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.