Shell

in natural history, the limy home of a mollusk. The body of the crayfish and the lobster secretes a limy covering; the coral builds a limestone home; the starfish, the sea-urchin, the sea-cucumber, and many other animals, chiefly marine, live in lime houses; but, in ordinary usage, a shell is the protection of a mollusk. The shells that live in the ocean, that strew the seashore, and that may be gathered in rivers and ponds may be classified in three groups: the shells of cephalopods, gastropods and bivalves.

Whatever the shape or size of a shell, it consists for the most part of carbon and lime, the constituents that enter largely into bone and the shell of eggs. This carbonate of lime, as the compound is called, forms from ninety up to ninety-nine per cent, by weight, of the matter in a shell. By burning, the carbon may be removed, leaving the lime. Lime obtained in this way is sometimes used by residents of the seacoast to make plaster. The material of the shell is obtained by the animal from its food and sea-water, quite as land animals obtain the material entering into the composition of bone.

Shells vary in size from miscroscopic shells to shells too heavy for a man to carry. The two valves of the giant clam weigh 500 pounds. Some mollusks, as the cuttlefish, do not have a true shell, but an interior plate, like the porous cuttlefish bone placed between the wires of a cage for the canary to pick at. Other shells are thick, hard, and capable of withstanding heavy blows. Many shells are covered with long, spiny projections, the use of which is not understood. The shells of bivalves are composed of two pieces or valves, as in the case of the oyster, clam, and mussel. The shells of gastropods are univalve or one valve. The shell of the limpet is shaped like a saucer; the shell of the snail and the whelk is conical and spiral or else coiled flat. The spiral mouth is coiled usually, but not always, in the direction opposite to the movement of the hands of a watch.

Shell has many uses in the arts. Seashore dwellers burn shells for the lime they contain. American roadmakers, in the tidewater region, use oyster shells for surfacing. Farmers in the maritime provinces of Canada use "mussel-mud," consisting of shells, mud and slime from decaying oyster beds, as a fertilizer. The pearly shell of the oyster and clam is much used for inlaid work. Ground into a powder it is employed by the Japanese for lacquer work. Several thousand tons of shells, American mussel, are converted into buttons yearly. London merchants buy shells to the value of $4,000,000 yearly. Oyster shells are crushed for laying hens. The cowry shell passes current in parts of Africa as money. Primitive people use shells for domestic utensils. The Filipino uses plates of a certain transparent shell as a substitute for window glass. Many shells afford beautiful material for cameos and numerous other ornaments.

See Conch; Cowry; Snail; Nautilus; Oyster; Clam; Mussel; Button; Pearl, etc.