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.