And all these gradations can be actually traced.
Besides the avicularia, the polyzoa possess curious organs called vibracula.

These generally consist of long bristles, capable of movement and easily excited.

In one species examined by me the vibracula were slightly curved and serrated along the outer margin, and all of them on the same polyzoary often moved simultaneously; so that, acting like long oars, they swept a branch rapidly across the object-glass of my microscope.

When a branch was placed on its face, the vibracula became entangled, and they made violent efforts to free themselves.

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.