The bodies of starfishes and brittle stars are drawn out into arms around a central disc. Stelleroids are mainly scavengers and carnivores feeding on molluscs, small crustaceans and many other suitably sized organisms. There are two main subclasses:
Subclass Asteroidea. The arms of true starfishes are not always distinct from the disc and contain lobes of digestive system and gonads. The podia or feet occur in ambulacral grooves, which extend along each arm, and are used in a stepping fashion when moving and for grasping prey.
The most infamous starfish, the Crown of Thorns starfish (Acanthaster planci) browses on the polyps of reef-building corals; population explosions of this predator can reduce productive reefs to underwater deserts.
Subclass Ophiuroidea. The arms of basket stars and brittle stars are distinct from the central disc, joining the disc on the oral (under) side and containing no organs. The arms are mostly vertebral ossicles and muscles, and are used in a rowing or looping fashion for moving and feeding. Brittle stars may cast off an arm if seized by a predator.