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any marine gastropod of the approximately 2,000 species of the subclass Opisthobranchia. These gastropods, sometimes called sea slugs, breathe either through gills, which are located behind the heart, or through the body surface. The shell and mantle cavity are reduced or lacking. Larval asymmetry may give way to bilateral symmetry in the adult through a partial untwisting of the embryonic viscera. Reproduction is hermaphroditic-i.e., both male and female reproductive organs occur in one individual.