A member of the largest class of phylum Mollusca. Characteristics generally include: a foot upon which the rest of the body (called the "visceral mass") sits, a well-developed head, a protective one-piece shell, and body "torsion" - where most of the visceral mass is twisted anticlockwise 180 degrees so that the back end of the animal is positioned over its head. The class includes the snails, slugs, sea hares, sea slugs, limpets, and abolone. Compare brachiopod.