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. For there is nothing in their
mode of life which imperatively requires it.
Fishes and Insects and the hard-skinned Crustacea present certain
differences in their eyes, but so far resemble each other as that none
of them have eyelids. As for the hard-skinned Crustacea it is
utterly out of the question that they should have any; for an
eyelid, to be of use, requires the action of the skin to be rapid.
These animals then have no eyelids and, in default of this protection,
their eyes are hard, just as though the lid were attached to the
surface of the eye, and the animal saw through it. Inasmuch,
however, as such hardness must necessarily blunt the sharpness of
vision, nature has endowed the eyes of Insects, and still more those
of Crustacea, with mobility (just as she has given some quadrupeds
movable ears), in order that they may be able to turn to the light and
catch its rays, and so see more plainly. Fishes, however, have eyes of
a fluid consistency. For animals that move much about have to use
their vision at considerable distances. If now they live on land,
the air in which they move is transparent enough. But the water in
which fishes live is a hindrance to sharp sight, though it has this
advantage over the air, that it does not contain so many objects to
knock against the eyes. The risk of collision being thus small,
nature, who makes nothing in vain, has given no eyelids to fishes,
while to counterbalance the opacity of the water she has made their
eyes of fluid consistency
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