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). The cylcostome family Eleidae also convergently evolved an opercular structure during the Early Cretaceous to Paleocene.