The avicularium is so called from its resemblance, in its most highly differentiated condition, to the head of a bird.
In Bugula, for instance, a calcareous avicularium of this type is attached by a narrow neck to each zooecium.
The avicularium can move as a whole by means of special muscles, and its chitinous lower jaw m- ect.
In its least differentiated form the avicularium occupies the place of an ordinary zooecium ("vicarious avicularium"), from which it is distinguished by the greater development of the operculum and its muscles, while the polypide is normally not functional.
They occur in particular in relation with the orifice of the zooecium, and with that of the compensation-sac. This delicate structure is frequently guarded by an avicularium at its entrance, while avicularia are also commonly found on either side of the operculum or in other positions close to that structure.
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