Sea urchin, any of a class (Echinoidea) of spiny marine animals related to the starfish and the sand dollar, occurring worldwide. The basic structure of the sea urchin is a sphere of 20 columns of calcareous plates, the “test,” within which the gut, gonads, and water-vascular system are looped around the inside wall. The center of the sphere is empty. The test bears tubercles and short spines, and pedicellaria—pincer-like organs that clear the surface of detritus. Tube feet protrude through pores in the test, arranged in double rows down the sides.