Sea urchins are small, spiny sea creatures of the class Echinoidea found in oceans all over the world. (The name urchin is an old 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, po… (More on Echinoidea)