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. 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. Such, for example, is the plant which is found on Parnassus, and
which some call the Epipetrum. This you may hang up on a peg and it
will yet live for a considerable time. Sometimes it is a matter of
doubt whether a given organism should be classed with plants or with
animals. The Ascidians, for instance, and the like so far resemble
plants as that they never live free and unattached, but, on the
other hand, inasmuch as they have a certain flesh-like substance, they
must be supposed to possess some degree of sensibility.
An Ascidian has a body divided by a single septum and with two
orifices, one where it takes in the fluid matter that ministers to its
nutrition, the other where it discharges the surplus of unused
juice, for it has no visible residual substance, such as have the
other Testacea
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