Echinoderms have a spiny body that usually divides into five equal parts. They walk on hundreds of tube feet that are full of water. If they lose part of their body, they can regrow it. They have a skeleton of calcium-carbonate plates.
Most echinoderms are mobile, although sea lilies are stationary. They live on shores, reefs, and the seabed. There are about 6,000 species.