Page:The New International Encyclopædia 1st ed. v. 15.djvu/220

OYSTER shell in successive layers within and on the margins. The mantle encloses a chamber (mantle cavity) open ventrally and posteriorly, into which project on each side a pair of gills, commonly 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 digestive, 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 mouth 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 stomach is the liver, a large dark green digestive gland opening into the stomach by numerous ducts. In front of the adductor lies the pericardium, containing the two-chambered heart and in proximity to the excretory organ. The simple degenerate nervous system consists of two pairs of ganglia, one above the gullet and the other beneath 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 occurs 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 or arrested. An average oyster will produce 16,000,000 eggs and a very large one 60,000,000. When ripe the sexual products ooze from the genital openings and fertilization results from their accidental meeting in the water. Segmentation results in five or 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 1.80 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, but those in clusters or 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 nucleus 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 1.5 fathoms, where the density is between 1.002 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° F. to 90° F. The embryos and fry require more equable and stable conditions, the temperature required being between 68° F. and 80° F. The best 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. Diatoms 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 (Urosalpinx), drumfish, rays, and other aggressive enemies, while it wages a passive fight against starvation and suffocation with mussels, barnacles, sponges, worms, aquatic vegetation, and other prolific- or luxuriant organisms growing on the beds.

Ostrea Virginica occurs from the Gulf of Saint Lawrence to the tropics, but between Cape Breton and Cape Cod the Sheepscot River, Maine, is its only locality. It has also been introduced in San Francisco Bay, where it breeds to a limited extent. The yield of Eastern oysters at the beginning of the present century was as follows:

The greatest production is in Chesapeake Bay, where the principal yield is from the natural beds. Most of the oysters from New England and from New York and the outer coast of New Jersey are produced by planted beds; the entire yield of the Pacific Coast is similarly derived, and there has been recently a considerable increase in oyster culture in New Jersey, Virginia, and other States. The number of persons engaged in the industry is estimated at upward of 60,000, but as many of them are employed part of the year in other fisheries, farming, etc., definite statistics are not available. Baltimore is the most extensive market and New York has a considerable export trade with Europe.

The native oyster of the Pacific Coast is Ostrea lurida, a small thin-shelled species. It is hermaphroditic, and, like the European oyster, retains its young for a time in the mantle cavity. In 1901 159,340 bushels, valued at $251,192, were marketed, principally on the Pacific Coast.



The European oyster (Ostrea edulis) is found from Italy to Norway. It is a round thin-shelled species, more shapely than the American species, and hermaphroditic, first female and afterwards male. It is less prolific than its American relative and the young undergo considerable development in the mantle chamber of the mother. It thrives in water of full, or almost full, organic density. The Portuguese oyster (Ostrea angulata) sexually and in its habits more closely resembles Ostrea Virginica.