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Green Island Hammerhead Trip (Feb11) C 582.jpg
Shi Lang, Green Island -- Close-up of a ea urchin (Echinothrix calamaris) during a night dive.<br />
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Clearly visible is the urchin's spherical, semi-transparent cloaca that protrudes from the anal opening. It serves to expel waste matter and can be withdrawn into the shell.<br />
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Sea urchins are members of the phylum Echinodermata, which also includes sea stars, sea cucumbers, brittle stars, and crinoids.  "Echinodermate" means "spiny skin" in Greek.<br />
Like other echinoderms they have a fivefold symmetry and move by means of hundreds of tiny, transparent, adhesive "tube feet".<br />
Sea urchins are nocturnal animals.<br />
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The spines, long and sharp in some species, protect the urchin from predators. The spines inflict a painful wound when they penetrate human skin, but are not dangerous. It is not clear if the spines are venomous.<br />
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Sea urchins feed mainly on algae, but can also feed on sea cucumbers, and a wide range of invertebrates such as mussels, polychaetes, sponges, brittle stars and crinoids.