- published: 24 Mar 2015
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Echinoderms (Phylum Echinodermata) are a phylum of marine animals. The adults are recognized easily by their (usually five-point) radial symmetry, and include such well known animals as sea stars, 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 70,000 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 traits), the first definitive members of the phylum appeared near the start of the Cambrian period. The word "Echinoderm" is derived from the Greek ἐχινοδέρματα (echinodermata), plural of ἐχινόδερμα (echinoderma), "spiny skin" from ἐχινός (echinos), "sea-urchin", originally "hedgehog," and δέρμα (derma), "skin".