GASTEROP'ODA (Gr. belly.rfooted), or GASTROPODS, a class of mollusks, inferior in organization to cephalopads, but far superior to almost all other mollusks, and containing a multitude of species, the greater number of which are marine, but some are inhabi tants of fresh water, and some are terrestrial. Snails, whelks, periwinkles, limpets,. cowries, and the greater number of mollusks with univalve shells belong to this class, and univalve molluscs constitute the greater part of it; but it contains also some mollusk§ with multivalve shells, as chitons, and some, as slugs, which have either only a rudimental internal shell, or no shell at all. Some aquatic kinds are destitute of shell in the adult state, but they are protected by a rudimentary shell on first issuing from the egg. No known gastropod has a bivalve Shell, unless the operculum,, which closed the mouth of the shell in many species, be regarded as a second valve.
Gastropods have a head, more or less fully developed, in which is situated the mouth, and which generally carries fleshy, retractile tentacula, varying from two to six in number. The tentacula do not encircle the mouth; they seem to he special and exquisitely sensitive organs of touch, a sense which the general surface of the body does not seem to possess in a high degree; and in some gasteropoda. as snails, they carry the eyes at their tips, hut in others the eyes—always small—are situated elsewhere on the head, and a few are destitute of eyes. They are believed to possess the senses of taste and smell, and at least some of them that also of hearing, as they not only have a nervous center analogous to the acoustic division of the brain in vertebrate animals, but a little sac on each side, apparently an organ of this sense. Their nervous system is more complex, and concentrated than that of the headless (acepluilous) mollusks; the principal nervous masses surround the gullet. In the highest gasteropoda, such as snails, there are only two principal nervous masses, one of which, supplying the nerves connected with sensation, is called the brain. The blood of gasteropoda is often opa lescent, With a few.colorless _corpuscles. The heart is always systemic only, and in almost all consists of one auricle and one ventricle, .altlibucli a few gasteropoda have two auricles, one set of gills. Near the commencement of the aorta, there is often a contractile muscular swelling (bulbus arteriosus), as in fishes. Respiration takes place generally by gills, which are very variously situated, sometimes externally, some times in a special cavity, and exhibit an equally great variety of form and structure; but some gasteropoda, as snails and slugs, have, instead of gills, a pulmonary sac or cavity, lined with a vascular these being either inhabitants of the land, or, if of the water, obliged to come occasionally to the surface for the purpose of breathing. A few of the lowest gastet•opoda, doubtfully placed in this class, are destitute of distinct respiratory organs. The digestive apparatus also exhibits much diversity. Sonic of the gasteropoda feed on vegetable, some on animal substances, and some of them on animals which •they themselves kill. Thus, whilst snails eat leaves and other soft parts of vegetables, whelks (Itecittion) prey on other mollusks, and are provided with a remark able apparatus at the end of a proboscis into which the mouth is elongated, for filine. a
hole—as nice as could be made by the drill of a mechanic—through the hardest shell. The mouth of the puail is, in like manner, admirably adapted to the cutting of leaves or similar 'substances by the action of the lips against a sharp horny plate. Other gasteropoda have the mouth furnished with two cutting blades, wrought by powerful muscles. The tongue of some is covered with minute recurved hooks, to prevent the possibility of anything escaping from the mouth; and the stomach of some is a muscular gizzard, provided with cartilaginous or sometimes calcareous projections, or stomachic teeth, to aid in the comminution of the food. The intestine is generally bent back, so that the anus is not far from the head. The liver is large, as are also the salivary glands of many gastropods. Very great diversities are found in the reproductive system. In some gasteropoda, the sexes are distinct (G. DitEctA); others are hermaphrodite (G. MonicciAl; and whilst self-impregnation takes place in some of these, others—as snails —mutually impregnate each other by copulation. In general, the reproductive organs are very largely 'developed, and are of complex and remarkahle structure. The gasteropoda are in general oviparous; a few are ovoviviparous. The young of aquatic gasteropoda at first swim about actively by means of ciliated fins attached to the head. Gasteropoda are generally unsymmetrical, one side of the body being developed without the other,.some of the principal organs of which—the gills and nerves—are atrophied; and thus the shell with which most of them are covered becomes, in the greater number, spiral, the spire turning towards the unatrophied side, which is generally the right side, although in some (refereed or sinistrorsal shells) it is the left. The head and the organ of locomotion are capable of being withdrawn into the last whorl of the shell, and in aquatic species generally, the mouth of the shell can be closed by an operculann (q.v.), exactly it, and attached to the foot, but in which many varieties of beautiful structure are exhibited, and which is generally horny, sometimes calcareous. Sonic shells are simply conical, and there are numerous diversities of form. The shell is secreted by the mantle. See Mom.usus, Sitsb.i.s, and UNFVALVES. The viscera are contained in whin sac—part of the mantle—which fills the upper part of the shell. The organ of locomotion, called the foot, is in general a muscular disk, developed from the ventral surface of the body; sometimes, as in limpets, capable of acting as a sucker, and exhibiting other even more remarkable modifications, so that in some it becomes an organ for swimming. Gasteropoda generally creep by means of this disk adhering to surfaces, and contracting in traverse wrinkles or undulations, which begin from behind. The gasteropoda generally secrete a peculiar kind of slime. Some of them also produce other peculiar secretions, of which the Tyrian purple affords an example. Gasteropoda have a great power of renewing lost ptirts; tentacles are thus restored, and even the eyes which they bear at their tips, the mouth with all its apparatus, or the head itself.