it is used by nature for an end, namely, to facilitate and to hasten
the concoction of food. For all that is hot aids concoction; and fat
is hot, and the omentum is fat. This too explains why it hangs from
the middle of the stomach; for the upper part of the stomach has no
need of it, being assisted in concoction by the adjacent liver. Thus
much as concerns the omentum.
4
The so-called mesentery is also a membrane; and extends continuously
from the long stretch of intestine to the great vessel and the
aorta. In it are numerous and close-packed vessels, which run from the
intestines to the great vessel and to the aorta. The formation of this
membrane we shall find to be the result of necessity, as is that of
the other [similar] parts. What, however, is the final cause of its
existence in sanguineous animals is manifest on reflection. For it
is necessary that animals shall get nutriment from without; and,
again, that this shall be converted into the ultimate nutriment, which
is then distributed as sustenance to the various parts; this
ultimate nutriment being, in sanguineous animals, what we call
blood, and having, in bloodless animals, no definite name. This
being so, there must be channels through which the nutriment shall
pass, as it were through roots, from the stomach into the
blood-vessels. Now the roots of plants are in the ground; for thence
their nutriment is derived. But in animals the stomach and
intestines represent the ground from which the nutriment is to be
taken. The mesentery, then, is an organ to contain the roots; and
these roots are the vessels that traverse it. This then is the final
cause of its existence. But how it absorbs nutriment, and how that
portion of the food which enters into the vessels is distributed by
them to the various parts of the body, are questions which will be
considered when we come to deal with the generation and nutrition of
animals.
The constitution of sanguineous animals, so far as the parts as
yet mentioned are concerned, and the reasons for such constitution,
have now been set forth. In natural sequence we should next go on to
the organs of generation, as yet undescribed, on which depend the
distinctions of male and female. But, inasmuch as we shall have to
deal specially with generation hereafter, it will be more convenient
to defer the consideration of these parts to that occasion.
5
Very different from the animals we have as yet considered are the
Cephalopoda and the Crustacea. For these have absolutely no viscera
whatsoever; as is indeed the case with all bloodless animals, in which
are included two other genera, namely the Testacea and the Insects.
For in none of them does the material out of which viscera are
formed exist. None of them, that is, have blood. The cause of this
lies in their essential constitution. For the presence of blood in
some animals, its absence from others, must be included in the
conception which determines their respective essences. Moreover, in
the animals we are now considering, none of those final causes will be
found to exist which in sanguineous animals determine the presence
of viscera. For they have no blood vessels nor urinary bladder, nor do
they breathe; the only part that it is necessary for them to have
being that which is analogous to a heart. For in all animals there
must be some central and commanding part of the body, to lodge the
sensory portion of the soul and the source of life. The organs of
nutrition are also of necessity present in them all. They differ,
however, in character because of differences of the habitats in
which they get their subsistence.
In the Cephalopoda there are two teeth, enclosing what is called the
mouth; and inside this mouth is a flesh-like substance which
represents a tongue and serves for the discrimination of pleasant
and unpleasant food. The Crustacea have teeth corresponding to those
of the Cephalopoda, namely their anterior teeth, and also have the
fleshy representative of a tongue. This latter part is found,
moreover, in all Testacea, and serves, as in sanguineous animals,
for gustatory sensations. Similarly provided also are the Insects. For
some of these, such as the Bees and the Flies, have, as already
described, their proboscis protruding from the mouth; while those
others that have no such instrument in front have a part which acts as
a tongue inside the mouth. Such, for instance, is the case in the Ants
and the like. As for teeth, some insects have them, the Bees and the
Ants for instance, though in a somewhat modified form, while others
that live on fluid nutriment are without them. For in many insects the
teeth are not meant to deal with the food, but to serve as weapons.
In some Testacea, as was said in the first treatise, the organ which
is called the tongue is of considerable strength; and in the Cochli
(Sea-snails) there are also two teeth, just as in the Crustacea. The
mouth in the Cephalopoda is succeeded by a long gullet. This leads
to a crop, like that of a bird, and directly continuous with this is
the stomach, from which a gut runs without windings to the vent. The
Sepias and the Poulps resemble each other completely, so far as
regards the shape and consistency of these parts. But not so the
Teuthides (Calamaries). Here, as in the other groups there are the two
stomach-like receptacles; but the first of these cavities has less
resemblance to a crop, and in neither is the form [or the consistency]
the same as in the other kinds, the whole body indeed being made of
a softer kind of flesh.
The object of this arrangement of the parts in question is the
same in the Cephalopoda as in Birds; for these also are all unable
to masticate their food; and therefore it is that a crop precedes
their stomach.
For purposes of defence, and to enable them to escape from their
foes, the Cephalopoda have what is called their ink. This is contained
in a membranous pouch, which is attached to the body and provided with
a terminal outlet just at the point where what is termed the funnel
gives issue to the residua of the stomach. This funnel is placed on
the ventral surface of the animal. All Cephalopoda alike have this
characteristic ink, but chief of all the Sepia, where it is more
abundant than in the rest. When the animal is disturbed and frightened
it uses this ink to make the surrounding water black and turbid, and
so, as it were, puts a shield in front of its body.
In the Calamaries and the Poulps the ink-bag is placed in the
upper part of the body, in close proximity to the mytis, whereas in
the Sepia it is lower down, against the stomach. For the Sepia has a
more plentiful supply of ink than the rest, inasmuch as it makes
more use of it. The reasons for this are, firstly, that it lives
near the shore, and, secondly, that it has no other means of
protection; whereas the Poulp has its long twining feet to use in
its defence, and is, moreover, endowed with the power of changing
&
nbsp; colour. This changing of colour, like the discharge of ink, occurs
as the result of fright. As to the Calamary, it lives far out at
sea, being the only one of the Cephalopoda that does so; and this
gives it protection. These then are the reasons why the ink is more
abundant in the Sepia than in the Calamary, and this greater abundance
explains the lower position; for it allows the ink to be ejected
with ease even from a distance. The ink itself is of an earthy
character, in this resembling the white deposit on the surface of a
bird's excrement and the explanation in both cases is the same,
namely, the absence of a urinary bladder. For, in default of this,
it is the ink that serves for the excretion of the earthiest matter.
And this is more especially the case in the Sepia, because there is
a greater proportion of earth in its composition than in that of the
other Cephalopoda. The earthy character of its bone is a clear
indication of this. For in the Poulp there is no bone at all, and in
the Calamary it is thin and cartilaginous. Why this bone should be
present in some Cephalopoda, and wanting in others, and how its
character varies in those that have it, has now been set forth.
These animals, having no blood, are in consequence cold and of a
timid character. Now, in some animals, fear causes a disturbance of
the bowels, and, in others, a flow of urine from the bladder.
Similarly in these it produces a discharge of ink, and, though the
ejection of this ink in fright, like that of the urine, is the
result of necessity, and, though it is of excremental character, yet
it is used by nature for a purpose, namely, the protection and
safety of the animal that excretes it.
The Crustacea also, both the Caraboid forms and the Crabs, are
provided with teeth, namely their two anterior teeth; and between
these they also present the tongue-like piece of flesh, as has
indeed been already mentioned. Directly after their mouth comes a
gullet, which, if we compare relative sizes, is but small in
proportion to the body: and then a stomach, which in the Carabi and
some of the Crabs is furnished with a second set of teeth, the
anterior teeth being insufficient for adequate mastication. From the
stomach a uniform gut runs in a direct line to the excremental vent.
The parts described are to be found also in all the various
Testacea. The degree of distinctness, however, with which they are
formed varies in the different kinds, and the larger the size of the
animal the more easily distinguishable are all these parts
severally. In the Sea-snails, for example, we find teeth, hard and
sharp, as before mentioned, and between them the flesh-like substance,
just as in the Crustacea and Cephalopoda, and again the proboscis,
which, as has been stated, is something between a sting and a
tongue. Directly after the mouth comes a kind of bird-like crop,
then a gullet, succeeded by a stomach, in which is the mecon, as it is
styled; and continuous with this mecon is an intestine, starting
directly from it. It is this residual substance which appears in all
the Testacea to form the most palatable morsel. Purpuras and Whelks,
and all other Testacea that have turbinate shells, in structure
resemble the Sea-snail. The genera and species of Testacea are very
numerous. For there are those with turbinate shells, of which some
have just been mentioned; and, besides these, there are bivalves and
univalves. Those with turbinate shells may, indeed, after a certain
fashion be said to resemble bivalves. For they all from their very
birth have an operculum to protect that part of their body which is
exposed to view. This is the case with the Purpuras, with Whelks, with
the Nerites, and the like. Were it not for this, the part which is
undefended by the shell would be very liable to injury by collision
with external objects. The univalves also are not without
protection. For on their dorsal surface they have a shell, and by
the under surface they attach themselves to the rocks, and so after
a manner become bivalved, the rock representing the second valve. Of
these the animals known as Limpets are an example. The bivalves,
scallops and mussels, for instance, are protected by the power they
have of closing their valves; and the Turbinata by the operculum
just mentioned, which transforms them, as it were, crom univalves into
bivalves. But of all there is none so perfectly protected as the
sea-urchin. For here there is a globular shell which encloses the body
completely, and which is, moreover, set with sharp spines. This
peculiarity distinguishes the sea-urchin from all other Testacea, as
has already been mentioned.
The structure of the Testacea and of the Crustacea is exactly the
reverse of that of the Cephalopoda. For in the latter the fleshy
substance is on the outside and the earthy substance within, whereas
in the former the soft parts are inside and the hard part without.
In the sea-urchin, however, there is no fleshy part whatsoever.
All the Testacea then, those that have not been mentioned as well as
those that have, agree as stated in possessing a mouth with the
tongue-like body, a stomach, and a vent for excrement, but they differ
from each other in the positions and proportions of these parts. The
details, however, of these differences must be looked for in the
Researches concerning Animals and the treatises on Anatomy. For
while there are some points which can be made clear by verbal
description, there are others which are more suited for ocular
demonstration.
Peculiar among the Testacea are the sea-urchins and the animals
known as Tethya (Ascidians). The sea-urchins have five teeth, and in
the centre of these the fleshy body which is common to all the animals
we have been discussing. Immediately after this comes a gullet, and
then the stomach, divided into a number of separate compartments,
which look like so many distinct stomachs; for the cavities are
separate and all contain abundant residual matter. They are all,
however, connected with one and the same oesophagus, and they all
end in one and the same excremental vent. There is nothing besides the
stomach of a fleshy character, as has already been stated. All that
can be seen are the so-called ova, of which there are several,
contained each in a separate membrane, and certain black bodies
which have no name, and which, beginning at the animal's mouth, are
scattered round its body here and there promiscuously. These
sea-urchins are not all of one species, but there are several
different kinds, and in all of them the parts mentioned are to be
found. It is not, however, in every kind that the so-called ova are
edible. Neither do these attain to any size in any other species
than that with which we are all familiar. A similar distinction may be
made generally in the case of all Testacea. For there is a great
difference in the edible qualities of the flesh of different kinds;
and in some, moreover, the residual substance known as the mecon is<
br />
good for food, while in others it is uneatable. This mecon in the
turbinated genera is lodged in the spiral part of the shell, while
in univalves, such as limpets, it occupies the fundus, and in bivalves
is placed near the hinge, the so-called ovum lying on the right; while
on the opposite side is the vent. The former is incorrectly termed
ovum, for it merely corresponds to what in well-fed sanguineous
animals is fat; and thus it is that it makes its appearance in
Testacea at those seasons of the year when they are in good condition,
namely, spring and autumn. For no Testacea can abide extremes of
temperature, and they are therefore in evil plight in seasons of great
cold or heat. This is clearly shown by what occurs in the case of
the sea-urchins. For though the ova are to be found in these animals
even directly they are born, yet they acquire a greater size than
usual at the time of full moon; not, as some think, because
sea-urchins eat more at that season, but because the nights are then
warmer, owing to the moonlight. For these creatures are bloodless, and
so are unable to stand cold and require warmth. Therefore it is that
they are found in better condition in summer than at any other season;
and this all over the world excepting in the Pyrrhean tidal strait.
There the sea-urchins flourish as well in winter as in summer. But the
reason for this is that they have a greater abundance of food in the
winter, because the fish desert the strait at that season.
The number of the ova is the same in all sea-urchins, and is an
odd one. For there are five ova, just as there are also five teeth and
five stomachs; and the explanation of this is to be found in the
fact that the so-called ova are not really ova, but merely, as was
said before, the result of the animal's well-fed condition. Oysters
also have a so-called ovum, corresponding in character to that of
the sea-urchins, but existing only on one side of their body. Now
inasmuch as the sea-urchin is of a spherical form, and not merely a
single disk like the oyster, and in virtue of its spherical shape is
the same from whatever side it be examined, its ovum must
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