An approximately spherical, spiny, marine invertebrate (class Echinoidea) with a body of five equal-sized parts (fivefold symmetry) radiating from a central axis, but some including sand dollars, with an oval shape. The mouth and anus are on the lower part of the body. The spines, up to 3 cm long, are defensive, often sharp and venomous in some species. A sea urchin moves by means of many transparent tube feet that arise from the grooves between the sections of the shell. Sea urchins feed mainly on algae, but also eat invertebrates. There are about 950 species found in all oceans.