Biology A lidlike structure covering an opening, especially:
a. A bony plate that covers and protects the gills of most bony fishes. Also called gill cover.
b. 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.
c. A covering at the top of the spore capsule of most mosses, falling off when mature spores are released.