opercula

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o·per·cu·lum

O0094300 (ō-pûr′kyə-ləm)n. pl. o·per·cu·la (-lə) or o·per·cu·lums 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.
[Latin, lid, from operīre, to cover; see wer- in Indo-European roots.]
o·per′cu·lar (-lər) adj.