Gasteropoda (Gr.Gasteropoda 700279 belly, and

Gasteropoda 700281foot), a class of the mollusca distinguished by the under side of the body forming a single muscular foot, on which the animal creeps or glides. The snails, limpets, and chitons are examples of this class. They are divided into two natural groups, one breathing air (pulmo-nifera), the other water (branchtfera). These form the four orders of prosobranchiata, jmlmonifera, opisthiobrancMata, and nucleo-branchiata. The shell is usually spiral and univalve, but sometimes tubular or conical; in the chiton it is multivalve. Some marine species, as the doris and ceolis, have no shells. Most are provided with a horny or shelly operculum, which forms the bottom of the foot, and when withdrawn closely shuts the aperture of the shell, to which it is firmly held by the strong muscles of this part of the body. In some species, as the limpet and patella, the animal uses the expanded surface of the foot for attaching the shell firmly to rocks and other surfaces. Almost all are unsymmetrical, the body being coiled up spirally, and the respiratory organs of the left side usually atrophied. A few, like the snails, are viviparous, but most are oviparous.

The shells are nearly all right-handed; the cavity is a single conical or spiral chamber, never many-chambered like the nautilus and the cephalopods; the apex is almost always directed backward. The lines between the whorls or turns of the shell are the sutures, the last or the body whorl being usually very large; the aperture is entire in most vegetable feeders, but notched or elongated into a canal or siphon which is respiratory in its office; there is sometimes a posterior or anal canal.