They
look as if they moved at two points only, where they touch before
and behind, but that is only because they are narrow in breadth. Even.
in them the right is the sovereign part, and there is an alternate
correspondence behind, exactly as in quadrupeds. The reason of their
flexions is their great length, for just as tall men walk with their
spines bellied (undulated) forward, and when their right shoulder is
leading in a forward direction their left hip rather inclined
backwards, so that their middle becomes hollow and bellied
(undulated), so we ought to conceive snakes as moving in concave
curves (undulations) upon the ground. And this is evidence that they
move themselves like the quadrupeds, for they make the concave in
its turn convex and the convex concave. When in its turn the left of
the forward parts is leading, the concavity is in its turn reversed,
for the right becomes the inner. (Let the right front point be A,
the left B, the right hind C, the left D.)
Among land animals this is the character of the movement of
snakes, and among water animals of eels, and conger-eels and also
lampreys, in fact of all that have their form snakelike. However, some
marine animals of this shape have no fin, lampreys for example, but
put the sea to the same use as snakes do both land and water (for
snakes swim precisely as they move on the ground). Others have two
fins only, for example conger-eels and eels and a kind of cestreus
which breeds in the lake of Siphae.
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