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any small, wormlike, marine mollusk of the class Aplacophora. Unlike most mollusks, solenogasters have no shell. They are covered instead by a spiny cuticle. Most are 2.5 cm (1 inch) or less in length. The largest is 30 cm long. The animals occur at ocean depths of 30 to 1,800 m (100 to 5,900 feet). About 150 species are known.