In the

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

n order

Cheilostomatida Cheilostomatida, also called Cheilostomata, is an order of Bryozoa in the class Gymnolaemata. They are exclusively marine, colonial invertebrate animals. Cheilostome colonies are composed of calcium carbonate and grow on a variety of surfaces, ...

, the operculum is a

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

chitin Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is the second most abundant polysaccharide in nature (behind only cell ...

ous 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. This includes the tentacles, tentacle sheath, U-shaped digestive tract, musculature and nerve cells. It is housed in the zooidal exoskeleton, which in ...

protrudes. Many species have modified the operculum in specialized

zooid A zooid or zoöid is an 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 zooids can ...

s (avicularia) to form a range of

mandible In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...

s (probably for defense) or hair-like

seta In biology, setae (; seta ; ) are any of a number of different bristle- or hair-like structures on living organisms. Animal setae Protostomes Depending partly on their form and function, protostome setae may be called macrotrichia, chaetae, ...

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 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

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

.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