Echinoderms are an ancient group of animals extending back to the Cambrian period (Hickman et al, 2004). Despite an excellent fossil record, the origin and early evolution of echinoderms are still obscure (Hickman et al, 2004). It seems clear that they descend from bilateral ancestors because their larvae are bilateral but become radially symmetrical later in their development (Hickman et al, 2004).
Sea urchins graze kelp and may reach population densities large enough to destroy kelp forests at the rate of 30 feet per month (Anon , 2005). Urchins move in "herds," and enough urchins may remain in the "barrens" of a former kelp forest to negate any attempt at regrowth (Anon , 2005). Sea otters, playing a critical role in containing the urchin populations, prey on urchins and thus control the numbers of kelp grazers (Anon , 2005).
Anonymous , (2005) downloaded from http://www.sanctuaries.nos.noaa.gov , 13/03/06
Hickman. P, Roberts. L, Larson. A, I'Anson. H. (2004) Integrated principles of zoology, twelfth edition, McGraw-Hill Companies, pg 443
Wray, G.A (date unknown).downloaded from http://tolweb.org , 11/03/06
www.Wikipedia.com , 11/03/06