Sea urchins belong to the class Echinoidea of the echinoderm phylum which also includes sea stars, sea cucumbers, brittle stars and crinoids. Also included in this class are three groups of "irregular" echinoids: sand dollars, sea biscuits and heart urchins. Echinodermate means "spiny skin" in Greek.

Sea urchins have fivefold symmetry, radiating from a central axes are five equally sized parts that are visible in their dried “test” (shell like orb of a dead urchin). They are covered with sharp spines that many divers know can be somewhat painful if you happen to land on one. Some urchins have spines up to 12 inches long.

Sea urchins move across the ocean floor using their many very small “tube feet” and eat mostly algae. The locomotion of these tube feet is powered by a water vascular system. Contracting muscles force water into the tube feet to extend and push against the ground, on relaxation of the muscles, the feet retract.

Sea urchins have many predators that include sea otters, wolf eels, trigger fish, and people to mention a few. Their roe is a delicacy for many cultures.