1. (in zoology) A lid or flap of skin covering an aperture, such as the gill slit cover of fish and larval amphibians and the horny calcareous operculum secreted by many gastropod molluscs, which closes the opening of the shell when the animal is inside.
2. (in botany) The cone-shaped lid of the capsule of mosses, which is forcibly detached to release the spores.
Geology and Earth Sciences
1. In animals, a lid or cover, sometimes hinged, occurring, for example, in some cylindrical rugose corals, some Bryozoans, and in gastropods.
2. A flap of skin covering the gills in bony fish (Osteichthyes).