necessarily be of a corresponding symmetry. For the spherical shape
has not the asymmetry of the disk-shaped body of the oysters. For in
all these animals the head is central, but in the sea-urchin the
so-called ovum is above [and symmetrical, while in the oyster it is
only one side]. Now the necessary symmetry would be observed were
the ovum to form a continuous ring. But this may not be. For it
would be in opposition to what prevails in the whole tribe of
Testacea; for in all the ovum is discontinuous, and in all excepting
the sea-urchins asymmetrical, being placed only on one side of the
body. Owing then to this necessary discontinuity of the ovum, which
belongs to the sea-urchin as a member of the class, and owing to the
spherical shape of its body, which is its individual peculiarity, this
animal cannot possibly have an even number of ova. For were they an
even number, they would have to be arranged exactly opposite to each
other, in pairs, so as to keep the necessary symmetry; one ovum of
each pair being placed at one end, the other ovum at the other end
of a transverse diameter. This again would violate the universal
provision in Testacea. For both in the oysters and in the scallops
we find the ovum only on one side of the circumference. The number
then of the ova must be uneven, three for instance, or five. But if
there were only three they would be much too far apart; while, if
there were more than five, they would come to form a continuous
mass. The former arrangement would be disadvantageous to the animal,
the latter an impossibility. There can therefore be neither more nor
less than five. For the same reason the stomach is divided into five
parts, and there is a corresponding number of teeth. For seeing that
the ova represent each of them a kind of body for the animal, their
disposition must conform to that of the stomach, seeing that it is
from this that they derive the material for their growth. Now if there
were only one stomach, either the ova would be too far off from it, or
it would be so big as to fill up the whole cavity, and the
sea-urchin would have great difficulty in moving about and finding due
nourishment for its repletion. As then there are five intervals
between the five ova, so are there of necessity five divisions of
the stomach, one for each interval. So also, and on like grounds,
there are five teeth. For nature is thus enabled to allot to each
stomachal compartment and ovum its separate and similar tooth.
These, then, are the reasons why the number of ova in the sea-urchin
is an odd one, and why that odd number is five. In some sea-urchins
the ova are excessively small, in others of considerable size, the
explanation being that the latter are of a warmer constitution, and so
are able to concoct their food more thoroughly; while in the former
concoction is less perfect, so that the stomach is found full of
residual matter, while the ova are small and uneatable. Those of a
warmer constitution are, moreover, in virtue of their warmth more
given to motion, so that they make expeditions in search of food,
instead of remaining stationary like the rest. As evidence of this, it
will be found that they always have something or other sticking to
their spines, as though they moved much about; for they use their
spines as feet.
The Ascidians differ but slightly from plants, and yet have more
of an animal nature than the sponges, which are virtually plants and
nothing more. For nature passes from lifeless objects to animals in
such unbroken sequence, interposing between them beings which live and
yet are not animals, that scarcely any difference seems to exist
between two neighbouring groups owing to their close proximity.
A sponge, then, as already said, in these respects completely
resembles a plant, that throughout its life it is attached to a
rock, and that when separated from this it dies. Slightly different
from the sponges are the so-called Holothurias and the sea-lungs, as
also sundry other sea-animals that resemble them. For these are free
and unattached. Yet they have no feeling, and their life is simply
that of a plant separated from the ground. For even among
land-plants there are some that are independent of the soil, and
that spring up and grow, either upon other plants, or even entirely
free. Such, for example, is the plant which is found on Parnassus, and
which some call the Epipetrum. This you may hang up on a peg and it
will yet live for a considerable time. Sometimes it is a matter of
doubt whether a given organism should be classed with plants or with
animals. The Ascidians, for instance, and the like so far resemble
plants as that they never live free and unattached, but, on the
other hand, inasmuch as they have a certain flesh-like substance, they
must be supposed to possess some degree of sensibility.
An Ascidian has a body divided by a single septum and with two
orifices, one where it takes in the fluid matter that ministers to its
nutrition, the other where it discharges the surplus of unused
juice, for it has no visible residual substance, such as have the
other Testacea. This is itself a very strong justification for
considering an Ascidian, and anything else there may be among
animals that resembles it, to be of a vegetable character; for
plants also never have any residuum. Across the middle of the body
of these Ascidians there runs a thin transverse partition, and here it
is that we may reasonably suppose the part on which life depends to be
situated.
The Acalephae, or Sea-nettles, as they are variously called, are not
Testacea at all, but lie outside the recognized groups. Their
constitution, like that of the Ascidians, approximates them on one
side to plants, on the other to animals. For seeing that some of
them can detach themselves and can fasten upon their food, and that
they are sensible of objects which come in contact with them, they
must be considered to have an animal nature. The like conclusion
follows from their using the asperity of their bodies as a
protection against their enemies. But, on the other hand, they are
closely allied to plants, firstly by the imperfection of their
structure, secondly by their being able to attach themselves to the
rocks, which they do with great rapidity, and lastly by their having
no visible residuum notwithstanding that they possess a mouth.
Very similar again to the Acalephae are the Starfishes. For these
also fasten on their prey, and suck out its juices, and thus destroy a
vast number of oysters. At the same time they present a certain
resemblance to such of the animals we have described as the
Cephalopoda and Crustacea, inasmuch as they are free and unattached.
The same may also be said of the Testacea.
Such, then, is the structure of the parts that minister to nutrition
and which every animal must possess. But besides these organs it is
quite plain that in every animal there must be some part or other
which shall be analogous to what i
n sanguineous animals is the
presiding seat of sensation. Whether an animal has or has not blood,
it cannot possibly be without this. In the Cephalopoda this part
consists of a fluid substance contained in a membrane, through which
runs the gullet on its way to the stomach. It is attached to the
body rather towards its dorsal surface, and by some is called the
mytis. Just such another organ is found also in the Crustacea and
there too is known by the same name. This part is at once fluid and
corporeal and, as before said, is traversed by the gullet. For had the
gullet been placed between the mytis and the dorsal surface of the
animal, the hardness of the back would have interfered with its due
dilatation in the act of deglutition. On the outer surface of the
mytis runs the intestine; and in contact with this latter is placed
the ink-bag, so that it may be removed as far as possible from the
mouth and its obnoxious fluid be kept at a distance from the nobler
and sovereign part. The position of the mytis shows that it
corresponds to the heart of sanguineous animals; for it occupies the
self-same place. The same is shown by the sweetness of its fluid,
which has the character of concocted matter and resembles blood.
In the Testacea the presiding seat of sensation is in a
corresponding position, but is less easily made out. It should,
however, always be looked for in some midway position; namely, in such
Testacea as are stationary, midway between the part by which food is
taken in and the channel through which either the excrement or the
spermatic fluid is voided, and, in those species which are capable
of locomotion, invariably midway between the right and left sides.
In Insects this organ, which is the seat of sensation, lies, as
was stated in the first treatise, between the head and the cavity
which contains the stomach. In most of them it consists of a single
part; but in others, for instance in such as have long bodies and
resemble the Juli (Millipedes), it is made up of several parts, so
that such insects continue to live after they have been cut in pieces.
For the aim of nature is to give to each animal only one such dominant
part; and when she is unable to carry out this intention she causes
the parts, though potentially many, to work together actually as
one. This is much more clearly marked in some insects than in others.
The parts concerned in nutrition are not alike in all insects, but
show considerable diversity. Thus some have what is called a sting
in the mouth, which is a kind of compound instrument that combines
in itself the character of a tongue and of lips. In others that have
no such instrument in front there is a part inside the mouth that
answers the same sensory purposes. Immediately after the mouth comes
the intestine, which is never wanting in any insect. This runs in a
straight line and without further complication to the vent;
occasionally, however, it has a spiral coil. There are, moreover, some
insects in which a stomach succeeds to the mouth, and is itself
succeeded by a convoluted intestine, so that the larger and more
voracious insects may be enabled to take in a more abundant supply
of food. More curious than any are the Cicadae. For here the mouth and
the tongue are united so as to form a single part, through which, as
through a root, the insect sucks up the fluids on which it lives.
Insects are always small eaters, not so much because of their
diminutive size as because of their cold temperament. For it is heat
which requires sustenance; just as it is heat which speedily
concocts it. But cold requires no sustenance. In no insects is this so
conspicuous as in these Cicadae. For they find enough to live on in
the moisture which is deposited from the air. So also do the
Ephemera that are found about the Black sea. But while these latter
only live for a single day, the Cicadae subsist on such food for
several days, though still not many.
We have now done with the internal parts of animals, and must
therefore return to the consideration of the external parts which have
not yet been described. It will be better to change our order of
exposition and begin with the animals we have just been describing, so
that proceeding from these, which require less discussion, our account
may have more time to spend on the perfect kinds of animals, those
namely that have blood.
6
We will begin with Insects. These animals, though they present no
great multiplicity of parts, are not without diversities when compared
with each other. They are all manyfooted; the object of this being
to compensate their natural slowness and frigidity, and give greater
activity to their motions. Accordingly we find that those which, as
the (Millipedes), have long bodies, and are therefore the most
liable to refrigeration, have also the greatest number of feet. Again,
the body in these animals is insected-the reason for this being that
they have not got one vital centre but many-and the number of their
feet corresponds to that of the insections.
Should the feet fall short of this, their deficiency is
compensated by the power of flight. Of such flying insects some live a
wandering life, and are forced to make long expeditions in search of
food. These have a body of light weight, and four feathers, two on
either side, to support it. Such are bees and the insects akin to
them. When, however, such insects are of very small bulk, their
feathers are reduced to two, as is the case with flies. Insects with
heavy bodies and of stationary habits, though not polypterous in the
same way as bees, yet have sheaths to their feathers to maintain their
efficiency. Such are the Melolonthae and the like. For their
stationary habits expose their feathers to much greater risks than are
run by those of insects that are more constantly in flight, and on
this account they are provided with this protecting shield. The
feather of an insect has neither barbs nor shaft. For, though it is
called a feather, it is no feather at all, but merely a skin-like
membrane that, owing to its dryness, necessarily becomes detached from
the surface of the body, as the fleshy substance grows cold.
These animals then have their bodies insected, not only for the
reasons already assigned, but also to enable them to curl round in
such a manner as may protect them from injury; for such insects as
have long bodies can roll themselves up, which would be impossible
were it not for the insections; and those that cannot do this can
yet draw their segments up into the insected spaces, and so increase
the hardness of their bodies. This can be felt quite plainly by
putting the finger on one of the insects, for instance, known as
Canthari. The touch frightens the insect, and it remains motionless,
while its body becomes hard. The division of the body into segments is
also a necessary result of there being several supreme organs in place
of one; and this again is a part of the essential constitution of
inse
cts, and is a character which approximates them to plants. For
as plants, though cut into pieces, can still live, so also can
insects. There is, however, this difference between the two cases,
that the portions of the divided insect live only for a limited
time, whereas the portions of the plant live on and attain the perfect
form of the whole, so that from one single plant you may obtain two or
more.
Some insects are also provided with another means of protection
against their enemies, namely a sting. In some this is in front,
connected with the tongue, in others behind at the posterior end.
For just as the organ of smell in elephants answers several uses,
serving alike as a weapon and for purposes of nutrition, so does
also the sting, when placed in connexion with the tongue, as in some
insects, answer more than one end. For it is the instrument through
which they derive their sensations of food, as well as that with which
they suck it up and bring it to the mouth. Such of these insects as
have no anterior sting are provided with teeth, which serve in some of
them for biting the food, and in others for its prehension and
conveyance to the mouth. Such are their uses, for instance, in ants
and all the various kinds of bees. As for the insects that have a
sting behind, this weapon is given them because they are of a fierce
disposition. In some of them the sting is lodged inside the body, in
bees, for example, and wasps. For these insects are made for flight,
and were their sting external and of delicate make it would soon get
spoiled; and if, on the other hand, it were of thicker build, as in
scorpions, its weight would be an incumbrance. As for scorpions that
live on the ground and have a tail, their sting must be set upon this,
as otherwise it would be of no use as a weapon. Dipterous insects
never have a posterior sting. For the very reason of their being
dipterous is that they are small and weak, and therefore require no
more than two feathers to support their light weight; and the same
reason which reduces their feathers to two causes their sting to be in
front; for their strength is not sufficient to allow them to strike