The so-called tethyum or ascidian has of all these animals the
most remarkable characteristics. It is the only mollusc that has its
entire body concealed within its shell, and the shell is a substance
intermediate between hide and shell, so that it cuts like a piece of
hard leather. It is attached to rocks by its shell, and is provided
with two passages placed at a distance from one another, very minute
and hard to see, whereby it admits and discharges the sea-water; for
it has no visible excretion (whereas of shell fish in general some
resemble the urchin in this matter of excretion, and others are
provided with the so-called mecon, or poppy-juice). If the animal be
opened, it is found to have, in the first place, a tendinous
membrane running round inside the shell-like substance, and within
this membrane is the flesh-like substance of the ascidian, not
resembling that in other molluscs; but this flesh, to which I now
allude, is the same in all ascidia. And this substance is attached
in two places to the membrane and the skin, obliquely; and at the
point of attachment the space is narrowed from side to side, where the
fleshy substance stretches towards the passages that lead outwards
through the shell; and here it discharges and admits food and liquid
matter, just as it would if one of the passages were a mouth and the
other an anal vent; and one of the passages is somewhat wider than the
other Inside it has a pair of cavities, one on either side, a small
partition separating them; and one of these two cavities contains
the liquid. The creature has no other organ whether motor or
sensory, nor, as was said in the case of the others, is it furnished
with any organ connected with excretion, as other shell-fish are.
The colour of the ascidian is in some cases sallow, and in other cases
red.

    There is, furthermore, the genus of the sea-nettles, peculiar in
its way. The sea-nettle, or sea-anemone, clings to rocks like
certain of the testaceans, but at times relaxes its hold. It has no
shell, but its entire body is fleshy. It is sensitive to touch, and,
if you put your hand to it, it will seize and cling to it, as the
cuttlefish would do with its feelers, and in such a way as to make the
flesh of your hand swell up. Its mouth is in the centre of its body,
and it lives adhering to the rock as an oyster to its shell. If any
little fish come up against it it it clings to it; in fact, just as
I described it above as doing to your hand, so it does to anything
edible that comes in its way; and it feeds upon sea-urchins and
scallops. Another species of the sea-nettle roams freely abroad. The
sea-nettle appears to be devoid altogether of excretion, and in this
respect it resembles a plant.

    Of sea-nettles there are two species, the lesser and more
edible, and the large hard ones, such as are found in the
neighbourhood of Chalcis. In winter time their flesh is firm, and
accordingly they are sought after as articles of food, but in summer
weather they are worthless, for they become thin and watery, and if
you catch at them they break at once into bits, and cannot be taken
off the rocks entire; and being oppressed by the heat they tend to
slip back into the crevices of the rocks.

    So much for the external and the internal organs of molluscs,
crustaceans, and testaceans.