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.
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