Some animals at first live in water, and by and by change their
shape and live out of water, as is the case with river worms, for
out of these the gadfly develops.
Furthermore, some animals are stationary, and some are erratic.
Stationary animals are found in water, but no such creature is found
on dry land. In the water are many creatures that live in close
adhesion to an external object, as is the case with several kinds of
oyster. And, by the way, the sponge appears to be endowed with a
certain sensibility: as a proof of which it is alleged that the
difficulty in detaching it from its moorings is increased if the
movement to detach it be not covertly applied.
Other creatures adhere at one time to an object and detach
themselves from it at other times, as is the case with a species of
the so-called sea-nettle; for some of these creatures seek their
food in the night-time loose and unattached.
Many creatures are unattached but motionless, as is the case with
oysters and the so-called holothuria. Some can swim, as, for instance,
fishes, molluscs, and crustaceans, such as the crawfish. But some of
these last move by walking, as the crab, for it is the nature of the
creature, though it lives in water, to move by walking.
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