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.

17



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.