Echinoderms (Phylum Echinodermata) are a phylum of marineanimals. The adults are recognizable by their (usually five-point) radial symmetry, and include such well-known animals as starfish, sea urchins, sand dollars, and sea cucumbers. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone. The phylum contains about 7000 living species, making it the second-largest grouping of deuterostomes (a superphylum), after the chordates (which include the vertebrates, such as humans, sharks and frogs). Echinoderms are also the largest phylum that has no freshwater or terrestrial (land-based) representatives.
Aside from the hard-to-classify Arkarua (a Precambrian animal with Echinoderm-like pentamerous radial symmetry), the first definitive members of the phylum appeared near the start of the Cambrian period. The word "echinoderm" is made up from Greekἐχινόδερμα (echinóderma), "spiny skin", cf. ἐχῖνος (echínos), "hedgehog; sea-urchin" and δέρμα (dérma), "skin", echinodérmata being the Greek plural form.
Brittle stars or ophiuroids are echinoderms in the class Ophiuroidea closely related to starfish. They crawl across the seafloor using their flexible arms for locomotion. The ophiuroids generally have five long slender, whip-like arms which may reach up to 60 centimetres (24 in) in length on the largest specimens. They are also known as serpent stars.
Ophiuroidea contains two large clades, Ophiurida (brittle stars) and Euryalida (basket stars). Many of the ophiuroids are rarely encountered in the relatively shallow depths normally visited by humans, but they are a diverse group. There are over 2,000 species of brittle stars living today. More than 1200 of these species are found in deep waters, greater than 200 metres deep.