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.