On The Gait Of Animals   

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.

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There is reason, too, for winged creatures having feet, but fish

none. The former have their home in the dry medium, and cannot

remain always in mid air; they must therefore have feet. 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.

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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.