. They
must therefore be the oldest of marine animals. By oldest is meant
that their kind appeared earliest in the history of the world, and as
it is certain that ocean life is older than terrestrial life--that is,
that the first animals lived in the ocean--it is obvious that the
marine Protozoa are the most ancient of all animal groups.
As already learned in the examination of examples of one-celled
animals, it is evident that life may be successfully maintained
without a complex body composed of many organs performing their
functions in a specialized way. The marine Protozoa illustrate this
fact admirably. Despite their lack of special organs and their
primitive way of performing the life-processes, that they live
successfully is shown by their existence in such extraordinary
numbers. They outnumber all other animals. The conditions of life in
the surface-waters of the ocean are easy and constant, and a simple
structure and simple method of performing the necessary life-processes
are wholly adequate for successful life under these conditions.
FOOTNOTE:
[7] In some Protozoa a number of similar cells temporarily unite to form
a colony, but each cell may still be regarded as an individual animal.
CHAPTER XVI
BRANCH PORIFERA: THE SPONGES
THE FRESH-WATER SPONGE (_Spongilla_ sp.)
TECHNICAL NOTE.--Fresh-water sponges may perhaps not be readily
found in the neighborhood of the school, but they occur over most
of the United States, and careful searching will usually result in
the finding of specimens. They are compact, solid-looking masses,
sometimes lobed, resting on and attached to rocks, logs, timbers,
etc., in clear water in creeks, ponds, or bayous. They are creamy,
yellowish-brown or even greenish in color and resemble some
cushion-like plant far more than any of the familiar animal forms.
They can be distinguished from plants, however, by the fact that
there are no leaves in the mass, nor long thread-like fibres such
as compose the masses of pond algæ (pond scum). When touched with
the fingers a gritty feeling is noticeable, due to the presence of
many small stiff spicules. Sponges should be removed entire from
the substance they are attached to, and may be taken alive to the
laboratory. They die soon, however, and should be put into alcohol
before decay begins.
Note the form of the sponge mass. Is it lobed or branched? Examine the
surface for openings. These are of two sizes; the larger are _osteoles_
or _exhalant openings_, while the smaller and more numerous are _pores_
or _inhalant openings_. The sponge-flesh is called _sarcode_. Examine a
bit of sarcode under the microscope; note the _spicules_. Have these
spicules a regular arrangement? Of what are they composed?
Draw the entire sponge, showing shape and openings; draw some of the
spicules.
Embedded in the body-substance, especially near the base, note (if
present) numerous small, yellowish, sub-spherical or disk-like
bodies, the _gemmules_. These are reproductive bodies. Each gemmule is
a sort of internal bud. It is composed of an interior group of
protoplasmic cells, enclosed by a crust thickly covered with spicules.
In winter the sponge dies down and the gemmules are set free in the
water. In spring the protoplasmic contents issue through an aperture
in the crust, called the _micropyle_ or _foraminal opening_, and
develop and grow into a new sponge.
For a good account of the fresh-water sponge, see Pott's "Fresh-water
Sponges."
A CALCAREOUS OCEAN-SPONGE (_Grantia_ sp.) (fig. 7, D, E, F.)
TECHNICAL NOTE.--For inland schools, specimens preserved in
alcohol or formalin must be used. They may be obtained from
dealers in naturalists' supplies (see p. 453). Specimens of some
species of this genus can be obtained at almost any point on the
Atlantic or Pacific coasts of this country.
Examine the external structure of a specimen. Note the elongate,
sub-cylindrical form, the attached base, the free end. Note the large
exhalant opening, _osteole_ or _osculum_, at the free end; the
numerous small inhalant openings elsewhere on the surface (best seen
in dried specimens). Note the _spicules_ covering the surface of the
body, and the longer ones surrounding the osculum. Cut the sponge in
two longitudinally and note the simple cylindrical body-cavity, the
_gastric cavity_ or _cloaca_. Note the thickness of the body-wall;
note the tubes running through the body-wall from cloaca to external
surface. Through these tubes water laden with food enters the gastric
cavity, where the food is digested, the water and undigested particles
passing out through the osculum. Crush a bit of dried sponge, or boil
a bit of soft sponge in caustic potash and mount on a glass slide.
Examine under a microscope and note the abundance of spicules and the
variety in their form. Two kinds may always be found, and sometimes
three. These spicules are composed of carbonate of lime and can be
dissolved by pouring on to them a drop of hydrochloric acid.
Some of the sponges may have buds growing out from them near the base.
These buds are young sponges developed asexually. If allowed to
develop fully the buds would have detached themselves from the parent
and each would have become a new sponge.
Make drawings showing the form of a whole sponge; the appearance of
the inner face of the sponge bisected longitudinally; the shape of the
spicules.
A COMMERCIAL SPONGE
TECHNICAL NOTE.--For the study of the skeleton of an ocean-sponge
with more complex body buy several common small bath-sponges without
large holes running entirely through them. The teacher should have
also a few specimens of small marine sponges preserved in alcohol or
formalin. Such specimens should be part of the laboratory equipment
(see account of laboratory equipment, p. 450), and can be readily
and cheaply obtained from dealers in naturalists' supplies.
The bath-sponge or slate-sponge consists simply of the hard parts or
skeleton of a sponge animal. In life all of the skeleton is enclosed
or covered by a soft, tough mass composed of layers of cells. Note the
many openings on the surface of the sponge. Crush a bit of the
skeleton and examine it under the microscope. Note that it is composed
of fine fibres of a tough, horny substance called _spongin_, instead
of tiny distinct calcareous spicules.
OTHER SPONGES
The sponges are fixed, plant-like aquatic animals. The members of a
single family live in fresh water, being found in lakes, rivers, and
canals in all parts of the world. All the other sponges, and there are
several thousand species known, live in the ocean. They are to be
found at all depths, some in shallow water near the shore and others
in deeper water, even to the deepest depths yet explored. They are
found in all seas, though especially abundantly in the Atlantic Ocean
and Mediterranean Sea.
=Form and size.=--The shape of the simplest sponges is that of a tiny
vase or nearly cylindrical cup, hollow and attached at its base. At the
free end there is a large opening. But there is a great deal of variety
in the form and size of different sponges. There is, indeed, much
variation in the shape and general character of different individuals of
the same species. Unlike most other animals, sponges are fixed, and the
character of the surface to which a sponge is attached has much
influence upon its shape. If this surface is rough and uneven the sponge
may follow in its growth the sinuosities of the surface and so become
uneven and distorted in shape. At best, only a few kinds of sponges have
any very even and symmetrical shape. Most of them are very unsymmetrical
and grow more like a low compact bushy plant than like the animals we
are familiar with. The smallest sponges are only 1 mm. (1/25 in.) high,
while the largest may be over a meter (39 in.) in height. In color
living sponges may be red, purple, orange, gray, and sometimes blue.
Most sponges have the whole body of one color.
[Illustration: FIG. 12.--The skeleton of a "glass" sponge (skeleton
composed of siliceous spicules) from Japan. (From specimen.)]
=Skeleton.=--A very few sponges have no skeleton at all. The others
have a skeleton or hard parts composed of interwoven fibres of the
tough, horny substance called spongin, or of hosts of fine needles or
spicules of silica or of carbonate of lime. The siliceous skeletons of
some of the so-called glass-sponges (fig. 12) are very beautiful. The
lime and siliceous sponge spicules exhibit a great variety of outline,
some being anchor-shaped, some cross-shaped, and some resembling tiny
spears or javelins.
=Structure of body.=--The skeleton of a sponge whether composed of
interlacing fibres or of short spicules is always invisible from the
outside when the sponge is alive. It is embedded in, or clothed by,
the soft, fleshy part of the body. The soft part of the sponge is
composed simply of two layers of cells, one constituting the external
surface of the body, and the other lining the interior cavities and
canals of the body. Between these two cell-layers there is a mass of
soft gelatinous substance all through which protoplasm ramifies, and
in which are embedded numerous scattered cells. There are, as seen in
the case of _Spongilla_ and _Grantia_, no systems of organs such as
characterize the higher animals. No heart, lungs, alimentary canal,
nervous system, organs of locomotion, eyes, ears, or other organs of
special sense; the sponge has none of these. It is simply an aggregate
of cells, arranged in two layers, and supported usually by a skeleton
of horny fibres or calcareous or siliceous spicules. Its body is
usually shapeless, unsymmetrical and without front or back, right or
left. It is not to be wondered at that sponges were for a long time
believed to be plants.
=Feeding habits.=--The sponges feed on minute bits of animal or plant
substance and on the microscopic unicellular plants or animals which
float in the water which bathes their bodies. The water entering the
sponge-body through the various openings of the surface is moved
along by the waving or lashing of the flagella of the cells which line
the canals, and these currents of water bear with them the tiny
organisms which are taken up by these same cells and digested. The
incoming currents of water meet in the central cavity or cavities of
the body and pass out through the large opening called the osculum at
the free end of the vase-like body, or if the body is branched,
through the large openings at the tips of these branches.
The same currents of water bring also oxygen for the sponge's breathing
and carry away the carbonic acid gas given out by the body-cells.
As a German naturalist has said, the one necessary condition for the
life of a sponge is the streaming of water through its body. All
sponges have a system of canals for this water-current and all have
means, in the waving flagella or cilia with which these canals are
lined, for producing these currents. When a live sponge is put into a
vessel of water, currents are immediately set up, and they always flow
into the body through the many fine openings and out of the body
through the osculum.
=Development and life-history.=--Although the sponge in its adult
condition is permanently attached by its base to the sea-bottom or to
some rock or shell, when it is first born it is an active free-swimming
creature. The sponges reproduce in two ways, asexually and sexually. The
asexual mode of reproduction of the fresh-water sponge by gemmules has
already been described. The ocean sponges also reproduce asexually
either by forming interior gemmules or external buds. In this latter
method a bud forms on the outer surface of the body which increases in
size and finally grows into a new sponge individual. In some species
this new sponge does not become separated from the body of the mother,
but remains attached to it like a branch to a tree-trunk. By the
continued production of such non-separating individuals, a colony of
sponges is formed which has the general appearance of a branching plant.
In other species the new sponge formed by the development and growth of
a bud falls off and becomes a distinct separate individual.
In the sexual mode of reproduction, male or sperm-cells and female or
egg-cells are developed in the same individual. The sperm-cells are
motile and swim about in the cavities and canals of the sponge-body
until they find egg-cells, which they fertilize. The fertilized eggs
begin to develop and pass through their first stages in the
sponge-body. Finally the embryo sponge, which is usually a tiny oval
or egg-shaped mass of cells, escapes from the body of the parent into
the water. The young sponge has some of its outer cells provided with
cilia, and by means of these it swims about. After a while it comes to
rest on the ocean-floor or on some rock or shell, attaches itself, and
begins to take on the form and character of the parent. It leads
hereafter a fixed sedentary life.
=The sponges of commerce.=--The sponge-skeletons which are the
"sponges" that we use all belong to a few species, not more than half
a dozen. Most of the commercial sponges come from the Mediterranean
Sea, though some come from the Bahama Islands, some from the Red Sea,
and a few from the coasts of Greece, Asia Minor, and Africa. The
commercial sponges do not live in very deep water; they are usually
found not deeper than 200 feet. The living sponges are collected by
divers, or are dragged up by men in boats using long-poled hooks, or
dredges. "When secured they are exposed to the air for a limited time,
either in the boats or on shore, and then thrown in heaps into the
water again in pens or tanks built for the purpose. Decay thus takes
place with great rapidity, and when fully decayed they are fished up
again, and the animal matter beaten, squeezed, or washed out, leaving
the cleaned skeleton ready for the market. In this condition after
being dried and sorted, they are sold to the dealers, who have them
trimmed, re-sorted and put up in bales or on strings ready for
exportation. There are many modifications of these processes in
different places, but in a general way these are the essential-steps
through which the sponge passes before it is considered suitable for
domestic purposes. Bleaching-powders or acids are sometimes used to
lighten the color, but these unless very delicately handled injure the
durability of the fibres."
=Classification.=--The sponges are classified according to the character
of the skeleton. In one group are put all those sponges which have a
skeleton of calcareous spicules, and this group is called the Calcarea.
All other sponges are grouped as Non-Calcarea, the members of this group
either having no skeleton at all, or having a skeleton composed of
siliceous spicules or of spongin fibres. According to the absence or
presence of a skeleton and the character of the skeleton when it exists
the Non-Calcarea are subdivided into smaller groups.
CHAPTER XVII
BRANCH CŒLENTERATA: THE POLYPS, SEA-ANEMONES,
CORALS, AND JELLYFISHES
The structure and life-history of an example of the polyps (the
Fresh-water Hydra, _Hydra_ sp.) has been studied in Chapters X and XI.
OTHER POLYPS, SEA-ANEMONES, CORALS, AND
JELLYFISHES
TECHNICAL NOTE.--The teacher should have, if possible, several
pieces of coral and a few specimens of Cœlenterates in alcohol or
formalin, which will show the external character, at least, of
these animals (see account of laboratory equipment, p. 450). If
the school is on the coast, the pupils should be shown the
sea-anemones of the tide-pools.
The animals which are included in the branch Cœlenterata are, at least
in living condition, unfamiliar to most of us. Like the sponges, they
are almost all inhabitants of the ocean; a few, like _Hydra_, live in
fresh water. Like the sponges, too, most of the members of this branch
are fixed, and in their general appearance suggest a plant rather than
an animal. The name zoophytes, or plant-animals, which is often
applied to these animals is based on this superficial resemblance. But
many of the Cœlenterates lead an active free-swimming life. This is
true of the jellyfishes which float or swim about on or near the
surface of the ocean. Many of the zoophytes spend part of their life
in an active free-swimming condition before settling down, becoming
attached and thereafter remaining fixed. In localities near the
seashore many animals belonging to this great group can be readily
found and observed. The beautiful sea-anemones with their
slowly-waving tentacles, the fine many-branched truly plant-like
hydroids with their hosts of little buds, and the soft colorless
masses of jelly, the jellyfishes, which are cast up on to the beaches
by the waves are all animals belonging to the branch Cœlenterata.
=General form and organization of body.=--The general or typical plan of
body-structure for the Cœlenterata, these animals which come next to the
sponges in degree of complexity, can best be understood by imagining the
typical cylindrical or vase-like body of the simple sponges to be
modified in the following way: The middle one of the three layers of the
body-wall not to be composed of scattered cells in a gelatinous matrix,
but to be simply a thin non-cellular membrane; the body-wall not to be
pierced by fine openings or pores, but connected with the outside only
by the single large opening at the free end, and this opening to be
surrounded by a circlet of arm-like processes or tentacles, which are
continuations of the body-wall and similarly composed. Such a
body-structure, which we saw well shown by _Hydra_, is the fundamental
one for all polyps, sea-anemones, corals, and jellyfishes. The variety
in shape of the body and the superficial modifications of this type-plan
are many and striking, but after all the type-plan is recognizable
throughout the whole of this great group of animals.
The two chief body-shapes represented in the branch are those of the
polyps on the one hand, and the jellyfishes or medusæ on the other. The
polyp-shape is that of a tube with a basal end blind or closed, attached
to some firm object in the water and with the free end with an opening,
the mouth-opening. At this mouth-end there is a circlet of movable, very
contractile tentacles. The mouth may open directly into the interior of
the body, which interior may be called the digestive cavity, or it may
lead into a simple short tube produced by the invagination or bending in
of the body-wall, which may be looked on as the simplest kind of
œsophagus. This œsophageal tube opens into the body-cavity or digestive
cavity. This cavity may be incompletely divided by longitudinal
partitions which project from the sides into the cavity.
The jellyfish or medusoid body-form corresponds in general to an
umbrella or bell. Around the edge of this umbrella are disposed
numerous threads or tentacles (corresponding to the circlet of
tentacles in the polyp). The mouth-opening is at the end of a longer
or shorter projection which hangs down from the middle of the under
side of the umbrella. The interior body-cavity or digestive cavity
extends out into the umbrella-shaped part of the body, usually in the
condition of canals radiating from the centre and a connecting canal
running around the margin of the umbrella.
=Structure.=--Although the Cœlenterata show little indication of the
complex composition of the body out of organs, as it exists among the
higher animals, yet they do show an unmistakable advance on the simple,
almost organless body of the sponges. This is chiefly shown by the
differentiation among the cells which compose the body. In the polyps
and jellyfishes some of the cells are specialized to be unmistakable
muscle-cells, some to be nerve-cells and fibres, and so on. A very
simple nervous system consisting of small groups of nerve-cells
connected by nerve-fibres exists. Some very simple special sense-organs
may occur. The digestive system, although in the simpler Cœlenterates
consisting merely of the cylindrical body-cavity enclosed by the
body-wall and opening by the single hole at the free end of the body, in
some is rather complex and is composed of different parts. Those
Cœlenterates which are not fixed but lead an active, free-swimming life,
viz., the jellyfishes or medusæ, are the most highly organized.
The tentacles which surround the mouth-opening and serve to grasp food
and carry it into the mouth, and the stinging or lasso threads with
which these tentacles are provided are special organs possessed by
most of these animals.
=Skeleton.=--Like the sponges, some of the Cœlenterata possess a hard
skeleton. This skeleton is always composed of calcium carbonate and is
called _coral_. Those polyps which form such a skeleton are called the
corals. Coral will be described in connection with the account of the
coral-polyps.
=Development and life-history.=--The polyps and jellyfishes reproduce
both asexually and sexually. The asexual mode is usually that of
budding. On a polyp a bud is formed by a hollow outgrowth of the
body-wall. The bud grows, an opening appears at its distal end, a
circlet of tentacles arises about this mouth-opening and a new polyp
individual is formed. This individual may separate from the parent or
it may remain attached to it. By the development of numerous buds, and
the remaining attached of all of the individuals developing from these
buds, a colony of polyp individuals may be formed, plant-like in
appearance. The various polyp individuals of a colony may differ
somewhat among themselves, and these differences are correlated with a
division of labor. Thus some of the individuals may devote themselves
to getting food for the colony, and these have mouth and tentacles.
Others may be devoted to the production of new individuals by budding
or by producing germ-cells, and may not have any mouth-opening or any
food-grasping tentacles.
In case of many polyps all or some of the new individuals which arise
by budding do not become polyps, but develop into medusæ or jellyfish,
which separate from the fixed polyp and swim off through the water.
These medusæ or jellyfish produce sperm-cells and egg-cells. The
sperm-cells fertilize the egg-cells and a new individual develops from
each fertilized egg. This new individual is at first an active
free-swimming larva called a _planula_, which does not resemble either
a medusa or polyp. After a while it settles down, becomes fixed and
develops into a polyp. Thus a polyp may produce a medusa or jellyfish
which, however, produces not a new jellyfish, but a polyp. This is
called an _alternation of generations_, and is not an uncommon
phenomenon among the lower animals. It results from such an
alternation of generations that a single species of animal may have
two distinct forms. This having two different forms is called
_dimorphism_. Sometimes, indeed, a species may appear in more than two
different forms; such a condition is called _polymorphism_.
Not all medusæ or jellyfish are produced by polyp individuals, nor do
jellyfish always produce polyps and not jellyfishes. There are some
jellyfishes (we might call them the true jellyfishes) which always
have the jellyfish form, producing new jellyfishes either by budding
or by eggs, and there are some polyps which always have the true polyp
form, producing new individuals, either by budding or by eggs, always
of polyp form and never of jellyfish form. That is, some species of
Cœlenterata exist only in polyp form, some species exist only in
jellyfish form, while some species (those having an alternation of
generations) exist in both polyp and jellyfish form, these two forms
appearing as alternate generations.
=Classification.=--The branch Cœlenterata is divided into four
classes: (1) the Hydrozoa, including the fresh-water polyps, numerous
marine polyps, many small jellyfishes and a few corals; (2) the
Scyphozoa, including most of the large jellyfishes; (3) the Actinozoa,
including the sea-anemones and most of the stony corals; (4) the
Ctenophora, including certain peculiar jellyfishes.
[Illustration: FIG. 13.--The Portuguese Man-of-War (_Physalia_ sp.).
(From specimen from Atlantic Coast.)]
=The polyps, colonial jellyfishes, etc. (Hydrozoa).=--To the class
Hydrozoa belongs the _Hydra_ already studied. There are a few other
fresh-water polyps and they all belong to this class. The most
interesting members of the class are the "colonial jellyfishes,"
constituting the order Siphonophora. These colonial jellyfishes are
floating or swimming colonies of polypoid and medusoid individuals in
which there is a marked division of labor among the individuals,
accompanied by marked differences in structural character. The
individuals are accordingly polymorphic, that is, appear in various
forms, although all belong to the same species. Because these various
individuals forming a colony have given up very largely their
individuality, combining together and acting together like the organs
of a complex animal, they are usually not called individuals, nor on
the other hand organs, but _zooids_, or animal-like structures. The
beautiful "Portuguese man-of-war" (fig. 13) is one of these colonial
jellyfishes. It appears as a delicate bladder-like float, brilliant
blue or orange in color, usually about six inches long, and bearing on
its upper surface which projects above the water a raised
parti-colored crest, and on its under surface a tangle of various
appendages, thread-like with grape-like clusters of little bell- or
pear-shaped bodies. Each of these parts is a peculiarly modified
polyp- or medusa-zooid produced by budding from an original central
zooid. The Portuguese man-of-war is very common in tropical oceans,
and sometimes vast numbers swimming together make the surface of the
ocean look like a splendid flower-garden.
[Illustration: FIG. 14.--A colonial jellyfish (Siphonophora). (After
Haeckel.)]
Usually the central zooid in a Siphonophore to which the other zooids
are attached is not a bladder-like float, but is an upright tube of
greater or less length. In the Siphonophore shown in figure 14, the
compound body is composed of a long central hollow stem with hundreds
or thousands of variously shaped parts, each of which is reducible to
either a polyp or medusazooid, attached around it. The upper end is
enlarged to form an air-filled chamber, a sac-like boat, by means of
which the whole colony is kept afloat. Around the upper end of the
central stem are many medusoid structures, the swimming-bells, by
means of whose opening and closing the whole colony is made to swim
through the water. Each swimming-bell is a modified medusa-zooid,
without tentacles, without digestive or reproductive organs, but
exercising the power of swimming by contracting and forcing the water
out of the hollow bell just as is done by the free medusæ. Below the
swimming-bells, at the lower end of the central stem, are grouped many
structures presenting at first sight a confusion of variety and
complexity, but on careful examination revealing themselves to be
polyp- and medusa-zooids modified to form at least five kinds of
particularly functioning structures. There are many flattened
scale-like parts whose function is simply that of affording a passive
protection, in times of danger, to the other structures. These
protecting-scales are greatly modified medusa-zooids, each consisting
of a simple cartilage-like gelatinous mass penetrated by a
food-carrying canal. Under the broad leaves of these protecting-zooids
are a number of pear-shaped bodies which have a wide octagonal
mouth-opening at their free end, and possess in their interior certain
digestive glands. Each one is provided with a very long flexible
tentacle which bears many fine stinging-threads. The tentacle waves
back and forth in the water, and on coming in contact with an enemy or
with prey its poisonous stinging-threads shoot out and paralyze or
wound the unfortunate animal. These pear-shaped bodies are the feeding
structures, each being a modified polyp-zooid. Scattered among these
dangerous structures are many somewhat similarly shaped but wholly
harmless structures, the sense-structures. Each of these has a
pear-shaped body but without mouth-opening, and also a long, very
sensitive, tentacle-like process. The sense of feeling is highly
developed in these tentacles, and they discover for the colony the
presence of any strange body. These sense-structures are modified
polyp-zooids. Finally there are two other kinds of structures, usually
arranged in groups like bunches of grapes, which are the reproductive
structures, male and female. They are modified medusa-zooids grown
together and without tentacles. This whole colony, or this compound
animal, floats or swims about at the surface of the ocean, and
performs all of the necessary functions of life as a single animal
composed of organs might. Yet the Siphonophore is more truly to be
regarded as a community in which the hundreds or thousands of animals,
representing five or six kinds of individuals, all of one species, are
fastened together. Each individual performs the particular duties
devolving upon its kind or class. Thus there are food-gathering
individuals, locomotor individuals, sense individuals, and
reproductive individuals. The modifications of the various kinds of
individuals are more extreme than in the case of the various kinds of
individuals composing a bee-community, for example, but the holding
together or fusing of all into one body or corporation is a condition
which makes this greater modification necessary and not unexpected.
And there is no difficulty in seeing that each of these parts is
really, structurally considered, a modified polyp or medusa.
[Illustration: FIG. 15.--A jellyfish or medusa, _Gonionema vertens_,
eating two small fishes. (From specimen from Atlantic Coast.)]
=The large jellyfishes, etc. (Scyphozoa).=--To the class Scyphozoa
belong most of the common large jellyfishes. When one walks along the
sea-beach soon after a storm one may find many shapeless masses of a
clear jelly-like substance scattered here and there on the sand. These
are the bodies or parts of bodies of jellyfishes which have been cast up
by the waves. Exposed to the sun and wind the jelly-like mass soon dries
or evaporates away to a small shrivelled mass. The body-substance of a
jellyfish contains a very large proportion of water; in fact there is
hardly more than 1 per cent of solid matter in it.
The jellyfishes occur in great numbers on the surface of the ocean and
are familiar to sailors under the name of "sea-bulbs." Some live in
the deeper waters; a few specimens have been dredged up from depths of
a mile below the surface. They range in size from "umbrellas" or disks
a few millimeters in diameter to disks of a diameter of two meters
(2-1/6 yards). They are all carnivorous, preying on other small ocean
animals which they catch by means of their tentacles provided with
stinging-threads. The tentacles of some of the largest jellyfishes
"reach the astonishing length of 40 meters, or about 130 feet." Many
of the jellyfishes are beautifully colored, although all are nearly
transparent. Almost all of them are phosphorescent, and when irritated
some emit a very strong light.
=The sea-anemones and corals (Actinozoa).=--Almost everywhere along
the seashore where there are rocks and tide-pools a host of various
kinds of sea-anemones can be found. When the tide is out, exposing the
dripping seaweed-covered rocks, and the little sand- or stone-floored
basins are left filled with clear sea-water, the brown and green and
purple "sea-flowers" may be found fixed to the rocks by the base with
the mouth-opening and circlet of slowly-moving tentacles hungrily
ready for food (fig. 16). Touch the fringe of tentacles with your
fingertip and feel how they cling to it and see how they close in so
as to carry what they feel into the mouth-opening. A host of
individuals there are, and scores of different kinds; some small,
some large, some with the body covered outside with tiny bits of stone
and shell so that they are hardly to be distinguished from the rock to
which they cling; some of bright and showy colors. These are the most
familiar members of the class Actinozoa.
[Illustration: FIG. 16.--Sea anemones, _Bunodes californica_, open and
closed individuals. The closed individuals in upper right-hand corner
show the external covering of small bits of rock and shell,
characteristic of most individuals of this species. (From living
specimens in a tide-pool on the Bay of Monterey, California.)]
But in other oceans, along the coasts of other lands, especially those
of the tropics and sub-tropics, there are some other members of the
class which are of unusual interest. They are the corals, or coral
polyps. We know these animals chiefly by their skeletons (fig. 17).
The specimens of corals which one sees in collections, or made into
ornaments, are the calcareous skeletons of various kinds of the coral
polyps. Some of the corals live together in enormous numbers, forming
branching colonies fixed as closely together as possible, and secrete
while living a stony skeleton of carbonate of lime. These skeletons
persist after the death of the animals, and because of their abundance
and close massing form great reefs or banks and islands. These coral
reefs and islands occur only in the warmer oceans. In the Atlantic
they are found along the coasts of Southern Florida, Brazil and the
West Indies; in the Pacific and Indian Oceans there are great coral
reefs on the coast of Australia, Madagascar and elsewhere, and certain
large groups of inhabited islands like the Fiji, Society, and Friendly
Islands are exclusively of coral formation. Coral islands have a great
variety of form, although the elongated, circular, ring-shaped and
crescent forms predominate. How such islands are first formed is
described as follows by a well-known student of corals:
[Illustration: FIG. 17.--Skeleton of a branching coral, _Madrepora
cervicornis_. (From specimen.)]
"A growing coral plantation, with its multitudinous life, oftentimes
arises from great depths of the ocean, and the sea-bed upon which it
rests is probably a submarine bank or mountain, upon which have lodged
and slowly aggregated the hard skeletons of pelagic forms of life.
When, through various sources of increase, this submarine bank
approaches the depth of from one hundred to one hundred and fifty feet
from the surface of the water, there begins on its top a most
wonderful vital activity. It is then within the bathymetric zone of
the reef-building corals. Of the many groups of marine life which then
take possession of the bank, corals are not the only animals, but
they are the most important, as far as its subsequent history goes. As
the bank slowly rises by their growth, it at last approaches the
surface of the water, and at low tide the tips of the growing branches
of coral are exposed to the air. This, however, only takes place in
sheltered localities, for long before it has reached this elevation it
has begun to be more or less changed and broken by the force of the
waves. As the submarine bank approaches the tide level, the delicate
branching forms have to meet a terrific wave-action. Fragments of the
branching corals are broken off from the bank by force of the waves,
and falling down into the midst of the growing coral below fill up the
interstices, and thus render the whole mass more compact. At the same
time larger fragments are broken and rolled about by the waves and are
eventually washed up into banks upon the coral plantation, so that the
island now appears slightly elevated above the tides. This may be
called a first stage in the development of a coral island. It is,
however, little more than a low ridge of worn fragments of coral
washed by the high tides and swept by the larger waves--a low, narrow
island resting on a large submarine bank."
When the coral island rises thus a little above the surface of the
water, the waves break up some of the coral into fine sand, which fills
in the interstices, and offers a sort of soil in which may germinate
seeds brought in the dried mud on the feet of ocean birds or carried by
the ocean currents. With the beginning of vegetable growth the soil is
more firmly held, is fertilized and ready for the seeds of plants which
need a better soil than lime sand. Flying insects find their way to the
island, especially if it be near the mainland, birds begin to nest on
it, and soon it may be the seat of a luxuriant plant and animal life.
For an account of coral islands see Darwin's "The Structure and
Distribution of Coral Reefs."
There are over 2000 kinds of coral polyp known, and their skeletons
vary much in appearance. Because of the appearance of the skeleton
certain corals have received common names, as the organ-pipe coral,
brain coral, etc. The red coral, of which jewelry is made, grows
chiefly in the Mediterranean. It is gathered especially on the western
coast of Italy, and on the coasts of Sicily and Sardinia. Most of this
coral is sent to Naples, where it is cut into ornaments.
There are other interesting members of the class Actinozoa like the
beautiful sea-pens, sea-feathers and sea-fans, delicate, branching,
tree-like forms found all over the world.
=Ctenophora.=--The members of this class are mostly small, peculiar
jellyfishes which do not form colonies, and are extremely delicate,
being usually perfectly transparent. They swim by means of cilia. They
never appear in a polyp condition, but are always medusoid in shape.
CHAPTER XVIII
BRANCH ECHINODERMATA: STARFISHES,
SEA-URCHINS, SEA-CUCUMBERS
STARFISH (_Asterias_ sp.)
TECHNICAL NOTE.--The species of _Asterias_ are widely distributed
on both coasts of the United States and may be procured on almost
any rocky shore at low tide. Teachers in inland schools can obtain
preserved material from the dealers mentioned on p. 453. Most of
the specimens should be placed in alcohol or 4% formalin. If fresh
material can be had it is well to place at least one specimen for
each student in a 20% solution of nitric acid in water for two or
three hours, when all of the calcareous parts will have been
dissolved, and after a thorough washing the specimen will be ready
for use.
=External structure= (figs. 18 and 19.)--In a fresh specimen or one
which has been preserved in alcohol or formalin note the raying out of
parts of the body from a common centre. This is characteristic of the
body organization of all Echinoderms, and is known as _radial symmetry_.
The lower surface of the body is called the _oral_ (because the mouth is
on this surface), while the upper is called the _aboral_ surface. The
central part of the body is called the _disk_. Note on the aboral
surface of the disk a small striated calcareous plate, the _madreporite_
or _madreporic plate_. In the middle (or very nearly in the middle) of
this surface of the disk there is a small pore, the _anal opening_. The
entire aboral surface as well as a greater part of the oral side is
thickly studded with the calcareous _ossicles_ of the body-wall. These
ossicles support numerous short stout _spines_ arranged in irregular
rows. Note that some of the ossicles support certain very small
pincer-like processes, the _pedicellariæ_. In the interspaces between
the calcareous plates are soft fringe-like projections of the inner
body-lining, the _respiratory cæca_. Note at the tip of each arm or ray
a cluster of small calcareous ossicles and within each cluster a small
speck of red pigment, the eye-spot or _ocellus_.
[Illustration: FIG. 18.--Dissection of a starfish (_Asterias_ sp.).]
Make a drawing of the aboral surface showing all these parts.
On the oral surface note the centrally-located _mouth_, the
_ambulacral grooves_, one running longitudinally along each ray, and
in each groove two double rows of soft tubular bodies with sucker-like
tips. These are called the _tube-feet_ and are organs of locomotion.
Make a drawing of the oral surface.
=Internal structure= (figs. 18 and 19).--TECHNICAL NOTE.--Take a
specimen which has been immersed for some time in the nitric acid
solution, and with a strong pair of scissors, or better,
bone-cutters, cut away all the aboral wall of the disk except that
immediately around the madreporite and the anus. Now begin at the
tip of each ray and cut away the aboral wall of each, leaving,
however, a single arm intact. When the roof of each arm has been
carefully dissected away the specimen should appear as in fig. 18.
Note the large _alimentary canal_, which is divided into several
regions. Note the short _œsophagus_ leading from the _mouth_ on the
oral surface directly into a large membranous pouch, the _cardiac_
portion of the _stomach_. By a short constriction the cardiac portion
is separated from the part which lies just above, i.e., the _pyloric_
portion of the stomach. From the pyloric portion large, pointed,
paired glandular appendages extend into each ray. These are the
_pyloric cæca_. Their function is digestive, and oftentimes they are
spoken of as the _digestive glands_ or "livers." The pyloric cæca, as
well as the cardiac portion of the stomach, are held in place by
paired muscles which extend into each arm. Note two sets of these
muscles, one set for thrusting the cardiac portion of the stomach out
through the mouth and another for pulling it back, the _protractor
muscles_ and _retractor muscles_, respectively. The starfish obtains
its food by enclosing it in its everted stomach and then withdrawing
stomach and food into the body. Note that the pyloric portion of the
stomach opens above into a short _intestine_ terminating in the
_anus_, and observe that there is attached to the intestine a
convoluted many-branched tube, the _intestinal cæcum_.
Carefully remove a pair of pyloric cæca from one of the rays and note
the short duct which connects them with the pyloric chamber of the
stomach. Note in the angle of each two adjoining rays paired glandular
masses which empty by a common duct on the aboral surface. These
glands are the _reproductive organs_. Note the small bulb-like
bladders extending in two double rows on the floor of each ray. These
are the water-sacs or _ampullæ_, and each one is connected directly
with one of the locomotor organs, the tube-feet.
Make a drawing of the organs in the dissection which have so far been
studied.
TECHNICAL NOTE.--For a careful study of the locomotor organs a fresh
starfish should be injected. This can usually be accomplished by
cutting one ray off squarely, and inserting the needle of a
hypodermic syringe (which has been previously filled with a watery
solution of carmine or Berlin blue), into the end of the radial
water-tube which runs along the floor of the ray. By injecting here,
the whole system of vessels, tube-feet, and ampullæ are filled.
Note a ring-shaped canal which passes around the alimentary canal near
the mouth from which radial vessels run out beneath the floor of each
ray and from which a hard tube extends to the madreporite. This hard
tube is the _stone canal_, so called because its walls contain a series
of calcareous rings, while the circular tube is the _ring canal_ or
_circum-oral water-ring_ from which radiate the _radial canals_. In some
species of starfish there are bladder-like reservoirs, _Polian
vesicles_, which extend interradially from the ring canal.
Note that the ampullæ and tube-feet are all connected with the radial
canals. By a contraction of the delicate muscles in the walls of the
ampullæ the fluid in the cavity is compressed, thereby forcing the
tube-feet out. By the contraction of muscles in the tube-feet they are
again shortened while the small disk-like terminal sucker clings to
some firm object. In this way the animal pulls itself along by
successive "steps." This entire system, called the _water-vascular
system_, is characteristic of the branch Echinodermata. In addition to
the fluid in the water-vascular system there is yet another
body-fluid, the _perivisceral fluid_, which bathes all of the tissues
and fills the body-cavity.
[Illustration: FIG. 19.--Semi-diagrammatic figure of cross-section of
the ray of a starfish, _Asterias_ sp.]
TECHNICAL NOTE.
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