Echinoderm means "spiny skin." always starts from a larvae stage and has approximately 7000 described living species and about 13,000 extinct species known from the fossil record. This phylum is the largest without any freshwater or terrestrial forms. Mainly a marine group, echinoderms are found in all the oceans.
An internal skeleton is present throughout members of the phylum. The skeletal and muscular arrangement varies among groups but mostly RADIALLY SYMMETRICAL. It also has a poorly defined open circulatory system. The OSSICLES are calcareous plates buried in the fleshy region beneath the outer skin. This represents the endoskeletonwhich gives the body rigidity and support. Echinoderms in general are most vulnerable in their larval stage. As adults, asteroids have an anti-predator adaptation where they can lose an arm to a predator and the arm is later regenerated. Sea urchin eggs are also edible and often served in sushi bars. Echinodermata includes the starfish, sea urchins, brittle stars, sea cucumbers and feather stars.