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.