The sea star is a member of phylum Echinodermata. Animals in this phylum have a distinctive mechanism of locomotion. Within the sea star is an internal water vascular system that terminates in branches called tube feet. As a sea star moves, it uses chemicals first to attach each tube foot to the substrate and then to release each tube foot from the substrate. Water enters the water vascular system via the madreporite, which is a strainer-like structure on the dorsal side of the animal. This video shows what happens when a sea star is placed on its back in an aquarium. The tip of the sea star's arm can bend back on itself, and once the tube feet contact the substrate, the animal slowly pulls itself onto its correct side. Credit: Courtesy of Graham R. Kent and Rebecca L. Turner, Smith College.