Most sea stars are carnivores (eating live animals) or scavengers (eating dead animals). They feed on prey that move slowly such as polychaete worms or not at all such as oysters. Most sea stars turn their own stomachs inside out onto the bodies of their prey to spread digestive juices. As the prey's body softens, the sea star pulls its stomach back into its mouth. The food travels to another stomach for further breakdown. To eat an oyster, the sea star uses its tube feet to pry open the shells, then pokes its stomach inside to begin digestion. Populations of sea stars can damage coral reefs by inching along and digesting everything they come across. Some sea stars keep their stomachs in their bodies and swallow whole prey. After digestion, shells and other waste are expelled out their mouths. While they do not look fierce, sea stars are top predators in many ecosystems.

Echinoderms move using rows of small tube feet powered by a fluid-filled (water vascular) system. The fluid travels from central, circular canals out through radial canals like the spokes of a wheel. Along the spokes are the tube feet, which typically end in adhesive pads. Each tube foot is kept firm by internal fluid pressure. By sticking down and unsticking in coordinated waves, the tube feet inch the animal along a surface. The water vascular system used for locomotion can also function for feeding. Some echinoderms pass small pieces of food along from foot to foot until it reaches their mouths. The otherwise soft echinoderm body gets the support it needs from a skeleton made of calcified pieces (ossicles). Ossicles often have spiny projections that give the overlying skin a prickly appearance (echino = spiny; dermis skin).