Sea urchins are small spiny sea creatures of the class Echinoidea found in oceans all over the world. (The name urchin is an Old English name for the round spiny hedgehogs sea urchins resemble.) Their shell, which is also called 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 that are 1 to 3 cm in length, 1 to 2 mm thick, and not terribly sharp. Diadema antillarum, familiar in the Caribbean, has thin spines that can be 10 to 20 cm long. Common colors include black and dull shades of green, olive, brown, purple, and red.
Sea urchins are members of the phylum Echinodermata, which also includes starfish, sea cucumbers, brittle stars, and crinoids. Like other echinoderms they have fivefold symmetry (called pentamerism) and move by means of hundreds of tiny, transparent, adhesive "tube feet". The pentamerous symmetry is not obvious at a casual glance but is easily seen in the dried shell of the urchin.
Within the echinoderms, sea urchins are classified as echinoids (class Echinoidea). Specifically, the term "sea urchin" refers to the "regular echinoids," which are symmetrical and globular. The ordinary phrase "sea urchin" actually includes several different taxonomic groups: the Echinoida and the Cidaroida or "slate-pencil urchins", which have very thick, blunt spines (see image at right), and others (see taxonomic box on the right). Besides sea urchins, the Echinoidea also includes three groups of "irregular" echinoids: flattened sand dollars, sea biscuits, and heart urchins. Wikipedia
