OYSTER, °street, a genus of lamellibranchiate mollusks. of the section with a single adductor muscle. See LAMELLIBRANCIIIATA. Tire shell consists of two unequal and sonic what irregularly slurped valves, of laminated and coarsely foliated structure; and the hinge is without tooth or ridge, the valves being held together by a ligament lodged in a little cavity in each. The animal is, in its organization, among the lowest and simplest of lamcllihrauchiate mollusks. It has no foot; and, except when very young, no power of locomotion, or organ of any kind adapted to that purpose. Its food consists of animal cules, and also of minute vegetable particles, brought to it by the water, a continual cur rent of which is directed toward the mouth by the action of the gills. The gills are seen in four rows when the valves of the shell are separated, a little within the fringed edge of the mantle. In the most central part is the adductor muscle; toward the hinge is the' liver, which is large; and between the adductor muscle and the liver is the heart, which may be recognized by the brown color of its auricle. The month—for, as in the other lamellibranehihta, there is no head—is situated beneath a kind of hood, formed by the union of the two edges of the mantle near the hinge. It is jawless and toothless. The . ovaries are very large the season of reproduction, which extends over certain months in summer, when oysters are out of season for the table. Oysters are hermaph rodite. They produce vast numbers of young. Leenwenhoek calculated that from 3,000 to 4,000 exist within an oyster at once when " " "milky." or full of spawn; and, according to Poli, one oyster produces about 1,200,000 eggs. The eggs are batched within the shell and mantle of the parent, and the young are to be seen swimming slowly in a whitish and mucous or creamy fluid surrounding the gills, which becomes darker and of a muddy appearance when they are ab6ut to be expelled. Each young oyster is then about ri-cr of an inch in length, and about two millions are capable of being closely packed in the space of a cubic inch. When the parent oyster expels the young, and this is done simultaneously by multitudes on an oyster-bank, the water becomes filled as with a thick cloud, and the spawn—called spat by fishermen—is wafted away by currents; the greater part, of course, to be generally lost, by being driven to unsuitable situations, as exposed rocks, muddy ground, or sand to which it cannot adhere, or to be devoured by fishes and other marine animals, but some to find an object to which it eau attach itself for life. The young come forth furnished with a temporary organ for swimming,
ciliated, and provided with powerful muscles for extending it beyond the valves and withdrawing it at pleasure; and when the oyster has become fixed in its permanent place of abode, this organ, being no longer of any use, has been supposed to drop off, or grad ually to dwindle away and disappear. But Dr. F. Buckland has recently expressed the opinion that the swimming organ of the young oyster is the "lungs." and remains as the "lungs" in the mature oyster. In very favorable situations, oysters grow rapidly, so that the common oyster is ready for tire table in a year and is half or two years; but in other places, a longer time is required, often about five years.
The species of oyster are numerous, and are found in the seas of all warm and tem• Aerate climates. None have been found in the c oldest parts of tire world. The COMMON OYSTER (0. edulis) is the only British species. Like it. the other species are generally found where the water is of no great depth; and some of them, also like it, are very abundant in estuaries, where the water is not very salt. The mangrove swamps of warns climates often abound in oysters of excellent flavor (0. pamsitica, etc.). adhering to the roots and branches of the trees, within the reach of the tide. Some of the species differ from the common oyster not a little in form, as the LONG-IITNGED OYSTER (0. Cab/If/en 148) of North America, which is very elongated; and some of them far exceed it in size. Sir J. E. Tennent states that ho measured the shell of an edible oyster in Ceylon,. and found it a little more than 11 in. in length by half as many in breadth; "thus unexpect edly attesting the correctness of one of the .stories related by the historians of Alexander's expedition, that in India they had found foot long." Some species of oyster have the valves plaited with strong longitudinal plaits.—For the illustrations here given we are indebted to the kindness of tire editor ot the Field.