Fishes also present diversities as regards the mouth. For in some
this is placed in front, at the very extremity of the body, while in
others, as the dolphin and the Selachia, it is placed on the under
surface; so that these fishes turn on the back in order to take
their food. The purpose of Nature in this was apparently not merely to
provide a means of salvation for other animals, by allowing them
opportunity of escape during the time lost in the act of turning-for
all the fishes with this kind of mouth prey on living animals-but also
to prevent these fishes from giving way too much to their gluttonous
ravening after food. For had they been able to seize their prey more
easily than they do, they would soon have perished from
over-repletion. An additional reason is that the projecting
extremity of the head in these fishes is round and small, and
therefore cannot admit of a wide opening.
Again, even when the mouth is not placed on the under surface, there
are differences in the extent to which it can open. For in some
cases it can gape widely, while in others it is set at the point of
a small tapering snout; the former being the case in carnivorous
fishes, such as those with sharp interfitting teeth, whose strength
lies in their mouth, while the latter is its form in all such as are
not carnivorous.
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