Sea urchins are echinoids. They have a body that is enclosed by a hard exoskeleton. They are usually found in shallow water. They sit on solid surfaces like rocks. Their body is armed with long, movable spines, ranging from about 6 to 12 centimeters in length. They help protect the urchin from predators. They move by means of their tube feet. Urchins feed on algae and some marine plants, they graze.