operculum

A lidlike structure covering an opening, especially.

A bony plate that covers and protects the gills of most bony fishes.

A horny or calcareous plate attached to the foot of most larval and many adult gastropods, used to close the aperture when the animal retracts into its shell.

A covering at the top of the spore capsule of most mosses, falling off when mature spores are released.

A lid or cover; in natural history, a part, organ, or structure which forms a lid, flap, or cover.