True starfish are classified in the Asteroidea, a group of echinoderms. Unlike the superficially similar brittle stars (Ophiuroidea), true starfish have no sharp demarcation between arms and central body, and they move using tube feet rather than wriggling movements of the whole arms. True starfish and ophiuroids shared a common ancestor in the Ordovician; the fossil starfish pictured above is much more recent, coming from the Eocene of California.
Most starfish are predators, feeding on sessile or slow-moving prey such as mollusks and barnacles. The aptly named crown-of-thorns starfish, Acanthaster (shown below), specializes on corals, and may do considerable damage to coral reefs. Many, but not all, starfish are able to turn a portion of their stomachs out through the mouth, and thus digest food outside of the body.