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1. One of several narrow external openings connecting with the pharynx, characteristic of sharks and related fishes, through which water passes to the exterior, thereby bathing the gills. Also called branchial cleft, gill cleft.
2. Embryology One of several rudimentary invaginations in the surface of the embryo, present during development of all air-breathing vertebrates and corresponding to the functional gill slits of aquatic species. Also called branchial cleft, branchial groove, pharyngeal cleft, pharyngeal groove.