Close-fitting skeletal plates lie under the animal's skin and form a shell called a test. The spines are attached to the test. Some sea urchins found on coral reefs have especially sharp spines that are dangerous to divers. Sea urchins may be black, brown, green, purple, red, or white. They live in oceans all over the world.
Sea urchins eat mostly plants found on rocks and on the sea bottom.
A sea urchin's mouth is on the underside of its body. The animal scrapes up its food with a set of five movable teeth. Some sea urchins can dig holes into rocks with their teeth. A sea urchin moves by pushing with its spines or by using its tentaclelike tube feet, which have suction disks at the ends. In addition, the animal's tube feet may function in respiration and as sense organs.