Echinoderms

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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, after the chordates (which include the vertebrates). Echinoderms are also the largest phylum that has no freshwater or terrestrial representatives.

Aside from the hard-to-classify Arkarua, the first definitive members of the phylum appeared near the start of the Cambrian period. The word is derived from the Greek ἐχινοδέρματα (echinodermata), plural of ἐχινόδερμα (echinoderma), "spiny skin" from ἐχινός (echinos), "sea-urchin", originally "hedgehog," and δέρμα (derma), "skin".

The echinoderms are important both biologically and geologically. Biologically, there are few other groupings so abundant in the biotic desert of the deep sea, as well as the shallower oceans, but the more notably distinct trait, which most echinoderms have, is their remarkable powers of regeneration of tissue, organs, limbs, asexual reproduction, and in some cases, complete regeneration from a single limb. Geologically, the value of echinoderms is for their ossified skeletons, which are major contributors to many limestone formations, and can provide valuable clues as to the geological environment. Further, it is held by some[citation needed][who?] that the radiation of echinoderms was responsible for the Mesozoic revolution of marine life.

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