From ZooTerms (Dictionary of Invertebrate Zoology)
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operculum (noun; plural opercula; Latin operculum, lid): 1. A lid or flaplike cover.

2. (Annelida: Polychaeta) In certain sedentary forms, a modified tentacle that closes the tube; in some Spirorbidae (Pileolaria and Janua) enlarges and serves as a brood pouch.

3. (Arthropoda) a. In Chelicerata, a plate covering the opening of the book-lungs of spiders. b. In Crustacea, scuta and terga and sometimes associated membrane forming the apparatus occluding an aperture. c. In Diplopoda, a plate-like anterior sclerite of the vulva. d. In Insecta, various plates, flaps and specialized structures of the genital segments.

4. (Bryozoa) A generally uncalcified membrane, hinged on its posterior lip that closes the zooidial orifice

See also: mandible. 5. (Cnidaria: Hydrozoa) A cover sealing the hydrotheca or gonotheca, may be up to four sections; the lid on the distal end of a nematocyst. 6. (Mollusca) A corneous or calcareous structure borne by the foot serving for closure of the aperture. 7. (Platyhelminthes) The lid-like opening of an egg-shell.