About the Sea Urchin

Sea urchins or urchins are small, spiny, globular animals which,with their close kin, such as sea cucumbers,sea stars,sand dollars, constitute the class Echinoidea of the echinoderm phylum. They inhabit all oceans. Their shell, or "test", is round and spiny, typically from 3 to 10 centimetres (1.2 to3.9 in) across.To move around coral reefs and rocks, sea urchins use spines in conjunction with tube feet on their bottom side as stilts. Most sea urchins that wash up on shore consist of only their shell, which is often sold in souvenir shops.About 700 sea urchin species are known throughout the world.
The Sea urchin's body is made up of five radial segments and develops a calcareous skeleton which protects and holds 'Aristotle's lantern', the intestines and the gonads. Just before reproduction,these gonads swell up to fill the whole shell. These gonads are very much sought after by the reef fishes that feed on the creature.They are the rich, yellow, paste-like, strong odoured and is served as a delicacy which is often used to top sushi in Japanese restaurants. The delicacy is called 'uni' in Japanese.
--http://www.journeymalaysia.com/seaurchins.htm
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