Brief Description: Echinoderms constitute a phylum of Metozoa. They have attracted the researchers? interest because of their extensive presence in the fossils archive, their ecological importance, their polymorphic and peculiar adult morphology and their unusual industrial properties. There are about 7,000 species of Echinoderms that are classified in five classes: Crinoidea (Crinoids - Feather Stars or Sea Lilies), Ophiuroidea (Brittle Stars and Basket Stars), Asteroidea (Asteroids or Sea Stars), Echinoidea (Echinoids and Sea Urchins) and Holothuroidea (Sea Cucumbers). Around 13,000 species of fossilized echinoderms have been identified by this day. All Mesozoic and Cenozoic forms are clearly classified in the five aforementioned classes, but the Paleozoic archive contains numerous distinct and often peculiar forms that have been classified in about 15 additional classes. Without a doubt, Echinoderms made their first appearance in the fossils archive during the Cambrian period, namely about 550 million years ago. 
Bibliography: Nielsen, C. 1995. Animal Evolution: Interrelationships of the Living Phyla. Oxford University Press, Oxford. Paul, C.R.C. and A.B. Smith. 1984. The early radiation and phylogeny of echinoderms. Biol. Rev. 59: 443-481.