OYSTER (OK oistre, ouistre, huistre, Fr. hnitre, from Lat. ostrco, ostreuui. from Gk. 6cr rpeov, oyster; connected with 6crrlov, 081e071, Lat. os, bone). A sessile bivalve mollusk of the fam ily Ostreidie, especially any of the numerous species. extinct and extant, of the genus Ostrea. The shells are irregular and unequal; the fixed left valve generally spacious, strongly convex without and excavated within; right valve gener ally plane or concave externally, always less con vex than its fellow; both shells beaked ; ligament al area elongate or triangular; hinge toothless; adductor impression single and shell subnaereous.
The oyster of the eastern coast of the United States is Ostrea Virginica, a valuable species of protean characters, formerly much subdivided by systematists and almost impossible to diagnose. The shells are lateral and hinged anteriorly, an elastic pad (ligament) causing them normally to gap. Closely applied to their inner faces and ex tensible beyond their margins are two thin folds (mantle) of the body-wall, which secrete the shell in successive layers within and on the margins. The mantle encloses a chamber (man tle cavity) open ventrally and posteriorly, into which project on each side a pair of gills, com monly called the 'beard.' and in front of these a pair of smaller fleshy lobes (palps). Above the gills and palps lies the body. containing the diges tive, reproductive, circulatory, excretory, and nervous systems, and the adductor muscle which closes the shells. The adductor (popularly. the 'eye' or 'heart') lies somewhat behind the middle of the body, the dark scars on the inside of empty shells marking its attachments. The funnel shaped month lies between the two pairs of palps. A short gullet leads into a spacious stomach, and this into the tubular intestine which opens by an anus above the adductor. Surrounding the stom ach is the liver, a large dark green digestive gland opening into the stomach by numerous ducts. In front of the adductor lies the pericar dium, containing the two-chambered heart and in proximity to the excretory organ. The simple (le generate nervous system consists of two pairs of ganglia, one above the gullet and the other be neath the adductor, connected by a pair of nerve cords.
The sexes are separate, but without external distinction. The sexual glands when ripe are creamy white organs surrounding the digestive system and opening on each side beneath the adductor. In Long Island Sound spawning oc
curs from May to August. in Chesapeake Bay from April to October, in South Carolina as early as March, and in Florida as early as February. In oysters transplanted during the spawning season reproduction is often interfered with o• arrested. An average oyster will produce 10, 000,000 eggs and a very large one 130,000.000. When ripe the sexual products ooze from the gen ital openings and fertilization results from their accidental meeting in the water. Segmentation results in five o• six hours in the production of a ciliated gastrula, a cup-shaped, free-swimming organism, often carried by the currents to found new and remote beds. An embryonic shell soon appears, and the little oyster sinks to the bottom. where. if favorably situated, it becomes attached by its left valve and gradually assumes the adult form. The recently attached spat is 1-S0 to 1-90 of an inch in diameter, and its subsequent growth varies with its environment. Single oysters on firm bottom become round and deep. hut those in clusters o• on soft bottom grow irregular and elongate. On undisturbed natural beds they grow in clusters, and the beds repose, as a rule. on a muddy substratum upon which they have been built up from a comparatively small nu cleus by the fixation, year after year. of the young upon the shells of their predecessors.
Oysters live from above low-water mark to a depth of 15 fathoms. where the density is between 1.00•L and 1.025. the optimum being from 1.011 to 1.022, and in a range of temperature which in Chesapeake Bay extends from 32' P. to 90° F. The embryos and fry require more equable and stable conditions, the temperature required being. between (IS° F. and SO' F. The hest and most productive beds are commonly in strong tidal currents, which desseminate the fry and food and keep the old shells clean enough to catch the spat. Diatonic constitute about 90 per cent. of the oyster's food. the rest consisting of other small plants and animals, and in the breeding season of its own eggs and fry. The latter are eaten by other mollusca also, and from its attachment until it reaches a large size the oyster is preyed upon by starfish, drills (Crosalpinx).