In the [[bryozoa]]n order [[Cheilostomatida]], the '''operculum''' is a [[calcareous]] or [[chitin]]ous lid-like structure that protects the opening through which the [[polypide]] protrudes.
Many species have modified the operculum in specialized [[zooid]]s (avicularia) to form a range of [[mandible]]s (probably for defense) or hair-like [[seta]]e (probably for cleaning, or in some unattached species, such as ''[[Selenaria]]'', for locomotion<ref>Cook, P.L. & Chimonides, P.J. 1987. Recent and fossil Lunulitidae (Bryozoa, Cheilostomata), 7. ''Selenaria maculata'' (Busk) and allied species from Australasia. Journal of Natural Historia 21: 933-966</ref>).
The [[Cyclostomatida|cyclostome]] family [[Eleidae]] also [[convergent evolution|convergently]] evolved an opercular structure during the Early [[Cretaceous]] to [[Paleocene]].<ref>Taylor, P.D. 1994. Systematics of the melicerititid cyclostome bryozoans; introduction and the genera ''Elea'', ''Semielea'' and ''Repromultelea''. Bulletin of the Natural history Museum, Geology Series 50:1-103</ref>
==References== {{reflist}}
[[Category:Protostome anatomy]] [[Category:Cheilostomatida]]
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