In the

bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic invertebrate animals, nearly all living in sedentary colonies. Typically about long, they have a special feeding structure called a ...

n order Cheilostomatida, the operculum is a

calcareous Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines. In zoology ''Calcareous'' is used as an ad ...

or chitinous lid-like structure that protects the opening through which the

polypide The polypide in bryozoans encompasses most of the organs and tissues of each individual zooid A zooid or zoöid is a single animal that is part of a colonial animal. This lifestyle has been adopted by animals from separate unrelated taxa. ...

protrudes. Many species have modified the operculum in specialized

zooid A zooid or zoöid is a single animal that is part of a colonial animal. This lifestyle has been adopted by animals from separate unrelated taxa. Zooids are multicellular; their structure is similar to that of other solitary animals. The zoo ...

s (avicularia) to form a range of

mandible In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...

s (probably for defense) or hair-like

seta In biology, setae (singular seta ; from the Latin word for " bristle") are any of a number of different bristle- or hair-like structures on living organisms. Animal setae Protostomes Annelid setae are stiff bristles present on the body. ...

e (probably for cleaning, or in some unattached species, such as '' Selenaria'', for locomotion). The cyclostome family Eleidae also convergently evolved an opercular structure during the Early

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

Paleocene The Paleocene, ( ) or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago (mya). It is the first epoch of the Paleogene Period in the modern Cenozoic Era. The name is a combination of the Ancient Greek ''pal ...

.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

References

Protostome anatomy Cheilostomatida {{bryozoan-stub