ZOOLOGY.
CLASSIFICATION OF ANIMALS—VERTEBRATES AND
INVERTEBRATES—PROTOZOA—HYDROZOA—ACTINOZOA.
Zoology treats of life—of organized beings which are capable of
voluntary motion. Plants exist, animals live and move. Both are organic
beings, but the latter possess the faculty of _will and spontaneous
movement_. The animal can leave a place and enjoy other surroundings,
the plant cannot. We have already crossed the borderland which connects
the plant and the animal. We have seen plants almost animals. We could
commence this section with animals which are almost plants, so closely
do the divisions approach each other. ZOOLOGY is the science of the
knowledge of animals as BOTANY is of the knowledge of plants.
Where there is vegetation there are animals, not quadrupeds or bipeds
necessarily, but numbers of small, it may be invisible, creatures
which exist upon the vegetable kingdom—the algæ and minute creations
of globules and cells, the infusoria already mentioned, the corals,
etc. And in the “protozoa,” or first specimens of animal life, we have
a similarity to the vegetable kingdom; we then get by gradual steps to
other more perfect beings, _each after his kind_, till we arrive at the
most perfect animal—MAN.
[Illustration: Fig. 826.—Echinus, or Sea-Urchin.]
Animals are divided into two families, the INVERTEBRATE and the
VERTEBRATE. The former has no spine nor skeleton; the latter has both.
These again are divided into sub-families, classes, and orders, as
follows.
Man is an animal—but what is an animal? We can scarcely tell in a
few words. Linnæus defined the _difference_ between the animal and
the plant, for the former, said he, live, grow, and feel, while the
latter live and grow. We have protozoa in the animal kingdom consisting
of a single cell or blood corpuscle, some others without mouths or
digestive organs, some have no head; some, as in the tape-worm, only a
so-called head, with suckers or attachments, after which it develops
joints, which are at first imperfect, but gradually mature as they are
pushed farther away by new-issuing joints.
Animals, therefore, do not all possess organs, nor is there any common
organ by which all animals can be classed. The indispensable in one is
absent in another, and while our mouths and digestive apparatus are
all important, in other animals suckers and no digestive apparatus
at all is quite sufficient. Some have one mouth, some several; some
have mouths and a proboscis to assist them, some only the trunk and no
mouth—so called—at all, as in some insects.
[Illustration: Fig. 827.—Polypidom.]
The organisms which could not be distinguished from vegetables were
termed zoophytes, or plant animals, and, were space available, a
comparison might be instituted between the extremes of growth of the
animals and plants, from the largest whales to the tiny microscopic
protozoa, and from the mould upon jam to the gigantic trees of
California, one leaf of which it is said will shelter twenty men from
rain.
Cuvier spent many years in perfecting his systematic arrangements of
animals, and this classification, though many rearrangements have been
made as modern discovery progressed, may be regarded as the fundamental
system of all. Professor Agassiz adopted it with modifications.
Professor Nicholson has made a somewhat different arrangement, but
essentially there will be found but slight difference between them. We
append both these arrangements for comparison:—
AGASSIZ-CUVIER.
INVERTEBRATA.
BRANCH I.—RADIATA.
Class I.—_Polypi_ 2 orders Including actinoids
and halcyonoids.
” II.—_Acalephs_ 3 ” hydroids, discophoræ, ctenophoræ.
” III.—_Echinoderms_ 4 ” crinoids, asteroids, echinoids.
BRANCH II.—MOLLUSCA.
Class I.—_Acephala_ 4 orders bryoza, brachiopods, tunicata,
and lamellibranchiata.
” II.—_Gasteropoda_ 3 ” pipteropoda, heteropoda,
and gasteropoda (proper).
” III.—_Cephalopoda_ 2 ” tetrabranchiata, and dibranchiata.
BRANCH III.—ARTICULATA.
Class I.—_Worms_ 3 orders trematods (including leeches,
etc.), nematoids, and annelides.
” II.—_Crustacea_ 4 ” rotifera, crinopods, tetradecapods,
and decapods.
” III.—_Insects_ 3 ” myriapods, arachnoids, and insects
proper.
BRANCH IV.—VERTEBRATA.
Class I.—_Myzontes_ 2 orders myxinoids and cyclostomes.
” II.—Fishes proper.
” III.—_Ganoids_ 3 ” cælacanths, axipenseroids, and
sauroids.
” IV.—_Selachiens_ 3 ” chimæræ, galeodes, and batides.
” V.—_Amphibians_ 3 ” cæciliæ, ichthyodi, and anoura.
” VI.—_Reptiles_ 4 ” serpents, saurii, rhizodontes,
and testudinata.
” VII.—_Birds_ 4 ” natatores, grallæ, rasores,
and incessores.
” VIII.—_Mammalia_ 3 ” marsupiaia, herbivora,
and carnivora.
In the vertebrated animals the blood is red in consequence of the
minute cells (_corpuscles_) which contain the colouring matter. In
invertebrate animals these red cells are absent, and so the animals are
white-blooded. Some animals, again, are cold-blooded like the fish;
birds and mammalia have warm blood. It is worthy of remark that the
higher we advance in the scale the fewer the offspring of the animal.
The animalcules multiply at the rate of many billions a day, and even
one codfish is stated to contain more than nine millions of eggs. A
mackerel will produce 500,000; and so on, as we rise, we find mammals
with seldom more than ten young at a time, down to one single offspring.
We could fill pages with the account of the differences existing
between animals created for such different purposes and fitted to
inhabit different climates, their mode of feeding and catching prey.
The manner of bringing forth and rearing the young, and the temperament
and temper of the animal creation would fill a volume, but we cannot
now stay to examine these various characteristics. The following is the
arrangement now usually adopted:—
NICHOLSON.
INVERTEBRATES.
PROTOZOA.
_The lowest forms of animal life—microscopic animals._
Gregarinida Cell forms; worm-like.
Rhizopoda Amebæ, foraminifera, radiolaria, sponges.
Infusoria. Suctoria, ciliata, etc.
CÆLENTERATA (THE OLD RADIATA).
HYDROZOA.
_Intermediary, having a mouth and receptacle separated by no alimentary
canal._
Polypes and Zoophytes, Actinozoa. Medusæ, millipores, corals,
sea-anemones, tubipora,
siphonophera, etc.
ECHINODERMATA.
Star-fishes, sea-urchins, sea-cucumbers, and crinoids.
ANNULOSA.
Entozoa Tape-worms, etc.
Rotifera Wheel-Animalculæ.
Annelida Worms, leeches.
Cerripida Barnacles.
Crustacea Shell-fish.
Insecta Various insects.
Arachnida Spiders, scorpions, etc.
MOLLUSCA.
MULLOSCOIDA—POLYZOA.
_Acephala of headless pupa._
Tunicata
Brachiopoda
Lamellibranchiata (bivalves).
_Encephala with head._
Pteropoda.
Gasteropoda (univalves).
Cephalopoda, cuttle-fish, etc.
VERTEBRATA.
ICHTHYOPSIDA.
Class I.—_Fishes_ Various orders.
” II.—_Amphibia_ Frogs, toads, salamanders, etc.
SAUROPSIDA.
” III.—_Reptilia_ Tortoises, smaller snakes, lizards, etc.
” IV.—_Birds_ Various orders.
MAMMALIA.
Class I.—_Non-Placental_ Marsupials.
” II.—_Placental_ Edentata, cetacea, ungulata, quadrumana,
bimana, etc.
We will adopt the latter order as being the more modern, and endeavour
to make the various classes of the invertebrates clear to the mind, if
we cannot present them to the vision, of the reader.
In our sketch of Botany we remarked upon the similarity existing
between the cells of plants and animals, and although there are, of
course, differences, there are many points of resemblance in these
cells.
Plants have their lowest representatives called Protophytes. Animals
which correspond to this class are termed Protozoa, from the Greek,
_proton_, first, and _zoön_, animal. The former are, as already
mentioned, seen amongst the algæ, consisting of simple cells, and
protozoa cannot easily be distinguished from them except in the matter
of nutriment, for some protozoa have no mouth except in the _infusoria_
class. The cells are very much alike, and Dr. Carpenter sums them up
briefly as follows:—
“The animal cell, in its most complete form, is comparable in most
parts of its structure to that of the plant, but differs from it
in the entire absence of the ‘cellulose wall’ or of anything that
represents it, the cell-contents being enclosed in only a single
limitary membrane, the chemical composition of which, being albuminous,
indicates its correspondence with the primordial utricle. In its
young state it seems always to contain a semi-fluid plasma, which is
essentially the same as the protoplasm of the plant, save that it
does not include chlorophyl granules, and this may either continue
to occupy its cavity (which is the case in cells whose entire energy
is directed to growth and multiplication), or may give place, either
wholly or in part, to the special product which it may be the function
of the cell to prepare. Like the vegetable cell, that of animals very
commonly multiplies by duplicative subdivision, it also (especially
among protozoa) may give origin to new cells by the breaking up of its
contents into several particles.”
[Illustration: Fig. 828.—Animalculæ found in stagnant water.
A, Cyclops Quadracornis.
B, Anguillula. Fluviatillis.
C, Actinophrys. Sol.
D, Coleps Hirtus.
E, Vorticella.
F, Ambœba princeps.
G, Acineta mystacina.
H, Oxytrycha.
I, Triophthalamus dorsalis.
J, Polyarthra.]
The protozoa are microscopic creatures consisting of one or more cells,
and are infinitely little, thousands existing in a drop of water.
They have no distinction of sexes, and their generation takes place
by subdivision or blending of cells. The infusoria are the highest of
the protozoa, and were formerly included amongst the radiata. Their
numbers are infinite, and in a drop of water (_see_ fig. 828) some
very interesting specimens will be found. These infusoria are merely
_sarcode_, or a jelly-like substance, and some have cilia, or hairy
appendages, with which they agitate the water and cause a kind of
current which brings them food. It is this partaking of food which
has served to divide the lowest animal from the lowest vegetable
creations. There is no progressive increase of development from the
lowest plant to the highest animal. The animal begins by himself, as
it were, as the plant, and both grow up in different directions. The
protozoa exist upon organic substances, while plants absorb inorganic
substances and assimilate them.
The _Gregarinidæ_ are very tiny cells, and though microscopically
minute, they sometimes develop into worm-like or elongated oval bodies.
They inhabit the intestines of crustacea, worms, and cockroaches, as
well as of higher animals. They are capable of certain motion, but
are not furnished even with the “false feet” (_pseudo podia_) of the
rhizopoda, the next animal in these very low scales of creation.
Of the Rhizopoda the AMEBÆ are very interesting, and we find them in
our veins as well as in the stagnant green water of the pond. They
are simply _sarcode_ or jelly, and, as the name implies, the amebæ
can change their appearance (_amoibos_, changing). They possess a
kind of crawling, progressive motion, and under the microscope will
be perceived to develop a tiny bud, as it were, which is the “false
foot” that assists its progress. These amebæ are in our blood moving
about, and are always altering their form, and when warm they move more
quickly in the red blood corpuscles or cells, but excessive heat will
kill them.
These curious creatures feed by the foot they protrude; and by drawing
in the “process” as it is termed, they can collect within themselves
the nourishment they require. Of course they have no mouth, and if we
can conceive a creature of this kind which thrusts out from a jelly-bag
a tiny lump, and pulls it in again at any time and place it likes, we
have an idea of an ameba.
The pond ameba is somewhat different from the others, inasmuch as it
possesses an outer and inner portion or layer which are different in
density. There is what is termed a contractile vesicle which “beats” as
a heart beats, but this is very primitive. There is really no structure
whatever in these rhizopoda, and, as we have seen, their shape is
always undergoing change. The outer and inner layers of the amebæ are
called “the ectosarc” and “endosarc” respectively; the latter contains
the darker portion—the nucleus.
The _Foraminifera_ have already been mentioned in the chapters on
Geology. We find these minute creatures must have had a great deal
to do with the building up of rocks, as they have the power to make
tiny coverings for themselves, which have been built into rocks by the
addition of sandy particles, and consolidated by pressure. Here we
have a most wonderful instance of the tiniest creatures producing the
greatest masses of the earth. The body is merely _sarcode_, the shell
is carbonate of lime. The foraminifera produce false feet in abundance,
which surround the cell like fine hairs or rays. They live in the sea,
and when they die the shells descend upon the ocean floor, where they
undergo many changes and become converted into rock. The ooze of the
great oceans is composed of these shells, and is practically a chalky
deposit; the shells are being built up as in former ages with the
curious nummulites of the Eocene formation, which are amongst the most
interesting of fossils.
SPONGES. We must go on at once to the _Sponges_, which form such an
interesting subject as they are so familiar to us all. Sponge is
not often regarded by the public as an animal, and though perhaps
authorities may not have yet concluded in what category they should be
placed, we may consider them here according to the list.
We find the spongida both in fresh and salt water, and they have
given rise to much discussion as to whether they should be classed as
animals at all. But that question having been finally settled, we can
proceed to examine a sponge in its native state, and we shall find both
skeleton and “flesh.”
The skeleton is hard and composed of needles of “tiny” texture. The
flesh is “sarcode,” and the animal possesses no mouth, but is full
of holes (pores) and canals through which the water is continually
distributed. The outer layer of the sponge is formed of ultimate
components of the living substance of the sponge (like the amebæ we
have been considering). Each contains a nucleus, and when joined
together form the outer layer of the body. Beneath is a wide cavity
communicating with the exterior by means of minute holes, and filled
with water. The cavity separates the superficial layer from the deeper
substance, which is of the same character. In the water passages of the
sponge are cilia which induce a cement, and the interior canals develop
into chambers lined with sponge particles, and the water carries
particles to the sponge, which represents a kind of sub-aqueous city,
where the people are arranged about the streets and roads in such a
manner that each can easily appropriate his food from the water as it
passes along.[38]
[Illustration: Fig. 829.—Fragment of sponge (magnified).]
Sponge, then, is a mass of living organisms—tiny living creatures
capable of feeding and of movement, resembling amebæ or perhaps
infusoria, with cilia, to enable them to obtain nourishment by a
kind of inhalation or respiration. They are reproductive by sexual
and a-sexual processes which produce _spongellæ_. The living sponge
is a beautifully coloured animal, and grows upon almost any solid
foundation; and in the autumn the parent sponge displays a number of
yellow dots or “gemmules,” which are the young. These are soon cast off
and left to shift for themselves, and seek their fortunes, helpless
as they appear, in the wide and stormy sea. At last they find a
resting-place, and fix themselves for ever, growing up and reproducing
their species until they are carried off to be sold and used in
civilized countries for domestic purposes.
We must leave these curious animalculæ and glance at the INFUSORIA,
which constitute a higher branch, but microscopic and universal, and
include those called Flagellate, Ciliate, and Tentaculate. The first
have whip-like cilia, or feelers, or filaments, which are ever in
motion to cause a movement of the water and carry food to the animal.
You will find plenty of infusoria in any stagnant water, and when
placed under the microscope a mouth may be perceived, but no stomach,
nor any apparent receptacle for food, which appears to enter at once
into the body substance. The other kinds capture their food by seizure
by the tentacles, or by agitating the cilia, like the flagellata, and
thus whipping the nourishment towards the mouth, as children will draw
in a toy boat to land by agitating the water in the given direction.
These cilia, or hairs, serve for organs of locomotion as well as of
capture. These creatures are called Infusoria, because they exist in
vegetable “infusions” exposed to the atmosphere.
[Illustration: Fig. 830.—Structure of polypidoms.]
Decaying vegetable or animal substances, such as the leaves of trees,
grass, a piece of flesh, etc., affused with water and exposed to air
and warmth, will speedily, upon microscopic examination, be found
peopled with numbers of most active minute creatures of the most varied
forms. These animalcules are found also in the stagnant pools around
our cities, in the waters of rivers, harbours, and lakes, and even in
the ocean.
In reference to the origin of these animalcules, the view was long
entertained that they were generated spontaneously, that the decaying
vegetable and animal substances were decomposed and resolved into
these simple beings. More accurate experiments have shown, however,
that the infusoria are produced from ova, or germs, which are probably
carried about in the dried-up state, in the form of minute particles of
dust,[39] ready to develop themselves in any spot which may afford them
the requisite moisture and nutriment. In this respect they resemble
the microscopic fungi, whose germs are diffused in the same way. When
once they have obtained the means of development, they multiply with
incredible celerity. If the decaying vegetable or animal substances be
carefully excluded from contact with the air, or if the air be heated
before it is admitted to them, no infusoria will appear. They are
rarely developed on mountains of a certain height, where the atmosphere
is free from foreign bodies.
Though these animalcules are so exceedingly minute, yet the forms
exhibited by them are extremely various, and some of them present also
considerable variety in the forms assumed by the same individual under
different circumstances. In many species the soft body is enclosed in a
firm integument, strengthened by a deposit of siliceous matter; these
envelopes, which are often preserved after the death of the animals,
are termed the _shields_, and the animalcules encased in them are
called _loricated_ infusoria. The remarkable discovery has been made
that large distinct beds of former formations are entirely made up of
the accumulated remains of these animalcules.
[Illustration: Fig. 831.—Volvox globator.]
We arrive at the HYDROZOA after leaving the Infusoria, and find
ourselves in the sea, and far from land, where it will be difficult
for us to ascertain the characteristics of these interesting animals.
But fortunately we can obtain much nearer home, and occasionally in
a private aquarium, a specimen of the hydrozoa which will serve our
purpose, as it has served before to introduce readers to the study of
these water-polypes, some of which are so like plants that they are
frequently mistaken for them.
The hydrozoa present a “definite histological structure,” says
Professor Huxley; “the body always exhibits a separation into at least
two distinct layers of tissue, an outer and an inner.” The Hydras, or
fresh-water polypes, which may be found in nearly every pond adhering
to the duckweed, appears like tubes, and if touched will curl up into
tiny knobs. But if let alone they will adhere to a glass by their
single foot, or sucker, which can be moved at pleasure.
[Illustration: Fig. 832.—The hydra.]
The foot, or sucker, is continued to a slender cylindrical stalk, from
the end of which radiate a number of tentacles, or “feelers,” growing
around the mouth, and serving to convey or attract food to the animal
which is, so to speak, all stomach. There is no breathing apparatus,
and what food it cannot digest is expelled from the mouth. The
peculiarity which has given the hydra its name is, that no matter into
how many pieces you cut this polype, the parts cut off will all develop
into little polypes perfect as their parent.[40] But germination is
carried on naturally by buds thrown out, and cast (by “gemmation”), or
by the ordinary sexual production of ova.
The outer and inner skins of the hydra are called the ectoderm and
endoderm, and the animal is quite capable of locomotion, walking,
or rather moving, backwards, by raising and planting its sucker or
foot, and by swimming. The prey is captured by the tentacles and by
the darting out of tiny spears from the cells or “thread cells” which
contain them on the surface of the body. The well-known “Portuguese
man-of-war,” an ocean polype, has these “harpoons” greatly developed,
and can inflict serious pain as of many stinging nettles; the sensation
is exceedingly painful, and lasts some time.
The MEDUSIDÆ are known to the seaside visitor as the jelly-fish, and
the other _Acalephæ_, the “hidden-eyed” medusæ, include the Portuguese
man-of-war mentioned above, and many other umbrella-like animals. They
have received the name of medusæ from Medusa, whose long, snaky locks
the tentacles of the animals are supposed to be like. Some of these
“floating umbrellas” are very dangerous, and will inflict severe stings
upon any one in their vicinity. The tentacles or filaments extend for
a long distance, and bathers should be cautious. We have often watched
them, and they are beautiful to contemplate particularly at night, and
in Kingstown Harbour, near Dublin, many exceedingly fine specimens have
been obtained. The pulsation of the “umbrella” or bell, enables the
animal to swim, and the even undulations of this beautiful covering are
apparently caused by nervous contractions.
The jelly-fish have no resemblance to “fish,” and scarcely appear to
exist; they are of no use to man, and when removed from the water
dwindle by little and little to a tiny film and nothing more—they
dissolve into air and water. Cases have been known and tales told of
how farmers collected hundreds of these jelly-fish for manure, and when
the cart reached the field, to the man’s astonishment, nothing was left
but what appeared cobweb in the place of the load of fish.
[Illustration: Fig. 833.—Medusa.]
The _Cyclippe_ is a very common specimen, and moves by means of its
cilia; _Cestum Veneris_—the zone or girdle of Venus—is another curious
example. It appears like a glass ribbon about five inches wide and
perhaps four or five feet long. The cilia when in motion are very
brilliant in colouring, and the creature undulates through the water
in a remarkable manner.
The luminosity of the medusæ is clearly perceived, the so-called
phosphorescence being due chiefly to the minute jelly-fish which
abound near the surface of the sea. It appears impossible, for most,
at any rate, if not all, these medusæ to sink beneath the surface, for
they can be found in hundreds cast ashore, melting away into film. We
might imagine that they would be provided with some means of sinking
themselves, but being apparently only air and water, it is necessary
for them to remain upon the surface to exist at all.
[Illustration: Fig. 834.—Sea cucumber.]
The term _Acalephæ_, by which they are known, means “stinging” fish or
sea-nettles, the Greek word meaning nettle.
The ACTINOZOA comprise corals and the popular sea anemones (actinidæ).
They resemble the hydrozoa in possessing tentacles, and also the two
inner and outer tissues of the body. But they differ from the hydrozoa
in their interior arrangement in the possession of a kind of stomach
between the “body cavity” and the mouth which the hydrozoa do not
possess. The appearance of the sea anemone is well known. It fixes
itself by the flat base and hangs out its tentacles to obtain food.
When we touch an anemone with a stick we perceive how it contracts
itself, but there is no nervous system nor any respiration. The
reproduction of its species is carried on within, not as in other
animals, like the hydra, by exterior budding.
[Illustration: Fig. 835.—Coral.]
The corals belong to the same class as the sea anemones, and are
called zoanthidæ. We have already in previous portions of this volume
mentioned the “coral” building polypes, but we may again describe them
here. We have the black coral or _antipathidæ_, which live in masses
and are united by a stem. They grow upon this fleshy trunk and cover
it in time “just as a trunk of a tree is covered by the bark.” This
stem is called a _cænosarc_, which secretes the coral, or skeleton. The
_madrepores_ are the greatest producers of the coral of commerce.
[Illustration: Fig. 836.—Coral.]
“If we examine a simple coral of this group,” says Professor Nicholson,
“we find that we have to deal with an animal in all important respects
identical with an ordinary sea anemone, but having a more or less
complicated skeleton developed in its interior.” This skeleton is the
corallum, and it is composed, as most people are aware, of calcareous
matter deposited within the polype itself; in the former case the
development or formation is exterior to the polype. A single polype
will thus secrete a deposit, and a colony of them produce a compound
skeleton, and as they throw out buds or young polypes, the manufacture
of skeletons goes on by secretion.
The _Tubipores_ are like pipes, and the coral has been termed the
“organ-pipe.” It is formed cylindrically and joined externally. As
under GEOLOGY we have examined the question of coral reefs, we need not
here recapitulate the descriptions given in that section.
[Illustration: Fig. 837.—Coral.]
Doctor Bariel writes of these animals as follows:—“By far the greater
part of the Zoanthoid polypes, as they grow, deposit in the cellular
substance of the flesh of their back an immense quantity of calcareous
matter which enlarges as the animal increases in size, and, in fact,
fills up those portions of the substance of the animal, which by the
growth of new parts are no longer wanted for its nourishment, and in
this manner they form a hard and strong case, amongst the folds of
which they contract themselves so as to be protected from external
injury, and by the same means they form for themselves a permanent
attachment which prevents their being tossed about by every wave of the
element in which they live. The stony substances so formed are called
corals, and their mode of formation causes them exactly to represent
the animal which secretes them. The upper surface is always furnished
with radiating plates, the remains of the calcareous particles which
are deposited in the longitudinal folds of the stomach.”
FOOTNOTES:
[38] Huxley. “Classification of Animals.”
[39] That the presence of millions of such ova in the air should not
be detected, will appear very natural indeed, if we reflect that the
animalcules are only 1/1500 to 1/2000 part of a line in diameter, and
that the ova are a thousand times smaller.
[40] Hercules conquered the “Hydra,” which is represented in mythology
as capable of reproducing two heads for each one cut off by the
warrior.