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.
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.