With the ostracoderma, or testaceans, such as the land-snails
and the sea-snails, and all the 'oysters' so-called, and also with the
sea-urchin genus, the fleshy part, in such as have flesh, is similarly
situated to the fleshy part in the crustaceans; in other words, it
is inside the animal, and the shell is outside, and there is no hard
substance in the interior. As compared with one another the testaceans
present many diversities both in regard to their shells and to the
flesh within. Some of them have no flesh at all, as the sea-urchin;
others have flesh, but it is inside and wholly hidden, except the
head, as in the land-snails, and the so-called cocalia, and, among
pelagic animals, in the purple murex, the ceryx or trumpet-shell,
the sea-snail, and the spiral-shaped testaceans in general. Of the
rest, some are bivalved and some univalved; and by 'bivalves' I mean
such as are enclosed within two shells, and by 'univalved' such as are
enclosed within a single shell, and in these last the fleshy part is
exposed, as in the case of the limpet. Of the bivalves, some can
open out, like the scallop and the mussel; for all such shells are
grown together on one side and are separate on the other, so as to
open and shut. Other bivalves are closed on both sides alike, like the
solen or razor-fish. Some testaceans there are, that are entirely
enveloped in shell and expose no portion of their flesh outside, as
the tethya or ascidians.