Mollusca, a branch of the invertebrate animal kingdom, so named from the general softness of the body; some of its members were first defined by Aristotle under the name of mala-Tcia (soft animals), of which the Latin mollusca and English mollusk are rude equivalents. Cu-vier, between 1792 and 1817, determined the characters and boundaries of this branch by investigating its anatomical structure; before his time the study of the shells with which most mollusks are provided, or conchology, had occupied almost the exclusive attention of classifiers. (See Conchology, and Malacology.) The microscopic anatomy and embryology of mollusks led to the separation of cir-ripeds, and to their being placed among articulates in the class of crustaceans; for the same reason the bryozoa were taken from polyps and placed among acephalous mollusks, and since, with the brachiopods and ascidians, among articulates. The mollusca (heterogan-gliata of Owen) include such animals as have one or more nervous ganglia below the entrance 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 Ilanley, 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 comparatively slightly developed, and the movements slow. Respiration is effected usually by gills; a heart is generally present, receiving the blood from the gills, and distributing it by arterial 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 expended chiefly in the perfection of those organs concerned in the preservation of the individual and the species; some mollusks are hermaphrodite and require mutual impregnation, and in others the sexes are distinct; most are oviparous; the eggs, often connected in bunches or adhering to each other by a gelatinous substance, have a thin outer shell or chorion, sometimes of a horny consistence.
The terrestrial species are few compared with those of fresh, and especially of salt water. - In the lowest class of aeephala or headless mollusks, in the old classifications, we had the orders of bryozoa, tunicata, brachiopoda, and lamelli-branchiata. As stated under Brachiopoda and Bryozoa, Prof. E. S. Morse regards the first of these orders as articulates, coming near the tubicolous worms; and there are good reasons, as there stated, for including also the bryozoa or polyzoa and the ascidians or tuni-cates among the articulates with molluscan affinities, which have been separated by recent 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 branchiae are at the sides of the body, mostly lamellar (whence their name) and placed under each lobe of the mantle, but sometimes pectinated; they are generally two on each side, and sometimes the triangular interval between 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 nwnomyaria, and in the second dimyaria. The heart is arterial, consisting 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 consisting 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 laminae around the mouth; and this class is frequently sensible of light. Some have a firm and muscular prolongation from the abdomen called the " foot," possessing great contractility, 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 bys-sus, secreted by a glandular tissue, and occasionally united into a common mass; a familiar example of this is seen in the common mussel (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 submarine 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 (joecten); among the dimyarians, the pearl oyster (melea-grina), hammer shell (malleus), wing shell (pinna), mussel (mytilus), ark shell (area), freshwater clam and mussel (unio and anodonta), cockle (cardium), the great clam or benitier (used in Roman Catholic churches to contain holy water, sometimes 2 ft. wide, genua tri-docno, the largest of the class), the horse-foot clam (hippopus), the edible quahaug (venus merctnaria, the shell from which the wampum of the American Indians was made), the small fresh-water cyclas, the common clam(wya are-naria), the razor shell (solen), the pholas (pid-dock or stone-borer), the ship worm (teredo), so destructive to timber in vessels and dock-vards, the waterpot shells (aspergillum), and the club shells (clamgella). All bivalves are very prolific; in those which, like the oyster, are fixed, the sperm cells of the male are carried by the currents of the water to the cavity of the mantle of the female. - The remaining three fourths of mollusks are called encephala, from having a distinct head, commonly with eyes ami tentacles, and a mouth with a complex masticatory apparatus; they have been divided into the classes of cephalophora (head bearers) and cephalopoda (with the head surrounded by the feet). The cephalophora have been subdivided, according to the modifications of the locomotive organs, into the orders of pteropoda, heteropoda, and gasteropoda.
The pteropoda are so called from two wing-like muscular expansions from the sides of the anterior part of the body, used as swimming organs, and not, according to Owen, homologous with the foot of gasteropods; they are small, marine floating, hermaphrodite, and oviparous; the form is very variable, some being globular, others long and slender; the heart, as in the whole class, is arterial; the urinary sac, within the mantle and near the heart, communicates with the respiratory cavity and with the peri-irdial sinus, introducing water into the blood; some are naked, others are provided with very delicate shells of various forms; the eyes are not well developed, but the acoustic sac exists in all; the naked species have four tentacles, the testaceous ones two. In the family the-cosomata, the head is indistinct, and the shell fragile; the best known genera of this family are hyulea and cleodora, found in the warmer temperate and tropical seas; some of them are beautiful objects, as they swim through the water like butterflies in the air; one of the largest and finest is the H. tridentata, three fourths of an inch long, commonly known as the "chariot of Venus." In the family gym-notomata, or naked pteropods, the head is distinct, and the tins are attached to the sides of the neck; it includes the genera clio and pneu-modermon; of the former, the C. borealis exists in such immense numbers in high northern latitudes, that it forms chief portion of the food or the Greenland whale, and is hence Railed whale bait by the fishermen; it is hardly an inch long. - The order heteropoda is characterized by a compressed fin-like foot having a suctorial disk; the branchiae are fringed or pinnate; the sexes are distinct All are marine, and usually are rapid swimmers with the back downward and the foot upward-the foot corresponds to the anterior portion of this organ in gasteropods.
They are sometimes called nucleobranchiates, and may be divided into the families atlantidm and Jirolidw. In the first family belongs the atlanta, with a delicate shell large enough to protect the body, found in great numbers in the midst of the tropical and temperate oceans; in these the foot supports the operculum. In the second family is placed carinaria, sometimes called the "glassy sailor," which has an elongated body, with a very small keeled shell at the posterior part, the apex turned backward; on the head are two long tentacles, and two sessile eyes behind their base; the middle part of the foot is reduced to a compressed fin-shaped lobe, with a small suctorial disk, by which they adhere to seaweeds, etc.; their motions are rapid and graceful, and they inhabit the temperate and tropical waters; a small species is found in the Mediterranean; the shell of the G. vitrea, from the Indian ocean, is highly prized. In the genus Jirola or pterotrachea there is no shell, and the animal is almost transparent; there are two eyes, and generally no tentacles, but a slight fleshy proboscis; they swim or float free in mid ocean in great numbers, and also in the Mediterranean. - In the order gasteropoda there is a large muscular disk for creeping developed from the ventral surface of the body (hence the name), as in the common slugs and snails.
They are usually un-symmetrical, the visceral portion of the body coiled spirally and protected by a univalve shell, the organs of respiration being generally atrophied; the shell is almost always closed by a calcareous, horny, or albuminous operculum. Most of them are marine, some inhabit fresh water, and a few are terrestrial; they have been divided according to the characters of the breathing apparatus. In some (monoscia) the male and female organs are in the same individual, in others (dicecia) the sexes are distinct; most are oviparous, but a few (certain snails) are ovoviviparous. In the water breathers the young are excluded with an operculated shell, which in the naked species is either shed or concealed by the mantle, and by means of ciliated fins on the sides of the head they move far away from their inactive parents, undergoing several metamorphoses in the process of growth; the air breathers pass through no such changes. They have the power of repairing injuries and of reproducing lost parts to a considerable degree.
Among the monoecious gasteropods are the following five divisions: I. Apneusta, having no distinct respiratory organs, but in their place an extensive aquiferous system, and no shell in the adult; the body is soft and elongated, the integument ciliated; they are marine; calliopcm and actceon are well known genera.
Nudibrancmata, with the branchiae extending freely from various parts of the body, as in glaucus, doris (sea lemons), in which the branchiae form a plume-like circle in the middle of the back, and eolis (sea slugs), in which they are papillose and arranged along the sides of the back.
Inferobranchiata, like phyl-lidia, in which the branchiae are at the lower part of the sides of the body.
Tectibran-Chiata, in which the leaf-like branchiae are covered by the mantle and a small shell; as, in aplysia (sea hares), formerly dreaded on account of their strange form, and the violet fluid they eject when molested, in umbrella, and in bulla (bubble shell).
Pulmonata, in which a part of the mantle cavity forms a vascular air sac or lung; most are terrestrial, and such as live in the water rise to the surface to breathe; a few are naked, but most are shell-bearing, without or with an operculum; in the inoperculated, with a well developed shell, are helix (snails), succinea (amber snail),vitrina, bu-limus, pupa, acliatina, and other land snails, the slugs (Umax), land soles (avion), pond snails (limnea), etc.; in the operculated are cyclostoma, helicina, acicula, etc. In the dioecious gastero-pods belong the following four divisions.
Tubulihranchiata, in which the branchiae are two, symmetrical, behind the heart, and enclosed with the other soft parts in a long shelly tube; as in dentalium (tooth shells).
Cyclobran-Chiata, in which the branchiae are a series of lamellae, surrounding the body between the foot and mantle; as the limpets (patella) used as food and for bait, and the sea wood-lice (chiton), with multivalve shell pieces like the carapace of articulates.
Dentibranchiata, in which the branchiae are plumose or pectinate, and with the body protected by a widely opened inoperculate shell; as in the ear shells (haliotis); the delicate violet shells (janthina), found abundantly in mid ocean, feeding upon the acalephan velellce, and suspended by a raft of air vesicles, to the under surface of which the egg capsules are attached; and the fissurel-la, or key-hole limpets.
Pectinibranchia-Ta, in which the two comb-like branchiae are contained in a dorsal cavity of the mantle opening widely above the head; they have two feelers and two eyes, and a proboscis capable of elongation in a tube form; the females secrete an albuminous matter in which the eggs are enveloped, a familiar example being the yellow grape-like bunches of the whelk (bucci-num). Here belong the bonnet limpet (calyp-trcea) and the slipper shell (crepidula); the top shells (turbo), and the pheasant shells (phasia-nella); the river snails (paludina), the periwinkles (Utorina), the turret shells (turritella), the wentletraps (scalaria), the cerithium, and natica; cowries (cyprcea), very handsome shells, and one species, C. moneta, used as money on the W. coast of Africa; marginella, wluta, mitra (mitre shells); the tans (dolium), harps (harpa), whelks (buccinum); rock shells (mu-rex), fig shells (pyrula), wing shells (strombns), the seraphs (terebellum), and numerous others. - The class of cephalopoda, the highest type of mollusks, is characterized by the locomotive and prehensile organs being attached to the head, whence they radiate in the form of muscular arms and tentacles, and by an internal skeleton combined in some with an external shell, though the integument in most is uncal-cified and flexible; the head is free and the body is covered by a muscular sac or mantle, with a transverse anterior aperture, from which projects the expiratory siphon or tube; the branchiae are concealed, the sexes distinct, and the animals oviparous, aquatic, marine, predatory and carnivorous, nocturnal, and social; the colors are changeable and brilliant; they emit an inky secretion when disturbed, which permits them to escape by the discoloration of the water; this is what the true India ink is made from.
This class have a rudimentary internal skeleton; in the head of most is a cartilaginous ring around the oesophagus, the upper part covering the cerebral ganglion, and containing the organs of vision and hearing; there is often an additional cartilage to which the muscles of the arms are attached, and others on the back and sides. The mouth is in the middle between the arms, and has two jaws like the bill of a parrot, the lower the larger; the head is separated from the body by a constriction like a neck; there is a well marked tongue. The sexual organs are at the base of the visceral sac, and the spermatophores are very active; in some of the octopods, one of the arms is deciduous, and becomes a male organ, described by Cuvier as hectocotylus and a parasite; the eggs are laid in heaps or bunches, attached to each other and to foreign bodies. The nervous system is largely developed. For respiration water is drawn in and expelled by the muscular action of the mantle and funnel, as the gills have no vibratile cilia; the water enters the branchial cavity at the anterior opening of the mantle, and is forced out through the funnel, propelling the animal backward.
In the first order, the tetrabranchiata, the branchiae are in two pairs, without branchial hearts, and the mantle is thin and not very muscular; the ink bag is absent; the arms are very numerous, hollow, and with retractile tentacles; eyes pedunculate; the head retractile within a many-chambered siphunculated cell. Among existing mollusks this order contains only the genus nautilus (see Nautilus); in past ages lived the ammonites, baculites, hamites, orthoceratites, turrilites, etc. In the second order, the dibranchiata, the branchiae are two, each with a branchial heart; the funnel is an entire tube, and the mantle is muscular; an ink bag is present; there are eight non-retractile arms, large and complicated, bearing sucking disks or acetabula, with usually two additional long arms; the eyes are sessile and in orbits; the shell is internal, except in the female argonauts. In the decapod tribe, with eight arms and two tentacles, belong the genus spirula, the extinct belemnites, the cuttle fishes (sepia), and the squids. (See Squid.) In the octopod or eight-armed tribe there are no tentacles, the arms have sessile suckers, and the branchial chamber is divided by a longitudinal partition; the arms arc more robust, and are often united by a web at the base, constituting a powerful swimming organ.
Among the naked octopods belong the so-called sea spiders. (See Octopus.) Eledone and tre-mectopus are allied genera. The genus argo-nauta or paper nautilus is well known for the delicate and beautiful shell of the female. (See Nautilus.) The shell is used only for protecting and hatching the eggs; the male has no shell, and impregnation is effected by a deciduous hertocotylnt - The local distribution of faumeand the distinctness of zoological regions are well illustrated by mollusks; while some are very limited in their range, others, like the cyprcea, are extensively spread even across ocean barriers; some are cosmopolite, wandering wherever their food is found; helix cella-rw, attaching itself to water casks, occurs in most seaports of the world, H. similaris wherever the coffee plant grows, and H. ritrinoides follows the taro or arum esculentum. As a general rule, according to Mr. Jeffreys, specimen- are larger toward the north than toward the south; colors are usually the brightest in the tropical seas, except in specimens from great depths. (For details on distribution, see "Mollusca and Shells of the United States Exploring Expedition," 1838-'42, by A. A. Gould, M. D, Boston, 1852.) The distribution of mollusks in time extends from the lower Silurian to the present epoch; all the classes are represented in the earliest fossiliferous strata; some families, like the ammonites and belemnites, have passed away; others, like the nautilus, are verging toward extinction; some have continued with slight specific modifications from the Silurian to the present day.
Lamel-libranchiate have succeeded palliobranchiate bivalves; siphonate have succeeded asiphonate univalves; and the dihranchiate now vastly outnumber the tetrabranchiate cephalopods. Whole strata of the earth's crust are made up principally of the shells of mollusks. - Mollusks supply an abundant, wholesome, and usually easily digestible article of food to nations civilized and savage, as well as to other animals; bivalves are considered the best, as having the least muscular fibre. The ornamental purposes to which the pearl and cameo slu-lb are put are well known; from the cuttle fish is obtained sepia and India ink; from the purpura and buccinum of the Mediterranean came the famous Tyrian dye of antiquity; fnim the filaments of the byssus of pinna are made tissues much esteemed on the shores of the Mediterranean. On the other hand, mol-lusks are sometimes injurious to man; slugs and snails do mischief in gardens; the teredo pierces ship timber, and the pholas bores into and weakens stone dikes.
The number of species of mollusks probably exceeds 25 000 surpassed only by the number of articulates.