For they all from their very
birth have an operculum to protect that part of their body which is
exposed to view. This is the case with the Purpuras, with Whelks, with
the Nerites, and the like. Were it not for this, the part which is
undefended by the shell would be very liable to injury by collision
with external objects. The univalves also are not without
protection. For on their dorsal surface they have a shell, and by
the under surface they attach themselves to the rocks, and so after
a manner become bivalved, the rock representing the second valve. Of
these the animals known as Limpets are an example. The bivalves,
scallops and mussels, for instance, are protected by the power they
have of closing their valves; and the Turbinata by the operculum
just mentioned, which transforms them, as it were, crom univalves into
bivalves. But of all there is none so perfectly protected as the
sea-urchin. For here there is a globular shell which encloses the body
completely, and which is, moreover, set with sharp spines. This
peculiarity distinguishes the sea-urchin from all other Testacea, as
has already been mentioned.
The structure of the Testacea and of the Crustacea is exactly the
reverse of that of the Cephalopoda. For in the latter the fleshy
substance is on the outside and the earthy substance within, whereas
in the former the soft parts are inside and the hard part without.
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