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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.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
References in classic literature ?
The opercular valves of sessile cirripedes (rock barnacles) are, in every sense of the word, very important structures, and they differ extremely little even in different genera; but in the several species of one genus, Pyrgoma, these valves present a marvellous amount of diversification: the homologous valves in the different species being sometimes wholly unlike in shape; and the amount of variation in the individuals of several of the species is so great, that it is no exaggeration to state that the varieties differ more from each other in the characters of these important valves than do other species of distinct genera.
Another group of tambaquis (n=96) was exposed to the same EO concentrations reported above to determine the ventilatory frequency (VF) through sedation and deep anesthesia; this was obtained through a visual observation of 20 successive opercular or buccal movements (ALVARENGA & VOLPATO, 1995).
Comparison of age estimates from scale, opercular bone, otolith, vertebrae and dorsal fin ray in Labeo rohita (Hamilton), Catla catla (Hamilton) and Channa marulius (Hamilton).
In general, increase in swimming and opercular movements and reversal cases were observed.
Foix-Chavany-Marie opercular syndrome is a severe form of pseudobulbar palsy caused by bilateral anterior opercular lesions (1,2).
Number of opercular movements min-1 of four fish in each aquarium was counted without moving them from aquarium Foraging as shown in Figure 1a and social behavior including aggression (fish bit another; Figure 1b), schooling (at least 4 fish swimming together; Figure 1c), and shoaling (3 fish swimming together; Figure 1d) were analyzed.
Among these, the most significant papers and research interests are dysphagia rehabilitation in opercular syndrome in patients with stroke, aortic atherosclerosis, inflammation and stroke, stroke incidence, gender differences and stroke, stroke in women, homocysteine and stroke, patent foramen ovale and stroke, Takatsubo cardiomyopathy and transient general amnesia, and cerebral vasculitis.
Antheridia consist of a basal cell, an annular cell, and an undivided asymmetrical opercular cell (Fig.