Echinoderms (Phylum Echinodermata) are a phylum of marine animals (including sea stars, urchins, sand dollars, and many other animals recognizable to most divers and beach goers) found at all ocean depths. The phylum contains about 7,000 living species, making it the second-largest grouping of deuterostomes, after the chordates; they are the largest phylum without freshwater or terrestrial representatives. The word derives from the Greek εχινοδέρματα (echinodermata), plural of εχινόδερμα (echinoderma), meaning "spiny skin" and that from εχινός (echinos), "sea-urchin", originally "hedgehog" + δέρμα (derma), "skin."
The Echinoderms are important both biologically and geologically: biologically because few other groupings are so abundant in the biotic desert of the deep sea, as well as in the shallower oceans, and geologically because their ossified skeletons are major contributors to many limestone formations, and can provide valuable clues as to the geological environment. The first echinoderms on the evolutionary ladder appeared near the start of the Cambrian period. Due to the nature of their bony ossicles, which make up their bodies, these animals fossilize relatively easily and are thus quite abundant in the fossil record. Additionally, some paleontologists maintain that the radiation of echinoderms was responsible for the Mesozoic revolution of marine life.
Two main subdivisions of Echinoderms are traditionally recognised: the more familiar, motile Eleutherozoa, which encompasses the Asteroidea (starfish), Ophiuroidea (brittle stars), Echinoidea (sea urchin and sand dollar) and Holothuroidea (sea cucumbers); and the sessile Pelmatazoa, which consist of the crinoids. Some crinoids, the feather stars, have secondarily re-evolved a free-living lifestyle.
A fifth class of Eleutherozoa consisting of just two species, the Concentricycloidea (sea daisies), were recently merged into the Asteroidea. The fossil record contains a host of other classes which do not appear to fall into any extant crown group.