OPERCULUM

n. (pl. opercula) 1 Zool. a a flaplike structure covering the gills in a fish. b a platelike structure closing the aperture of a gastropod mollusc's shell when the organism is retracted. c any of various other parts covering or closing an aperture, such as a flap over the nostrils in some birds. 2 Bot. a lidlike structure of the spore-containing capsule of mosses. øøopercular adj. operculate adj. operculi- comb. form. [L f. operire cover]