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. they have their legs too, not like the rest of birds in the centre of their body, but rather set back. their legs are short, and being set back are serviceable for swimming. the reason for their having short legs is that nature has added to their feet by subtracting from the length of their limbs; instead of length she gives stoutness to the legs and breadth to the feet. broad feet are more useful than long for pushing away the water when they are swimming.