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The radially symmetrical body cavity contains a system of water-filled canals unique to echinoderms. Called the water-vascular, or ambulacral, system, it connects with the tube feet, or podia, which are extensions of the body wall that generally protrude through holes in the skeleton. The areas with such holes are called ambulacra. The tube feet often have suction cups on their tips and are used for locomotion in most echinoderms; they also function in feeding, respiration, and sensory reception.
The water-vascular system consists of a circular passageway, the ring canal, that 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 foot, causing the foot to extend. The foot is retracted by the contraction of the attached muscles, thereby forcing the water back into the ampulla. Sea stars, sea cucumbers, and sea urchins move by alternately extending and retracting groups of tube feet, gripping with the suction cups and pulling themselves along. Because the tube feet are very thin-walled, their surface is suitable for the diffusion of oxygen into the body cavity and the diffusion outward of carbon dioxide and wastes.
The tube feet perform at least part of the respiratory function in most echinoderms; however, many groups have developed auxiliary respiratory structures. Echinoderms have no special excretory organs. Circulation occurs in an open system of channels and sinuses and in the body cavity, which is lined with flagellated cells that create an internal current. The cavity contains large phagocytic cells (amoebocytes) that function in the transport of food and the storage of insoluble wastes. There is a simple nervous system sensitive to temperature, light, and vibrations, with the various body projections serving as sensory receptors. Echinoderms have extensive powers of regeneration of lost or injured parts.
Most species reproduce sexually, and species have separate sexes. Fertilization is external; the gametes are simply shed into the water at spawning time. The floating embryo develops into a ciliated, free-swimming, bilaterally symmetrical larva, which undergoes metamorphosis into the radially symmetrical adult.
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