Fish on the
contrary live in the wet medium, and take in water, not air. Fins
are useful for swimming, but feet not. And if they had both they would
be non-sanguineous. There is a broad similarity between birds and
fishes in the organs of locomotion. Birds have their wings on the
superior part, similarly fish have two pectoral fins; again, birds
have legs on their under parts and near the wings; similarly, most
fish have two fins on the under parts and near the pectorals. Birds,
too, have a tail and fish a tail-fin.
19
A difficulty may be suggested as to the movements of molluscs,
that is, as to where that movement originates; for they have no
distinction of left and right. Now observation shows them moving. We
must, I think, treat all this class as mutilated, and as moving in the
way in which limbed creatures do when one cuts off their legs, or as
analogous with the seal and the bat. Both the latter are quadrupeds
but misshapen. Now molluscs do move, but move in a manner contrary
to nature. They are not moving things, but are moving if as
sedentary creatures they are compared with zoophytes, and sedentary if
classed with progressing animals.
As to right and left, crabs, too, show the distinction poorly, still
they do show it. You can see it in the claw; the right claw is
larger and stronger, as though the right and left sides were trying to
get distinguished.
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