On The Gait Of Animals
that they use them not for swimming but for walking; they always
keep on the ground. However, the flexion of the limbs of all
polypods is oblique, like that of the quadrupeds which live in
holes-for example lizards and crocodiles and most of the oviparous
quadrupeds. And the explanation is that some of them in their breeding
periods, and some all their life, live in holes.
Now the rest have bandy legs because they are soft-skinned, but
the crayfish is hard-skinned and its limbs are for swimming and not
for walking (and so are not bandy). Crabs, too, have their limbs
bent obliquely, but not bandy like oviparous quadrupeds and
non-sanguineous polypods, because their limbs have a hard and
shell-like skin, although they don't swim but live in holes; they live
in fact on the ground. Moreover, their shape is like a disk, as
compared with the crayfish which is elongated, and they haven't a tail
like the crayfish; a tail is useful to the crayfish for swimming,
but the crab is not a swimming creature. Further, it alone has its
side equivalent to a hinder part, because it has many leading feet.
The explanation of this is that its flexions are not forward nor its
legs turned in under (bandy). We have given above the reason why its
legs are not turned in under, that is the hardness and shell-like
character of its integument.
For these reasons then it must lead off with more than one limb, and
move obliquely; obliquely, because the flexion is oblique; and with
more than one limb, because otherwise the limbs that were still
would have got in the way of those that were moving.
Fishes of the flat kind swim with their heads twisted, as one-eyed
men walk; they have their natural shape distorted. Web-footed birds
swim with their feet; because they breath the air and have lungs
they are bipeds, but because they have their home in the water they
are webbed; by this arrangement their feet serve them instead of fins.