Sea urchins are spiny sea creatures of the class Echinoidea found in oceans all over the world. Their shell, which biologists call the test, is globular in shape and covered with spines. The size of an adult test is typically from 3 to 10 cm. Typical sea urchins have spines 1-2 cm in length a millimeter or two thick, and not terribly sharp. Sea urchins are members of the phylum Echinodermata, and like other echinoderms they have five-fold symmetry and move by means of hundreds of tiny, transparent, adhesive "tube feet". The Echinoidea class also includes three groups of "irregular" echinoids: flattened sand dollars, sea biscuits, and heart urchins.
Photo Credit: Photograph of a sea urchin, Raphaël Rigo: 08/05/2005