And, by the way,

the animal found in the shell of the neritae is a
separate species, like to the other in most respects; but of its
bifurcate feet or claws, the right-hand one is small and the left-hand
one is large, and it progresses chiefly by the aid of this latter
and larger one. (In the shells of these animals, and in certain
others, there is found a parasite whose mode of attachment is similar.
The particular one which we have just described is named the
cyllarus.)

The nerites has a smooth large round shell, and resembles the
ceryx in shape, only the poppy-juice is, in its case, not black but
red. It clings with great force near the middle. In calm weather,
then, they go free afield, but when the wind blows the carcinia take
shelter against the rocks: the neritae themselves cling fast like
limpets; and the same is the case with the haemorrhoid or aporrhaid
and all others of the like kind. And, by the way, they cling to the
rock, when they turn back their operculum, for this operculum seems
like a lid; in fact this structure represents the one part, in the
stromboids, of that which in the bivalves is a duplicate shell. The
interior of the animal is fleshy, and the mouth is inside. And it is
the same with the haemorrhoid, the purple murex, and all suchlike
animals.

Such of the little crabs as have the left foot or claw the
bigger of the two are found in the neritae, but not in the stromboids.
are some snail-shells which have inside them creatures resembling
those little crayfish that are also found in fresh water. These
creatures, however, differ in having the part inside the shells But as
to the characters, you are referred to my Treatise on Anatomy.