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.

Most Ophiuroidea (brittle stars and basket stars) move by making rowing motions along a surface with their long, flexible arms. Typically, 2 arms row, while the other 3 arms stabilize, but arms may switch roles from stroke to stroke. A system of muscles and nerves controls arm movement. Support is provided to each arm by an internal skeleton of tiny calcium carbonate plates (vertebral ossicles) linked together by ball and socket joints. Four outer chains of calcium carbonate plates run down the top, bottom, and each side of the arm. The side plates are often spiny, giving the animal traction as it rows. Despite the layers of support, Ophiuroidea arms are skinny and frequently break (thus, brittle star). Indeed, when attacked by a predator a brittle star will lose (autotomize) part or all of an arm as a defense tactic, and later regrow it.

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).