There are three main appendages covering the outside of the animal: spines, tube feet, and pedicellariae. Spines help deter predators while the tube feet attach the urchin to rock and aid in feeding. The pedicellariae are simillar to pinchers and are rarely seen by the average viewer (and aren't used on humans).
The Aristotle's lantern or mouth is located on the underside of the sea urchin. Five plates radially connected make up the main section of the mouth. Kelp is guided into the teeth by tube feet and is then digested in the gut. A special enzyme is secreted to aid in digestion.
Sea otters feed on urchins and have been know to eat smaller ones whole. Some wild otters can have both skull and teeth stained from consuming a steady diet of purple sea urchins.