. Atoll research bulletin. Coral reefs and islands; Marine biology; Marine sciences. operculum. Muscles contract and increase fluid pressure inside the zooid, slowly pushing the compressed lophophore and introvert region of the polypide out of the zooid through the opening operculum, so that the ciliated tentacles of the lophophore can expand to feed. Disturbance, a sudden water current or the movement of a trespassing organism on the colony surface, for example, results in the polypide's rapid retraction back into the zooid. Avicularia are polymorphic zooids found in many species of cheilosto
. Atoll research bulletin. Coral reefs and islands; Marine biology; Marine sciences. operculum. Muscles contract and increase fluid pressure inside the zooid, slowly pushing the compressed lophophore and introvert region of the polypide out of the zooid through the opening operculum, so that the ciliated tentacles of the lophophore can expand to feed. Disturbance, a sudden water current or the movement of a trespassing organism on the colony surface, for example, results in the polypide's rapid retraction back into the zooid. Avicularia are polymorphic zooids found in many species of cheilostome bryozoans. The most primitive type of avicularia have been usually been considered to be the B zooids found in some species, which may be larger than A zooids (autozooids), and have a feeding polypide like those of A zooids, but have an enlarged or thickened operculum. Most avicularia are smaller than zooids, and may either replace them in budding sequence in the colony (vicarious or interzooecial avicularia) or be budded from the frontal wall of a supporting autozooid (frontal avicularia). Avicularian zooids have an modified operculum called a mandible. It is enlarged compared to an autozooid operculum, often covering most of the frontal wall of the avicularium, and is usually more rigidly chitinized, strengthened, and modified, sometimes edged with a sharp pointed tip (, Fig. 2), rows of chitnous teeth, or shaped into a bristle or paddle form. The avicularium zooid may have enlarged muscles to operate the heavy mandible and retains some membranous frontal wall area, but most of it is essentially a reinforced socket for the closed mandible. There is no functional polypide, although sometime a polypide rudiment tipped with sensory cilia is present (Fig. 2), especially in the bird's head or pedunculate avicularia that are usually considered the most derived type. Figure 2. Pedunculate avicularium of Synnotum circinatum with open mandible. PR= opening of polypide rudiment.
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