They are supposed to serve as a defence, and may be seen, as Mr.

Busk remarks, "to sweep slowly and carefully over the surface of the polyzoary, removing what might be noxious to the delicate inhabitants of the cells when their tentacula are protruded.

" The avicularia, like the vibracula, probably serve for defence, but they also catch and kill small living animals, which, it is believed, are afterwards swept by the currents within reach of the tentacula of the zooids.

Some species are provided with avicularia and vibracula, some with avicularia alone and a few with vibracula alone.