Those of a
warmer constitution are, moreover, in virtue of their warmth more
given to motion, so that they make expeditions in search of food,
instead of remaining stationary like the rest. As evidence of this, it
will be found that they always have something or other sticking to
their spines, as though they moved much about; for they use their
spines as feet.
The Ascidians differ but slightly from plants, and yet have more
of an animal nature than the sponges, which are virtually plants and
nothing more. For nature passes from lifeless objects to animals in
such unbroken sequence, interposing between them beings which live and
yet are not animals, that scarcely any difference seems to exist
between two neighbouring groups owing to their close proximity.
A sponge, then, as already said, in these respects completely
resembles a plant, that throughout its life it is attached to a
rock, and that when separated from this it dies. Slightly different
from the sponges are the so-called Holothurias and the sea-lungs, as
also sundry other sea-animals that resemble them. For these are free
and unattached. Yet they have no feeling, and their life is simply
that of a plant separated from the ground. For even among
land-plants there are some that are independent of the soil, and
that spring up and grow, either upon other plants, or even entirely
free.
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