The avicularium in cheilostome

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...

 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...

ns is a modified, non-feeding zooid. The operculum

Operculum (bryozoa)

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

, which normally closes the orifice when the zooids tentacles are retracted, has been modified to become a mandible. Strong muscles operate it. 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....

 is greatly reduced, and the indiviual receives nourishment from neighboring zooids. The shape of the avicularian zooid can be identical to the feeding autozooid, but is usually elongated in the direction of the mandible.

Avicularia are categorised by their position relative to the autozooids. Vicarious avicularia effectively replace an autozooid in the colony structure and are usually a similar size and shape as the autozooids. Interzooidal avicularia are wedged between autozooids but do not replace an autozooid. Adventitious avicularia are placed somewhere on the external (frontal) wall of an autozooid and are usually much smaller. The 'birds-head' avicularium (e.g. Bugula

Bugula

Spiral tufted bryozoa or Bugula turrita are a very common colonial marine animal found from Maine to North Carolina.Bugula neritina is of current interest as a source of cytotoxic chemicals, bryostatins, under clinical investigation as anti-cancer agents.Dried Bugula are commonly used as...

) is elevated above the colony by a stalk (peduncle).

A zooid where the operculum is modified into a very long, hair-like structure is called a vibraculum.

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, which 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...

, also possess avicularia-like zooids.