The number of muscles by which the shells are moved have caused this Class to be divided into two orders: the Bimusculosa, in which there are two pair of muscles to perform this office; and the Unimusculosa, with only one pair.
ORDER UNIMUSCULOSA.
The Conchiferous animals which possess but one pair of muscles, are much more limited in number than those which possess two or more; but they contain in their ranks several well-known and useful species, as, for instance, the oyster, the mussel, and the animal which produces the oriental pearl.
The Horse-Foot Bowl Shell,
([Anomia ephippium].)
The shells of the Anomiæ are exceedingly irregular in their form; like the oysters, they remain during the whole of their existence attached to one spot, either on a rock, or on the shell of some larger inhabitant of the deep. These shells are more frequently found in the same places as the oyster, and very commonly attached to the shell of the latter; as an article of food, the Anomia is of little or no value. Its organization and manner of living are much the same as those of the oyster. The most singular part of its construction consists in the use made of one of the muscles with which it is furnished, which, instead of being attached to the shell, is fixed to a solid piece of shelly substance, in the form of a cone with the top cut off; this operculum, or lid, closes a singular opening in one of the valves of the shell itself. The animal adheres to the rock, or other substance, by means of this lid, and is detached with great difficulty.
Anomia ephippium.
[Anomia, a little bowl; ephippium, a horse’s foot.]
The Oyster, ([Ostrea edulis].)
Oysters, like all other creatures that have been destined to become food for man, are found in great abundance in most parts of the globe; they are inhabitants of salt waters only, and are always found in rocky ground, in no great depth from the surface.