Merriam-Webster Collegiate Dictionary - operculum

 noun  (plural opercula; also -lums)  Etymology: New Latin, from Latin, cover, from operire to shut, cover  Date: 1681  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 — see fish illustration  2. a lid or covering flap (as of a moss capsule)