Onchidiidae are a family of small, air-breathing sea (and land) slugs. They are shell-less marine (except for 2 species) pulmonate gastropod molluscs. Onchidiidae is the only family within the superfamily Onchidioidea. These animals are quite unusual in that they are emphatically not opisthobranchs with gills, as are almost all of the sea slugs. Instead these creatures are pulmonates. They are more closely related to air-breathing land and freshwater snails and slugs than they are to most other sea snails and sea slugs.