Starfish or sea stars are echinoderms belonging to the classAsteroidea. 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. Starfish occur across a broad depth range from the intertidal to abyssal depths of greater than 6,000 m (20,000 ft).
Starfish are among the most familiar of marine animals found on the seabed. They typically have a central disc and five arms, though some species have many more arms than this. The aboral or upper surface may be smooth, granular or spiny, and is covered with overlapping plates. Many species are brightly coloured in various shades of red or orange, while others are blue, grey, brown, or drab. Starfish have tube feet operated by a hydraulic system and a mouth at the centre of the oral or lower surface. They are opportunistic feeders and are mostly predators on benthic invertebrates. Several species having specialized feeding behaviours, including suspension feeding and adaptations for feeding on specific prey. They have complex life cycles and can reproduce both sexually and asexually. Most can regenerate damaged or lost arms.
Echinoderms (Phylum Echinodermata) are a phylum of marineanimals. The adults are recognizable by their (usually five-point) radial symmetry, and include such well-known animals as starfish, sea urchins, sand dollars, and sea cucumbers. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone. The phylum contains about 7000 living species, making it the second-largest grouping of deuterostomes (a superphylum), after the chordates (which include the vertebrates, such as humans, sharks and frogs). Echinoderms are also the largest phylum that has no freshwater or terrestrial (land-based) representatives.
Aside from the hard-to-classify Arkarua (a Precambrian animal with Echinoderm-like pentamerous radial symmetry), the first definitive members of the phylum appeared near the start of the Cambrian period. The word "echinoderm" is made up from Greekἐχινόδερμα (echinóderma), "spiny skin", cf. ἐχῖνος (echínos), "hedgehog; sea-urchin" and δέρμα (dérma), "skin", echinodérmata being the Greek plural form.