A subclass of the molluscan class Gastropoda, containing about 23,500 species of snails that are grouped into three orders (or superorders): Systellommatophora, Basommatophora, and Stylommatophora. Pulmonates include most of the common snails found on land or in fresh waters, although a few prosobranchs have invaded both habitats. Certain pulmonates, the Onchidiidae, Amphibolidae, Otinidae, Siphonariidae, Trimusculidae, and some members of the Ellobiidae are intertidal to subtidal on rocky shores of tropical and temperate regions.