Green Island Hammerhead Trip (Feb11) C 552-2.jpg
Shi Lang, Green Island -- Sea urchin (Echinothrix calamaris) during a night dive.
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.
Like other echinoderms they have a fivefold symmetry and move by means of hundreds of tiny, transparent, adhesive "tube feet".
Sea urchins are nocturnal animals.
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.