(1):

(n.) The fold of integument, usually supported by bony plates, which protects the gills of most fishes and some amphibians; the gill cover; the gill lid.

(2):

(n.) Any lid-shaped structure closing the aperture of a tube or shell.

(3):

(n.) The lid closing the aperture of various species of shells, as the common whelk. See Illust. of Gastropoda.

(4):

(n.) The principal opercular bone in the upper and posterior part of the gill cover.

(5):

(n.) The lid of a pitcherform leaf.

(6):

(n.) Any lidlike or operculiform process or part; as, the opercula of a dental follicle.

(7):

(n.) The lid of the urnlike capsule of mosses.