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More About Sea urchins ...
Sea urchins are spiny sea creatures 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.

Sea urchins are echinoderms (phylum Echinodermata), which also includes starfish, sea cucumbers, brittle-stars, and crinoids. Like other echinoderms they have five-fold or pentamerous radial symmetry 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 (see picture below, right).

Within the echinoderms, sea urchins are classified as echinoids (class Echinoidea). Specifically, the term "sea urchin" refer to the "regular echinoids," which are symmetrical and globular. The ordinary phrase "sea urchin" actually includes two different taxonomic classes: the Echinoida, and the Cidaroida or "slate-pencil urchins" which have very thick, blunt spines (see image at right). Besides sea urchins, the Echinodea include three classes of "irregular" echinoids: flattened sand dollars, sea biscuits, and heart urchins.

At first glance a sea urchin often appears to be an inanimate object, or one which is incapable of moving. Sometimes the most visible sign of life is the spines, which are attached at their bases to ball-and-socket joints and can be pointed in any direction. In most urchins, a light touch elicits a prompt and visible reaction from the spines, which converge toward the point that has been touched. A sea urchin has no visible eyes, legs or means of propulsion, but it can move freely over surfaces by means of its adhesive tube feet, working in conjunction with its spines.
Source: Wikipedia Read more about Sea urchins
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