1) A body process or part that suggests a lid: as a) a horny or shelly plate on the posterior dorsal surface of the foot in many gastropod mollusks that closes the shell when the animal is retracted b) the covering of the gills of a fish. 2)a lid or covering flap ( as of a moss capsule or pyxidium in a seed plant).
Source: Encyclopaedia Britannica 2002