OYSTER

This article appears in Volume V20, Page 424 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: ORC-PAI

OYSTER . The use of this name in the vernacular is equivalent to that of Ostrea (Lat. from Gr. &rTaeov, oyster, so called from its shell, 5omov, bone, shell) in zoological nomenclature; there are no genera so similar to Ostrea as to be confounded with it in

ordinary

ORDINARY (med. Lat. ordinarius, Fr. ordinaire)

language. Ostrea is a genus of Lamellibranch Molluscs. The degeneration produced by sedentary habits in all lamellibranchs has in the oyster reached its most advanced stage. The valves of the shell are closed by a single large adductor muscle, the anterior adductor being absent. The muscular projection of the ventral surface called the foot, whose various modifications characterize the different classes of Mollusca, is almost entirely aborted. The two valves of the shell are unequal in size, and of different shape; the left valve is larger, thicker and more convex, and on it the animal rests in its natural state. This valve, in the

young

oyster, is attached to some object on the sea-bottom; in the adult it is sometimes attached, sometimes free. The right valve is flat, and smaller and thinner than the left. In a corresponding manner the right side of the animal's

body

BODY

is somewhat less developed than the left, and to this extent there is a departure from the bilateral symmetry characteristic of Lamellibranchs.
The organization of the oyster, as compared with that of a typical lamellibranch such as Anodon (see

LAMELLIBRANCHIA

LAMELLIBRANCHIA (Lat. lamella, a small or thin plate, and Gr. /3payxia, gills)

), is brought about by the reduction of the anterior part of the

body

BODY

accompanying the loss of the anterior adductor, and the enlargement of the posterior region. The pedal ganglia and auditory organs have disappeared with the foot, at all events have never been detected; the cerebral ganglia are very minute, while the parieto-splanchnic are well developed, and constitute dangerous. It acquired considerable application in platinum the

principal

PRINCIPAL

part of the

nervous

NERVOUS

system

SYSTEM

.
works, this metal being only fusible in the oxyhydrogen flame According to Spengel, the pair of ganglia near the mouth, and the electric furnace; and also for the production of limelight, variously called labial or cerebral, represent the cerebral pair as in optical (magic) lanterns. But these applications are being and pleural pair of a gastropod combined, and the parieto-
superseded by the electric furnace, and electric light. splanchnic pair correspond to the visceral ganglia, the corn-

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