Page:The American Cyclopædia (1879) Volume XI.djvu/739

 MOLLUSCA 721 most mollusks are provided, or conchology, had occupied almost the exclusive attention of classifiers. (See CONCHOLOGY, and MALACOL- OGY.) The microscopic anatomy and embry- ology of mollusks led to the separation of cir- ripeds, and to their being placed among articu- lates in the class of crustaceans ; for the same reason the Iryozoa were taken from polyps and placed among acephalous mollusks, and since, with the brachiopods and ascidians, among articulates. The mollusca (Jieterogan- gliata of Owen) include such animals as have one or more nervous ganglia below the en- trance to the alimentary canal, from which radiate cords which form a collar round the oesophagus and supply the other organs of the body; in the higher forms other ganglia are added above the oesophagus and unsymmetrical- ly in different parts of the body. From the sac which invests the body they have been called saccata by Prof. Hyatt. In addition to the writers alluded to under MALACOLOGY, may be mentioned Poli, Rathke, Savigny, Chamisso, Pfeiffer, Deshayes, Forbes and Hanley, Loven, Quatrefages, Kiener, Chenu, Chemnitz, Rang, Alder and Hancock, Ferussac, D'Orbigny; Philippi, Sowerby, Johnston, Martini, Huxley, Eschricht, and Delle Chiaje; and in the United States, Say, Conrad, Lea, Couthouy, Binney, Adams, Jay, Haldeman, Gould, Morse, and Hyatt. In mollusca the body is covered by a soft moist skin, in or on which a shell is usually secreted; many have no head distinct from the rest of the body ; the organs of sense are com- paratively slightly developed, and the move- ments slow. Respiration is effected usually by gills ; a heart is generally present, receiving the blood from the gills, and distributing it by ar- terial tubes ; the capillaries are wanting, and the veins are replaced by sinuses; the blood is commonly whitish or whitish blue. The de- velopmental energies seem to have been ex- pended chiefly in the perfection of those organs concerned in the preservation of the individual and the species ; some mollusks are hermaph- rodite and require mutual impregnation, and in others the sexes are distinct ; most are ovipa- rous ; the eggs, often connected in bunches or adhering to each other by a gelatinous sub- stance, have a thin outer shell or chorion, sometimes of a horny consistence. The ter- restrial species are few compared with those of fresh, and especially of salt water. In the lowest class of acephala or headless mollusks, in the old classifications, we had the orders of firyozoa, tunicata, firacTiiopoda, and lamelli- branchiata. As stated under BEACHIOPODA and BRYOZOA, Prof. E. S. Morse regards the first of these orders as articulates, coming near the tubicolous worms ; and there are good rea- sons, as there stated, for including also the Iryozoa or polyzoa and the ascidians or tuni- cates among the articulates with molluscan affinities, which have been separated by re- cent authors under the division molluscoids. (See MOLLUSCOIDS.) The acephalous mollusks, the lowest of the branch, the lamellibran- chiata, are characterized by a right and left shell, enclosing a depressed body, covered on both sides by a layer of the mantle ; the bran- chiae are at the sides of the body, mostly lamellar (whence their name) and placed un- der each lobe of the mantle, but sometimes pectinated ; they are generally two on each side, and sometimes the triangular interval be- tween them on the dorsal surface is used as a temporary deposit for the eggs. Most have four lamelliform tentacles, in pairs on the sides of the mouth; the shells are opened by an elastic ligament at the back, and are closed by one or two internal muscles, in the former case being called monomyaria, and in the sec- ond dimyaria. The heart is arterial, consist- ing of a ventricle and usually of two auricles, the former, being generally traversed by the end of the intestine. They inhabit both salt and fresh water, and usually live with the back uppermost, resting on the ventral edge of the shell ; the sexes are in most cases distinct, and may often be recognized by the shape of the shell ; some are hermaphrodite, and the young are sometimes considerably different from the adults; they are ovoviviparous. As a rule there are three central nerve masses, each con- sisting of two lateral ganglia, of which the first two are always distinct from each other. The valves of the shell are in most of the same shape and size, but in some of the fixed species the lower is the deeper ; in the oyster, the lower and larger is the left valve ; in some the valves close tightly, in others they are open at one or both ends for the passage of the foot and other organs. Along a part or the whole of the margin of the mantle are conical cirri or organs of touch, and also tactile gill-like lami- nse around the mouth ; and this class is fre- quently sensible of light. Some have a firm and muscular prolongation from the abdomen called the " foot," possessing great contractil- ity, by means of which they move about at the bottom of the water ; at the base of the foot in others is a bundle of filaments, called the lys- sus, secreted by a glandular tissue, and occa- sionally united into a common mass ; a famil- iar example of this is seen in the common mus- sel (mytilus borealis, Lamarck), which attaches itself by its silken threads very firmly to rocks, shells, and seaweeds ; a few, unprovided with a byssus, grow fast by one of the shells to sub- marine objects. Many of this class are entirely fossil, and of some genera the extinct species are more numerous than those now living, the latter being in this case usually found in the Indian and South Pacific oceans. Among the monomyarians may be mentioned the common oyster and the comb or scallop shell (pecteri) ; among the dimyarians, the pearl oyster (melea- grina), hammer shell (malleus), wing shell ( pin- na), mussel (mytilus), ark shell (area), fresh- water clam and mussel (unio and anodonta), cockle (cardium), the great clam or fienitier (used in Roman Catholic churches to contain