Starfish or sea stars are echinoderms belonging to the class Asterriodea. The names "starfish" and "sea star" essentially refer to members of this class. However, common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as "brittle stars" or "basket stars". About 1,500 living species of starfish occur on the seabed in all the world's oceans, from the tropics to subzero polar waters. They are found from the intertical zone down to abyssal depths, 6,000 m (20,000 ft) below the surface.