GASTROPODA, the largest and most typical and familiar of the four classes of mol lusks (phylum Mollusca). The name refers to the most prominent tribal characteristics, namely, that the inferior surface of the body forms a flattened sole or disc called by the contractions of which the animal ad vances. In all these animals the primitive symmetry of the body is obscured by the un equal development of parts, whence results the spiral disposition of tile majority. The simplest gasteropods, however, such as the chiton, are symmetrical, not lop-sided like the higher forms. They have the mouth at one end of the long axis of the body, the anus at the other ; the gills, kidneys, genital ducts, and circulatory organs are paired; there are two pairs (pedal and visceral) of nerve cords run ning parallel to one another along the body, and the ganglia are slightly developed. Of all mol lusks these simplest gasteropods are probably nearest the hypothetical worm-like ancestor. When a shell is present it consists of only one piece, whence the name °univalve? formerly applied to the class; or if of more than one piece the separate portions are placed one be hind the other in the axis of the body (Amphi neura, chitons). The gasteropods agree with the cephalopods in possessing a distinct head, con taining a feeding instrument or °tongue') in the form of a lingual ribbon, but are separated from that class by the mode of formation of the shell, and by the absence of arms around the head. The lingual strap or odontophore consists of a central portion (rachis) and lateral pieces (pleura.). On all three of these, on the central, or only on the lateral regions are placed sili cious denticles, whose number, form, and ar rangement have been made the basis of classi fication of genera.
Mode of Though the number of ter restrial gasteropods, breathing the air directly by means of a pulmonary chamber, is very large — over 6,000 living species — those living is water are greatly in the majority, including over 10,000 forms, mostly marine. Of these, some 9,000 or so belong to the prosobranchs or Strep toneura, a relatively small minority being opisthobranchs and nudibranchs. The hetero
pods and some opisthobranchs enjoy a free swimming pelagic life, but most marine forms frequent the coasts either on the shores or along the bottom. Deep-sea gasteropods are compara tively few. The locomotion effected by the con tractions of the muscular ((foot') is in almost all cases very leisurely, and the average tendency is toward sluggishness. As to diet, the greatest variety obtains ; most prosobranchs with a respir atory siphon and a corresponding notch in the shell are carnivorous, and so are the active heteropods; most of the rest are vegetarian in diet. Numerous genera, both marine and terres trial, are very indiscriminate in their feeding; others, are as markedly specialists, keeping almost exclusively to some one vegetable or animal diet. Some marine snails partial to echinoderms have got over the digestive diffi culty presented by the calcareous character of the skins of their victims by a secretion of free sulphuric acid from the mouth. This acid changes the carbonate of lime into sulphate, which is brittle and readily pulverized by the rasping tongue. A few are parasitic — for ex ample, eulima, stylifer, and the very degenerate Entoconcha mirabilis, all occurring in or on holothu rians.
The eggs of gasteropods are usually small, and are surrounded with albumen, the surface of which becomes firm, while in the common snail (Helix) and some others there is an egg-shell of lime. The eggs not unfrequently develop into embryos within the parent, but in most cases they are laid, •either singly or in masses, and often with cocoons which take on various forms. Inside each of the numerous egg-cases are many embryos, but only a few reach maturity, the others serving as food ma terial, an infantile cannibalism or struggle for existence not uncommon in the class. As to development it may be noted that the ovum divides more or less unequally, according to the amount of yolk, that a gastrula-stage occurs as usual, and that this is succeeded in typical cases, first by a “trochophore)) and afterward by a aveligerp larva (see MOLLUSCA ).