As a general rule,

then, all testaceans grow by spontaneous
generation in mud, differing from one another according to the
differences of the material; oysters growing in slime, and cockles and
the other testaceans above mentioned on sandy bottoms; and in the
hollows of the rocks the ascidian and the barnacle, and common
sorts, such as the limpet and the nerites. All these animals grow with
great rapidity, especially the murex and the scallop; for the murex
and the scallop attain their full growth in a year. In some of the
testaceans white crabs are found, very diminutive in size; they are
most numerous in the trough shaped mussel. In the pinna also is
found the so-called pinna-guard. They are found also in the scallop
and in the oyster; these parasites never appear to grow in size.
Fishermen declare that the parasite is congenital with the larger
animal. (Scallops burrow for a time in the sand, like the murex.)