In the bryozoa

Bryozoa

The Bryozoa, also known as Ectoprocta or commonly as moss animals, are a phylum of aquatic invertebrate animals. Typically about long, they are filter feeders that sieve food particles out of the water using a retractable lophophore, a "crown" of tentacles lined with cilia...

n order Cheilostomata

Cheilostomata

Cheilostomata, an order of Bryozoa in the class Gymnolaemata, are exclusively marine, colonial invertebrate animals. Cheilostome colonies are composed of calcium carbonate and grow on a variety of surfaces, including rocks, shells, seagrass and kelps. The colony shapes range from simple encrusting...

, the operculum is a calcareous

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:...

 or chitinous

Chitin

Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...

 lid-like structure that protects the opening through which the polypide

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 skeleton, which in cyclostomes is tubular and in cheilostomes is box-shaped....

 protrudes.

Many species have modified the operculum in specialized zooid

Zooid

A zooid or zoöid is a single animal that is part of a colonial animal. The zooids can either be directly connected by tissue or share a common exoskeleton...

s (avicularia) to form a range of mandible

Mandible

The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...

s (probably for defense) or hair-like seta

Seta

Seta is a biological term derived from the Latin word for "bristle". It refers to a number of different bristle- or hair-like structures on living organisms.-Animal setae:In zoology, most "setae" occur in invertebrates....

e (probably for cleaning, or in some unattached species, such as Selenaria, for locomotion).

The cylcostome

Cyclostomatida

Cyclostomatida, or cyclostomes, are an ancient order of stenolaemate bryozoans which first appeared in the Lower Ordovician. It consists of 7+ suborders, 59+ families, 373+ genera, and 666+ species. The cyclostome bryozoans were dominant in the Mesozoic; since that era, they have decreased...

 family Eleidae also convergently

Convergent evolution

Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...

 evolved an opercular structure during the Early Cretaceous

Cretaceous

The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 to Paleocene

Paleocene

The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...

.