The avicularium is so called from its resemblance, in its most highly differentiated condition, to the head of a bird.
The avicularium can move as a whole by means of special muscles, and its chitinous lower jaw m- ect.
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.
In Bugula, for instance, a calcareous avicularium of this type is attached by a narrow neck to each zooecium.