On The Parts Of Animals   

All fishes have teeth of the serrated form, with the single exception
of the fish known as the Scarus. In many of them there are teeth even
on the tongue and on the roof of the mouth. The reason for this is
that, living as they do in the water, they cannot but allow this fluid
to pass into the mouth with the food. The fluid thus admitted they
must necessarily discharge again without delay. For were they not to
do so, but to retain it for a time while triturating the food, the
water would run into their digestive cavities. Their teeth therefore
are all sharp, being adapted only for cutting, and are numerous and
set in many parts, that their abundance may serve in lieu of any
grinding faculty, to mince the food into small bits. They are also
curved, because these are almost the only weapons which fishes
possess.
In all these offices of the teeth the mouth also takes its part; but
besides these functions it is subservient to respiration, in all such
animals as breathe and are cooled by external agency. For nature, as
already said, uses the parts which are common to all animals for many
special purposes, and this of her own accord. Thus the mouth has one
universal function in all animals alike, namely its alimentary office;
but in some, besides this, the special duty of serving as a weapon is
attached to it; in others that of ministering to speech; and again in
many, though not in all, the office of respiration. All these
functions are thrown by nature upon one single organ, the construction
of which she varies so as to suit the variations of office. Therefore
it is that in some animals the mouth is contracted, while in others it
is of wide dimensions. The contracted form belongs to such animals as
use the mouth merely for nutritive, respiratory, and vocal purposes;
whereas in such as use it as a means of defence it has a wide gape.
This is its invariable form in such animals as are saw-toothed. For
seeing that their mode of warfare consists in biting, it is
advantageous to them that their mouth shall have a wide opening; for
the wider it opens, the greater will be the extent of the bite, and
the more numerous will be the teeth called into play.
What has just been said applies to fishes as well as to other animals;
and thus in such of them as are carnivorous, and made for biting, the
mouth has a wide gape; whereas in the rest it is small, being placed
at the extremity of a tapering snout. For this form is suited for
their purposes, while the other would be useless.
In birds the mouth consists of what is called the beak, which in them
is a substitute for lips and teeth. This beak presents variations in
harmony with the functions and protective purposes which it serves.
Thus in those birds that are called Crooked-clawed it is invariably
hooked, inasmuch as these birds are carnivorous, and eat no kind of
vegetable food whatsoever. For this form renders it serviceable to
them in obtaining the mastery over their prey, and is better suited
for deeds of violence than any other. Moreover, as their weapons of
offence consist of this beak and of their claws, these latter also are
more crooked in them than in the generality of birds. Similarly in
each other kind of bird the beak is suited to the mode of life. Thus,
in woodpeckers it is hard and strong, as also in crows and birds of
crowlike habit, while in the smaller birds it is delicate, so as to be
of use in collecting seeds and picking up minute animals. In such
birds, again, as eat herbage, and such as live about marshes-those,
for example, that swim and have webbed feet-the bill is broad, or
adapted in some other way to the mode of life. For a broad bill
enables a bird to dig into the ground with ease, just as, among
quadrupeds, does the broad snout of the pig, an animal which, like the
birds in question, lives on roots. Moreover, in these root-eating
birds and in some others of like habits of life, the tips of the bill
end in hard points, which gives them additional facility in dealing
with herbaceous food.
The several parts which are set on the head have now, pretty nearly
all, been considered. In man, however, the part which lies between the
head and the neck is called the face, this name, (prosopon) being, it