Sea stars belong to a group of animals having radial symmetry, meaning they are the same on both sides of an imaginary axis, no matter how the animal is rotated. Two other characteristics of echinoderms are spiny skin (echino=spiny + derma=skin), and tubular feet which they use for locomotion. The picture on the left shows the tube feet on the underside of the arm. The feet are operated by an internal hydraulic system that inflates and deflates the feet. A seastar is capable of using the hundreds of tube feet on its arms to create a powerful grip on corals, rock, or the shell of a bivalve such as a mussel. By wrapping the arms around the shell, a sea star is able to slowly pry it open. Then it takes its digestive system and slips it into the shell of its prey, thereby digesting the bivalve's soft tissues. This is an unusual way for an animal to feed, taking its stomach to the food rather than bringing the food to the stomach. Other members of the Echinodermata, the phylum name, includes brittle stars, sea cucumbers, sea urchins and sand dollars.