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. There are four main genera, viz.
the Carabi, as they are called, the Astaci, the Carides, and the
Carcini. In each of these genera, again, there are numerous species,
which differ from each other not only as regards shape, but also
very considerably as regards size. For, while in some species the
individuals are large, in others they are excessively minute. The
Carcinoid and Caraboid Crustacea resemble each other in possessing
claws. These claws are not for locomotion, but to serve in place of
hands for seizing and holding objects; and they are therefore bent
in the opposite direction to the feet, being so twisted as to turn
their convexity towards the body, while their feet turn towards it
their concavity. For in this position the claws are best suited for
laying hold of the food and carrying it to the mouth. The
distinction between the Carabi and the Carcini (Crabs) consists in the
former having a tail while the latter have none. For the Carabi swim
about and a tail is therefore of use to them, serving for their
propulsion like the blade of an oar. But it would be of no use to
the Crabs; for these animals live habitually close to the shore, and
creep into holes and corners. In such of them as live out at sea,
the feet are much less adapted for locomotion than in the rest,
because they are little given to moving about but depend for
protection on their shell-like covering. The Maiae and the crabs known
as Heracleotic are examples of this; the legs in the former being very
thin, in the latter very short
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