Polymorphism usually occurs, certain individuals having the form of avicularia or vibracula.
Although at first sight the arrangement of the avicularia in Cheilostomes appears to follow no general law some method is probably to be made out on closer study.
In the majority of Cheilostomes, the avicularia are, so to speak, forced out of the ordinary series of zooecia, with which they are rigidly connected.
It can hardly be doubted that the function of these avicularia is the protection of the tentacles and compensation-sac. The suggestion that they are concerned in feeding does not rest on any definite evidence, and is probably erroneous.
The most singular of the external appendages found in the Polyzoa are the avicularia and vibracula of the Cheilostomata.
The avicularia and vibracula give valuable aid to the systematic study of the Cheilostomata.
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.
But avicularia or vibracula may also occur in other places - on the backs of unilaminar erect forms, along the sutural lines of the zooecia and on their frontal surface.