Molluscs include terrestrial snails and slugs, sea slugs, chitons, shellfish - bivalves and univalves, tusk shells, octopuses and squid, and the "living fossil" group known as monoplacophorans. Although diverse morphologically, molluscs exhibit a variety of common characteristics, including a soft body (usually) surrounded by some sort of shell secreted by the mantle and a strong muscular foot used for locomotion. The shell may be external or internal or secondarily lost.