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.