This
tail differs in different fishes, in some approximating in character
to the fins, while in others, namely in some of the flat kinds, it
is spinous and elongated, because the material which should have
gone to the tail has been diverted thence and used to increase the
breadth of the body. Such, for instance, is the case with the
Torpedos, the Trygons, and whatever other Selachia there may be of
like nature. In such fishes, then, the tail is spinous and long; while
in some others it is short and fleshy, for the same reason which makes
it spinous and long in the Torpedo. For to be short and fleshy comes
to the same thing as to be long and less amply furnished with flesh.
What has occurred in the Fishing-frog is the reverse of what has
occurred in the other instances just given. For here the anterior
and broad part of the body is not of a fleshy character, and so all
the fleshy substance which has been thence diverted has been placed by
nature in the tail and hinder portion of the body.
In fishes there are no limbs attached to the body. For in accordance
with their essential constitution they are swimming animals; and
nature never makes anything superfluous or void of use. Now inasmuch
as fishes are made swimming they have fins, and as they are not made
for walking they are without feet; for feet are attached to the body
that they may be of use in progression on land.
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