The same considerations
explain why crabs also move the upper division of each claw and not
the lower. For their claws are substitutes for hands, and so require
to be suitable for the prehension of food, and not for its
comminution; for such comminution and biting is the office of teeth.
In crabs, then, and in such other animals as are able to seize their
food in a leisurely manner, inasmuch as their mouth is not called on
to perform its office while they are still in the water, the two
functions are assigned to different parts, prehension to the hands
or feet, biting and comminution of food to the mouth. But in
crocodiles the mouth has been so framed by nature as to serve both
purposes, the jaws being made to move in the manner just described.
Another part present in these animals is a neck, this being the
necessary consequence of their having a lung. For the windpipe by
which the air is admitted to the lung is of some length. If,
however, the definition of a neck be correct, which calls it the
portion between the head and the shoulders, a serpent can scarcely
be said with the same right as the rest of these animals to have a
neck, but only to have something analogous to that part of the body.
It is a peculiarity of serpents, as compared with other animals allied
to them, that they are able to turn their head backwards without
stirring the rest of the body.
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