Operculum (bryozoa)

In the bryozoan order Cheilostomatida, the operculum is a calcareous or chitinous lid-like structure that protects the opening through which the polypide protrudes.

Many species have modified the operculum in specialized zooids (avicularia) to form a range of mandibles (probably for defense) or hair-like setae (probably for cleaning, or in some unattached species, such as Selenaria, for locomotion^[1]).

The cyclostome family Eleidae also convergently evolved an opercular structure during the Early Cretaceous to Paleocene.^[2]