In addition to their many tubed feet, sea urchins sometimes use their spines to help with movement across the ocean floor.  These spines are also used for sensing the environment and protection from predators.  If a predator is prodding an urchin, all of the spines of the urchin will rotate and point in the direction of the threat.  Sea urchin spines are made of calcium carbonate and can grow back within a few days if broken off.