Echinderms use their water vascular system to aid in their locomotion. The madreporite is located next to the central disk and takes in water. The water continues to the stone canal to the ring canal and into the radial canals, located in each arm. The radial canals carry water to the ampullae and provide suction to the tube feet. The tube feet work by attaching temporarily to surfaces and move in a wave, as one area attaches to the substrate, another releases. This action allows the arms to propel the starfish and the suction can also be used to apply pressure to bivalves like clams to cause the muscles keeping the clam shell closed to tire, thus opening the shell. The sea star can then feed on its prey.