Sea urchins are spiny sea creatures of the class Echinoidea
found in oceans all over the world. (The name sea urchin
means sea hedgehog, hedgehog being one meaning of the
word urchin). Their shell, which biologists call the test,
is globular in shape, and covered with spines. The size
of the test in adults is typically in the range of 3 to
10 cm. Typical sea urchins have spines 1-2 cm in length
(e.g. "Sea urchin", right), a millimeter or
two thick, and not terribly sharp. Diadema antillarum,
familiar in the Caribbean, has thin spines that can be
10-20 cm long. Sea urchins are usually dull in color,
common colors including green, olive, brown, purple, and
black.
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