Sea urchins have ball-shaped bodies covered with long, movable spines. 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.