Sea urchin – New World Encyclopedia

Sea urchin is the common name for various spiky echinoderms within the class Echinoidea, characterized by pentamerous radial symmetry; a hard calcareous shell, or test, made of regularly arranged, tightly fused plates through which rows of slender, extensible tube feet extend; movable spines covering the shell; and a mouth on the lower side, facing the substratum, with a complex chewing …

: http://www.newworldencyclopedia.org/entry/Sea_urchin