The clams (Fig. 83) differ from the oysters in having a pronounced foot (f) which protrudes from the large end of the shell; and with it the animal digs its burrow. It also hears indirectly by its foot, as its ears are in this organ, little transparent sacs containing a clear fluid in which floats a glassy globule. The clam also has a siphon (s), which in the common clam is very long. It has a black head or tip and the clam may rest some distance down in its hole and take in water through its siphon, which is double-barreled. One opening (in.) receives water containing food and oxygen; the other (ex.) expels the water. In strolling along the sands at low tide one often sees a spurt of water shoot out of a hole, and may assume that a clam has been alarmed and has retracted its siphon so suddenly that it has shot a stream of water above the surface. The shells increase by eggs, the oyster depositing a vast number, which at first are curious little free-swimming objects (Fig. 84) paddling by the aid of cilia or whips, but soon attaching themselves to the bottom and taking the oyster form.
Fig. 84.—Free-swimming young of a bivalve.
The oyster is perhaps the most valuable bivalve to man, being a favorite article of food, for which $1,500,000 is paid annually in New York alone. Thousands of men find employment collecting them in various parts of the world. In this country the most valuable oyster beds are in the vicinity of New York, at the mouth of the Shrewsbury River, in the Chesapeake Bay, and at various points alongshore to Florida, where there are large banks at the mouths of the rivers. In watching the excavation of a cellar at the town of Mayport at the mouth of the St. John River I saw oyster shells thrown up as deep as the men went. The town is built on an ancient oyster bed. Among the old shells numerous pieces of pottery have been found, showing that the early natives frequented the spot. The living oyster bed here to-day is some distance out in the stream. When sailing up a small river in Maine some years ago, I found, about ten miles from its mouth, a mound of oyster shells thirty or forty feet high. The river appeared to have cut the bed in two, and out of the top of the mound, which was of solid shells, grew a tree which must have been a century old. I believe there are no oysters on the Maine coast to-day, and the great pile was accumulated ages ago when Maine had oyster beds and the Indians carried the oysters ten miles up the river to this spot which must have been the site of an ancient Indian town or city. The pearl oyster is another valuable shell (Fig. 85). It is common in warmer waters. Near La Paz in the Gulf of California is a famous fishery, which is owned by the government and farmed out. In Ceylon it is estimated that 17,000,000 oysters are destroyed to obtain $80,000 worth of pearls. The shells are also very valuable, being made into buttons and various other objects. Liverpool is the great receiving port for these, and many tons are used annually. In diving for pearls the Ceylonese, who are able to remain beneath the water several minutes, place as many shells as possible in a basket and then ascend, leaving the crew to haul the basket up. In Lower California many divers of to-day go down in armor.
Fig. 85.—Pearl oysters.
Pearls are generally valued according to their symmetry and color. Some are perfect, and when of large size bring vast sums. One of the shahs of Persia owned a necklace in which the pearls were perfect and as large as hazelnuts. The pearl is the result of the oyster's attempts to protect itself from injury. If we should take one of these beautiful pearl oysters and with a gimlet bore a hole through the shell from the outside and replace it in the water, we should find, months after, if the oyster was examined, that it had, by using its mantle, secreted a large amount of pearly nacre over the wound, not only filling up the hole, but heaping the pearly secretion over it until a projection a quarter of an inch high was the result, resembling a pearl attached to the shell. This is the way imperfect pearls are formed; they are the attempts on the part of the oyster to prevent injury to itself. Occasionally some foreign body, like a grain of sand, will enter the shell. Its sharp edges will cut the soft flesh of the delicate creature, which immediately covers it with pearly nacre. The larger it grows the more the oyster notices it among its folds, and the more it instinctively covers it with pearl. In this way the pearls grow.
The seed pearls are those in which some impurity has been covered but a few times, while the very large pearls are those which have been bathed in nacre time and again. If a large pearl is cut in halves, the various layers can be counted, the sections recalling the interior of an onion. The skillful native fakirs of the East take advantage of this industry of the pearl oyster to introduce metal beads and figures of the Buddha into shells, which are then marked. The objects finally become covered, when they are removed from the shells and sold to the unsuspecting natives as "miracles."
One of the interesting shells of the seashore is the Pinna. I have found the shores of the outside islands of Texas scattered with them. They are also called fan shells, and are attached to the bottom by a peculiar cable, or byssus, formed of a silklike substance which has been woven. Gloves and hose of pinna silk may be seen in the British Museum.
