The Molluscs ( Molluska ) Snails, Cuttlefishes, Squids, Octopus, Tusk Shells, Bivalve Molluscs, Oysters
The Molluscs have no limbs. The body is surrounded by a membraneous sac, from the secretions of which in many species a chalky shell is formed. The organs of circulation, digestion, and respiration are well developed. The under side of the body is thickened into a fleshy “foot,” by which locomotion is effected, and there is a well-marked head.
The Molluscs are divided into five classes: (1) Snails and Slugs (Gastropoda). (2) Cuttlefishes (Cephalopoda); (3) Tusk Shells (Scaphopoda); (4) Bivalves (Lamellibranchia); (5) Mail Shells (Protomollusca).
Argonaut (Argonauta) belongs to the two-gilled cuttle-fishes, and are distinguished by the females possessing a single-chambered external shell not organically connected with the body of the animal. The males have no shell and are of much smaller size than the females. The shell is fragile, translucent, and boat-like in shape; it serves as the receptacle of the eggs of the female, which sits in it with the respiratory tube or “funnel” turned toward the carina or “keel.” This famed mollusk swims only by ejecting water from its funnel, and it can crawl in a reversed position, carrying its shell over its back like a snail. The argonaut, or paper-nautilus, must be carefully distinguished from the pearly-nautilus or nautilus proper.
Cuttlefish.—One of the mollusks in which there are ten arms around the mouth. The internal shell is calcified and is used as a supply of lime for cage birds. They have also an ink bag, the secretion of which furnishes the pigment sepia. Cuttle-fish are an important article of food in southern Europe.
Octopus.—A mollusk with a rounded body, and a small head bearing a pair of well-developed eyes, the mouth surrounded by eight long arms, each arm bearing numbers of suckers by which the animals hold their prey. Inside the mouth is a pair of jaws, shaped much like those of a parrot. Most of the species are small, possibly averaging a weight of five pounds, but some on the Pacific coast spread nearly twenty-eight feet. The octopus is eaten extensively in the Mediterranean countries.
Oyster.—Possibly the most valuable of all of the mollusks. There are various species in all parts of the world, but the best is the American species, which now occurs from Cape Cod to the Gulf of Mexico. Formerly it extended to the coast of Maine, and even now there are scattered beds in the Gulf of St. Lawrence. The oyster grows in shallow water, fastening its shell to some rock or shell, and in this way large beds are formed. They are also planted; that is, the young are taken and placed in favorable situations for rapid growth.
The oyster contains but comparatively little nourishment, though eaten extensively. The European oyster is smaller than ours and has a coppery taste.
Allied to the true oysters are the Pearl Oysters, especially abundant around Ceylon. These have the interior of the shell lined with mother-of-pearl, and when foreign particles get between body and shell they are covered with the same substance, thus forming the pearls used for adornment. These oysters are obtained by diving; the animal matter is allowed to rot, leaving the pearls behind. The shell is also of value, furnishing material for knife handles, buttons, etc., though most of our pearl buttons are now made from the shells of fresh-water mussels from the Mississippi valley.
Scallop (Pecten), a well-known bivalve, one of those with a single muscle closing the shell. The valves are fan-shaped, the left often more or less flat, the right more markedly arched; both are marked with sinuous radiating ridges, to which the name pecten (Lat. “a comb”) refers. The hinge-line is without teeth, and is extended laterally in two ears. The small finger-shaped foot is usually marked with bright orange or red color. The scallops are widely distributed in all seas, at depths of three to forty fathoms.
Snails.—A common name used for any mollusk with a coiled shell. In the narrower meaning it includes only those forms which occur on land. These land-dwelling forms have a slight shell, into which the whole body can be retracted. They feed exclusively on vegetation, which they rasp by means of a long ribbon, just inside the mouth, the surface of which is covered with thousands of minute teeth, so that the whole is a flexible file. The animal creeps about on a broad sole, and has four tentacles on the head, one pair of them bearing the simple eyes at the tip. Snails do considerable damage where they are numerous. One species is eaten by many in Europe, especially in France and Italy. Over ten thousand species are known.
The shells of sea snails are often of great beauty, and large sums have been given by [230] collectors for specimens of unusual elegance or rarity, fifty pounds having been paid for a single example of a species of cone shell (Conus). The helmet shells (Cassis) are made up of differently colored layers, and on account of their beauty have been largely employed for the carving of cameos.
Squid.—A mollusk nearly related to the cuttlefish. It has a barrel-shaped body, with a head in front bearing ten pairs of tapering tentacles, each with numerous suckers. On the side of the head is a well-developed eye. Squid live largely on small fishes which they catch with the tentacles, biting them with a pair of parrot-like jaws. They are largely used as bait in fishing, and to a limited extent as food. The average length is a foot or two, but in the seas around Newfoundland and Japan giants are occasionally found with bodies a dozen feet in length and tentacles adding thirty feet to this.
Tusk Shells are a small group of burrowing marine forms, in which the body is covered by a long, curved shell resembling a tusk in shape. There is a small hole at its tip, through which the water which has been used in breathing makes its exit. An imperfectly developed head and a rasping organ are present, and burrowing is effected by a long foot with a three-lobed end. The food consists of small organisms, which are apparently secured by the agency of a bunch of filaments with thickened sticky tips that can be protruded from the mouth of the shell. In some respects these animals are intermediate in structure between typical sea snails and bivalve molluscs.