Echinodermata
. Echinoderm, common name for about 6000 living species constituting a phylum of marine animals, such as starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers. They usually show a superficial five-part radial symmetry, and generally are equipped with peculiar tube feet. The phylum name is derived from the spiny skin. An echinoderm such as the starfish typically has a mouth surrounded by five arms that bear minute, fleshy tube feet with which the animal clings and crawls. The tube feet are supported by a complex internal hydraulic system that is inflated with seawater. Some echinoderms, especially brittle stars, crawl or swim by moving their arms. Often, as in sea urchins, rows of tube feet extend along the body surface, and arms are absent. The spines are particularly well developed in sea urchins. The skeleton, made up of calcium carbonate, may form a large proportion of the body, or, as in some sea cucumbers, it may be greatly reduced. The fossil record shows that th