Encyclopaedia Britannica > Volume 3, Anatomy-Astronomy

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AiNIMAL KINGDOM.
171
Imrnal sects. Among many molluscous animals, however, such bark, while serpents and cray-fish cast ofF and reproduce Animal
i ngdom. as the oyster and other bivalves, both sexes are found on their scaly and crustaceous covering. To be produced Kingdom
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the same individual, which is consequently sufficient of it¬
self for the purposes of reproduction, and may be regard¬
ed as a genuine hermaphrodite. This singular order of
things is in fact indispensable to the nature of bivalve
testacea, which, being almost entirely deprived of the
power of locomotion, and destitute of eyes or other organs
by which to distinguish each other, require to possess
within themselves the power of reproduction, lest their
kind should cease. The species is both represented and
continued by a single individual. In the earthworm,
again, the same union of the sexes occurs, but modified
in such a way that the concurrence of two individuals is
required for the continuance of the race, and each acts in
relation to the other both as male and female. This is
also the case with slugs, and a great proportion of uni¬
valve and turbinated shells. Lastly, the zoophytical ani¬
mals are not distinguished by any sex, but are multiplied
by separation or excision of parts of their own bodies.
The law which establishes a perfect distinction and se¬
paration of the sexes in animals seems likewise to produce
a double and symmetrical structure, and is of great extent
in that kingdom ; whilst the circular or radiated form more
especially distinguishes plants, and is also characteristic
of those zoophytical tribes which, both by name and na¬
ture, claim an alliance to the vegetable world.
The grain, and the fruit or kernel, may be said to bear
the same relation to a vegetable as the egg or the embryo
does to an animal,—with this difference to be borne in
mind, that the concurrence of the sexes is necessary to
the formation of the vegetable egg, whereas in the animal
kingdom that circumstance is indispensable only to the
fecundation of the pre-existing germ. The perfection
of a plant, and the ultimate aim of its existence, if we
may use such a phrase to an inanimate structure, con¬
sist in the continuance of its kind. In such as are
named annuals especially, the term of whose existence is
limited in many instances to a small portion of the year,
the ripening of the seeds is speedily effected, and, after a
very brief period, death succeeds “ the bright consum¬
mate flower.” In numerous tribes of insects the same
fleeting existence is observable, though the ephemeral
nature of these last-named tribes is rather apparent than
real, as the wonderful metamorphoses to which they are
subjected conceal their identity from the eye of the un¬
initiated, and greatly interfere with a continuous tracing
of the same individual from the egg to the perfect form.
For example, many aquatic flies, such as the Ephemerae
and others, whose declared and more obvious existence
does not exceed a few hours, have, previous to their as¬
suming the winged state, spent months or even years in
the banks of rivers, and beneath the surface of the stream.
Even the mode of reproduction among the lower tribes
of the animal kingdom bears some analogy to that of ve¬
getables ; and as the vital principle in the smallest branch
or portion of a willow-tree is easily continued and increas¬
ed though separated from the parent stem, so in many
zoophytical animals a bud, branch, or other section, re¬
moved from the full-grown individual, suffers no injury
from such partition, but, on the contrary, acquires almost
immediately a complete and independent power of exist¬
ence within itself, and is ere long capable of exercising or
enduring a like division in favour of posterity. Animals
as well as plants are liaole to be affected by the revolu¬
tions of the seasons; for the period of flowering in the
one class is answered by the season of love in the other,
and the fall of the leaf is only analogous to the periodical
lenewal of the feathers of birds and the hair of qua¬
drupeds. Ihe platanus quits and renews its superficial
and nourished, to increase, to engender, and to die, are
characters common to every class of organized existence ;
but animals properly so called are alone endowed with in¬
stinct and voluntary motion,—they alone possess nerves,
muscles, digestive organs, and blood, with the faculties of
perception, &c. consequent on these attributes.
The preceding observations will serve to illustrate the
principal relations and analogies which exist between
animal and vegetable life. But there are other characters
proper to animals, the whole of which, however, are not
universally bestowed on that class, of which it will now be
proper to say a few words. The substance of which even
the most perfect animal is composed may be resolved
into four tissues,—the cellular, the muscular, the fibrous,
and the medullary.
The cellular tissue is the most extensively bestowed,
and forms, according to an expression of the French phy¬
siologists, the canvass of all the organs, and of every ani¬
mal. It is even common to vegetables, and serves at
once to compose, to unite, and to separate the organs.
Formed of laminae or plates interlacing each other in
every direction, and perforated by small cavities which
have intimate communication, it also sometimes presents
itself under the form of membranes, which, when they
assume a tunnelled or cylindrical shape, are denominated
vessels. It is in this tissue that the gelatine accumulates
for the formation of the cartilages, and the calcareous salts
for that of the bones. It is amongst its meshes that the
fat is amassed, the small vessels distributed, and warmth
developed. It forms the basis of the organs.
The muscular tissue is composed of fibrin, and its chief
property is that of contraction. It forms what are called
the fleshy parts of bodies. Bundles of this tissue crossing
each other compose the heart, and, under another aspect,
form the stomach and intestines. It is the agent of move¬
ment.
The medullary or nervous substance is composed of a
soft albuminous pulp, and is protected by powerful mem¬
branes. It enjoys the admirable faculties of perceiving,
comparing, judging, remembering, and willing; it gives
to the senses their special properties, to the muscles their
moving force, and is the seat of that mysterious union be¬
tween mind and body through which the intellectual fa¬
culties result or become manifested, and the exact nature
of which the most acute of metaphysical inquirers, and
the most accurate and thoughtful observers of nature,
have as yet sought in vain to illustrate. Sensation is the
attribute of the nerves.
The fibrous tissue, the most resistant and unfeeling of
all, fastens the bones to each other, and connects the
bones and the muscles. It forms the ligaments, the ten¬
dons, many vessels, and some resisting membranes em¬
ployed for the protection of the more important organs.
In composition it approaches the cellular tissue, but its
properties are dissimilar. Its character is resistance.
Each of these tissues is destined to the performance of
a special purpose: The cellular organizes, the muscular
moves, the nervous perceives, the fibrous attaches and
resists; but one and all are under the influence of that
nourishing fluid, so different in various animals, known by
the name of blood. This fluid is red, circulating, of a
high temperature, in animals of the superior classes—that
is, the mammalia and birds; less red, colder, and not so
charged with oxygen, in fishes and reptiles; colourless,
but still circulating, in the mollusca; without either co¬
lour or movement in insects; scarcely perceptible in cer¬
tain worms; and apparently wanting in zoophytes. It is
this fluid which animates all the organs, and presides over