It is generally admitted that the ordinary spines serve as a protection; and if so, there can be no reason to doubt that those furnished with serrated and movable branches likewise serve for the same purpose; and they would thus serve still more effectively as soon as by meeting together they acted as a prehensile or snapping apparatus.

Thus every gradation, from an ordinary fixed spine to a fixed pedicellariae, would be of service.
In certain genera of star-fishes these organs, instead of being fixed or borne on an immovable support, are placed on the summit of a flexible and muscular, though short, stem; and in this case they probably subserve some additional function besides defence.

In the sea-urchins the steps can be followed by which a fixed spine becomes articulated to the shell, and is thus rendered movable.

I wish I had space here to give a fuller abstract of Mr.Agassiz's interesting observations on the development of the pedicellariae.

All possible gradations, as he adds, may likewise be found between the pedicellariae of the star-fishes and the hooks of the Ophiurians, another group of the Echinodermata; and again between the pedicellariae of sea-urchins and the anchors of the Holothuriae, also belonging to the same great class.
Certain compound animals, or zoophytes, as they have been termed, namely the Polyzoa, are provided with curious organs called avicularia.