[This diary was first posted to Daily Kos on August 11, 2006.]
Sea urchins are Echinoderms, spiny-skinned animals related to starfish and sea cucumbers. Echinodermata is a rather small phyla of animals and is unique in that virtually all members of this group are strictly marine.
They also, as a group, employ nearly every type of feeding method exhibited by animals. Sea stars are predators, crinoids are filter feeders, as are some sea cucumbers. Many types of sea cucumbers are deposit feeders (swallowing sand, digesting the organic material and passing the undigestible bits). Sea urchins alone are mainly herbivores. The animal above is my local Purple Sea Urchin (Arbacia punctulata).