Internal TRansport and Circulation

Internally, echinoderms have a thickened vessel which acts as the heart, pumping blood through a closed circulatory tract. In the body cavity, circulation occurs in an open system of channels and sinuses, which is lined with flagellated cells that create an internal current. The water-vascular system consists of a circular passageway, the ring canal, which surrounds the digestive tract and five radial canals that radiate from the ring canal like spokes of a wheel. Each radial canal underlies an ambulacral area. The ring canal is usually connected to a porous plate in the body wall, the madreporite (by a lime-walled tube called the stone canal). The position of the madreporite varies in the different groups. Seawater enters the system through the madreporite, which is regulated by the animal. Short lateral canals equipped with valves lead from the radial canals into the tube feet. Generally a muscular, water-filled bulb, the ampulla, is connected to each tube foot. When the valve closes and the ampulla contracts, water is squeezed into the tube feet, causing the feet to extend. Feet are retracted by the contraction of the attached muscles, thereby forcing the water back into the ampulla.