Pronunciation: (ō-pûr'kyu-lum),[key] —n., —pl.-laPronunciation: (-lu),[key]-lums. 1. Bot.,Zool.a part or organ serving as a lid or cover, as a covering flap on a seed vessel.
2. Zool. a. the gill cover of fishes and amphibians. See diag. under fish. b. (in many gastropods) a horny plate that closes the opening of the shell when the animal is retracted.