Starfish or sea stars are echinoderms belonging to the class Asteroidea. The names "starfish" and "sea star" essentially refer to members of the class Asteroidea. However, common usage frequently finds these names also applied to ophiuroids, which are correctly referred to as "brittle stars" or "basket stars". About 1,800 living species of starfish occur in all the world's oceans, including the Atlantic, Pacific, Indian, Arctic and Southern Ocean regions. The fossil record for starfish is rather poor, probably because their exoskeletons are relatively fragile; the oldest known starfish are from the Ordovician. Starfish occur across a broad depth range from the intertidal to abyssal depths of greater than 6,000 m (20,000 ft).[Source]