The New International Encyclopædia/Sea-Urchin
SEA-URCHIN. The name applied to species of the echinoderm class Echinoidea. The sea-urchin of the coast north of Cape Cod (Echinus or Strongylocentrotus Drobachiensis) is common among rocks, ranging from low-water mark to fifty or more fathoms. It eats seaweeds, and is also a scavenger, feeding on dead fish and the like. Certain kinds are known to bore for a little way into limestone rocks or coral reefs, where they are protected from the waves. Sea-urchins have scattered over the surface, among the spines, microscopic button-like bodies called sphæridia, which are thought to be organs probably of taste or smell. They evidently react to odors. The eggs are numerous and small. After hatching the young sea-urchin enters the free-swimming larval or pluteus stage, passing through a complicated metamorphosis. On the other hand, certain forms (Anochanus Sinensis) have a direct development, the larval stage being suppressed. A Chilean form and also a South Pacific species of Hemiaster carry their young in brood-pouches, and they also directly develop, for no pluteus sea-urchin larvae were captured by the Challenger expedition in the Southern Ocean. The large sea-urchin of the Mediterranean is an article of food, and the Indians of the northwest coast eat the large local species. See Echinoidea; Echinodermata.