The main feature of Gastropods is torsion. At some stage in their development part of the body containing the internal organs rotates through 90 to 180 degrees relative to the foot. Typically they have a large muscular creeping foot which they can retract into a coiled shell and often close the opening with a lid, the operculum. The head has one or two pairs of tentacles, eyes and a rasping tongue, the radula.
The Pulmonates include snails and slugs, the latter having lost their shells, which live on land and can breathe air. The Nudibranchs include the sea slugs and sea hares in which the shells have largely been lost or become internal. The Prosobranchs include other aquatic species with a single shell such as the top shell, periwinkle, cowrie and whelk.
The Nudibranchs are marine gastropods in which the body is generally uncoiled and the shells have largely been lost or become internal. Some do not crawl like other gastropods but glide through the water by undulations of their extended mantle. They include the sea slugs and sea hares.
Subclass of the gastropods containing the majority of the conspicuous marine species the empty shells of which get washed up on the beach, including the limpets, conchs, whelks, cone shells, periwinkles and cowries.
The pulmonates are nearly all land-living gastropods and include about 15,000 species of slugs and snails. They have eyes on the tips of retractible, sensory stalks. Their mantle cavity forms a lung with a contractile opening. They are hermaphrodites and most have coiled shells but various groups have lost these.