Echinoderms Division

Echinoderms Categories

Echinoderms (Phylum Echinodermata) are a phylum of marine animals (including sea stars) found at all ocean depths. Aside from the problematic Arkarua, the first definitive members of the phylum appeared near the start of the Cambrian period, and 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), "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 the shallower oceans, and geologically as their ossified skeletons are major contributors to many limestone formations, and can provide valuable clues as to the geological environment. Further, it is held by some 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, mobile echinids, which encompasses the Asteroidea (starfish), Ophiuroidea (brittle stars), Echinoidea (sea urchin and sand dollar) and Holothuroidea (sea cucumbers); and the sessile echinids, which consist of the blastoids, cystoids and crinoids.

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