Any of various marine invertebrates (phylum Echinodermata) characterized by a hard spiny covering, a calcite skeleton, and five-rayed radial body symmetry. About 6,000 existing species are grouped in six classes: feather stars and sea lilies (Crinoidea), starfishes (Asteroidea), brittle stars and basket stars (Ophiuroidea), sea urchins (Echinoidea), sea daisies (Concentricycloidea), and sea cucumbers (Holothurioidea). Echinoderms are found in all the oceans, from the intertidal zone to the deepest oceanic trenches. Most species have numerous tube feet that are modified for locomotion, respiration, tunneling, sensory perception, feeding, and grasping. Movement of water through a water vascular system composed of five major canals and smaller branches controls extension and retraction of the tube feet. Most echinoderms feed on microscopic detritus or suspended matter, but some eat plants.