Digestive Organs.—The alimentary canal is typically a straight tube, but in most forms it becomes convoluted to increase the amount of digestive surface, while not infrequently it is so flexed on itself that mouth and vent, instead of being at opposite ends of the body, are in close proximity to each other. In all except the Pelecypoda the region of the mouth contains a peculiar structure, variously known as the lin gual ribbon, radufa or odontophore. This con sists of a strong hand, having upon its upper surface numbers of rows of hard horny teeth —in fact, a flexible file; and of muscles adapted to draw it back and forth over any object to which the mouth may be applied. In this way. the snails rasp away vegetation, or, as in the case of shells of other mollusks, are perforated in order that the soft parts may be reached and devoured. In the cone-shells a poison-gland is connected with the lingual ribbon. Wear at one end of the ribbon is made good by constant growth at the other end. Behind the gullet is a large sacular stomach, and closely connected with it are the ducts of the voluminous liver. The intestine is long, without enlargements, and in many acephala is remarkable for passing through the heart.
Circulation.— The heart lies dorsal to the digestive tract enclosed in a special sac, the pericardium, which is to be regarded as the sole representative of the true body-cavity or cce lom. (See EMURYOLOGy). In the heart two parts are always to be distinguished, a muscular ven tricle which forces the blood through the arte ries to all parts of the body, and One or two auricles which receive the blood as it comes from the gills and force it into the ventricle. As will be seen, the heart thus receives only oxidized or arterial blood. With the loss of the gill of one side the corresponding auricle disappears. When four gills are present as in the nautilus, there are four auricles. In the cephalopods branchial hearts, which force the blood through the gills, occur. At one time it was thought that some of the blood-vessels opened to the exterior, but this has been shown to be a mistake. The blood is usually yellowish or colorless, but occasionally is red, the color being due to the plasma and not to the con tained corpuscles which resemble the leucocytes of the blood of man.
Gills.—As was mentioned above, there may be two kinds of gills, the ctenidia and the see ondary or adaptive gills, the latter occurring only in the gasteropoda. The ctenidia, which always occur in the angle between mantle and body-wall, consist, typically, of a series of fila ments with blood-vessels in the interior, the fila ments and the ridge from which they spring resembling somewhat the teeth and back of a comb, whence the name (Greek Krevk comb). The ctenidia are typically paired, but in the nautilus there are two pairs, while in many gasteropods one ctenidium (that of the left side) is lost. The adaptive gills occur on vari ous parts of the body, usually upon the back. In most land snails (Pulmonata) the gills en tirely disappear and an air-breathing organ, the lung, is developed on the walls of the mantle cavity, the opening to it being usually on the right side of the body.
Nerves.— The nervous system was described above. It is only necessary to say that the gan glia may coalesce into a smaller number. There are usually present three pairs of sense organs, a pair of eyes connected with the cerebrum a pair of so-called ears (really organs,. of equili
bration) connected with the pedal ganglia, and a pair of organs of smell (osphradia) with the visceral ganglia. Of these the eyes are the least constant. In some cases they are replaced by numerous other eyes developed upon the back or upon the edges of the mantle.
Viscera.— The excretory organs are true ne phi'idia, that is, coiled tubes opening at one end into the ccelom (pericardium), and at the other to the external world. They are also known as the organs of Bojanus. The reproductive or gans are large. Usually the sexes are separate, but some, like the land-snails, are hermaph roditic. In no case is an asexual reproduction (fission, budding, etc.) known. A few bring forth living young. In many species a meta morphosis occurs during the development. In these a peculiar larva, known as the veliger, hatches from the egg, a larva which develops from a utrochosphere" form like that of the annelids, a resemblance which points to a rela tionship between the two groups. The veliger receives its name from the a circle of cilia upon the dorsal side of the head above and in front of the mouth, by means of which the larva swims.
The mollusks are divided into five classes, Amphineura, Pelecypoda or Acephala, Scaphopoda, Gastropoda and Ceph alopoda.
Amphineura.— This, the most primitive group of mollusks, contains the chitons (Placo phora) and the Solenogastres, in both of which the body is markedly bilaterally symmetrical, while the nervous system is of a very low type. The chitons (q.v.) are flattened and covered with eight transverse plates of shell. The Solenogastres arc worm-like shell-less forms of the deeper seas.
Pelecypoda or Acephala.— These are the mollusks which have the shell in two parts or "valves," no head, and the filaments of the gills more or less completely united into a couple of leaves (lamellm) on either side of the body. See BIVALVES.
Scaphopoda.— This class includes a few ma rine forms of small size known as tooth-shells, from having shells shaped somewhat like an ele phant's tusk, and open at both ends. See DEN TALTI/M.
Gastropoda.— In these, the so-called snails, the foot is usually a broad creeping disc and the head is well developed. See GASTROPODA.
Cephalopoda.— In these the head is well marked off from the body, and the mantles of the two sides are united so that a single mantle cavity results, which is open to the exterior in front. See CEPHALOPODA.
Formerly two other orders of mollusks were recognized, the Heteropoda and the Ptero poda, but the heteropods are now known to be prosobranchs, the pteropods to be opistho branchs, both being modified for a life on the surface of the sea.
Bibliography.— Hescheler, in Lang's 'Com parative Anatomy' (2d ed., 1896) ; Woodward, 'Manual of the Mollusca' (4th ed., 1880) ; Cooke, 'The Mollusca' (in Cambridge Natural History, Vol. III, 1895); Kingsley, 'Standard Natural History' (Vol. I, 1885) ; part V of 'A Treatise on Zoology,' edited by E. Ray Lankester, by Paul Pelseneer (London 1906); Tryon and Pilsbury began in 1896 a work in which they proposed to deScribe and figure every known species of mollusk. For mollusks of the United States consult writings of Gould, Binney, Morse, Tryon, Dalt, Verrill, Bush, Stearns, etc.