Several genera (Flustra, Eschara, Cellaria, Crisia, and others) agree in having singular movable organs (like those of Flustra avicularia, found in the European seas) attached to their cells.

The organ, in the greater number of cases, very closely resembles the head of a vulture; but the lower mandible can be opened much wider than in a real bird's beak.

The head itself possesses considerable powers of movement, by means of a short neck.

In one zoophyte the head itself was fixed, but the lower jaw free: in another it was replaced by a triangular hood, with a beautifully-fitted trap-door, which evidently answered to the lower mandible.

In the greater number of species, each cell was provided with one head, but in others each cell had two.
The young cells at the end of the branches of these corallines contain quite immature polypi, yet the vulture-heads attached to them, though small, are in every respect perfect.