An echinoderm characterized by radial symmetry, and usually with five arms (rays) radiating from a central body. Minute pincer-like structures on the arms, called pedicellaria, ensure that the surface of the arms stay free from algae. There are a few sea stars that have 6 or 7 arms, and some with even more. Injuries will also cause more arms to grow. The majority of sea stars are carnivorous and feed on sponges, bryozoans, ascidians and mollusks. Other are detritus feeders or scavengers. Some sea stars, for example the crown-of-thorns that feeds on coral polyps, are specialized feeders; also called "starfish" .