The underside of the body bears a mouth at the center and a groove running along each arm. The grooves contain rows of tiny, flexible appendages called tube feet. Sea stars move by means of the tube feet, which are operated by a hydraulic, or water-vascular, system unique to echinoderms. Seawater, circulated through the radiating canals of this system, enters and extends the tube feet. Each tube foot can be withdrawn by its attached muscles. The tube feet are equipped with suction cups, and the animal moves in any direction by gripping with some of its tube feet and pulling itself forward. A sea star that is turned upside down can right itself by turning an arm under and walking with the tube feet.