Testaceans also differ from one another in regard to the thickness
or thinness of their shell, both as regards the shell in its
entirety and as regards specific parts of the shell, for instance, the
lips; for some have thin-lipped shells, like the mussel, and others
have thick-lipped shells, like the oyster. A property common to the
above mentioned, and, in fact, to all testaceans, is the smoothness of
their shells inside. Some also are capable of motion, like the
scallop, and indeed some aver that scallops can actually fly, owing to
the circumstance that they often jump right out of the apparatus by
means of which they are caught; others are incapable of motion and are
attached fast to some external object, as is the case with the
pinna. All the spiral-shaped testaceans can move and creep, and even
the limpet relaxes its hold to go in quest of food. In the case of the
univalves and the bivalves, the fleshy substance adheres to the
shell so tenaciously that it can only be removed by an effort; in
the case of the stromboids, it is more loosely attached. And a
peculiarity of all the stromboids is the spiral twist of the shell
in the part farthest away from the head; they are also furnished
from birth with an operculum.
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