This image shows a partially dissected view of the aboral (upper) surface of a plastic model of a starfish. The image contains the principal internal organs and some elements of the water-vascular system. Asteroids normally have five rays, or arms, (1) arranged around a central disc (2).   Most echinoderms have a complete digestive system with a mouth (found on the oral surface of the starfish), a cardiac stomach (3), a pyloric stomach (4), pyloric ceca (5) that take up most of the space in the arms and an anus (6). Although radially symmetrical as adults, echinoderms possess a well developed coelom (7) lined with cilia that keep water moving through the organism.   Structures of the water vascular system that can be seen include the entrance to the system called a madreporite (8) that leads to a vertical stone canal (9). From there water enters a ring canal that surround the mouth, then into radial canals in each arm that deliver water via lateral canals to the bulb-like structures called ampullae (10). It is the contractions of these ampullae that deliver fluid under pressure to extend the tube feet so that they can make contact with the substrate or a food item. The lateral canals are contained within bony elements called ambulacral ossicles (11) that form an ambulacral ridge in each arm (seen as ambulacral grooves on the oral surface of each arm). Starfish also have a well developed internal skeleton made up of bony elements called dermal ossicles (12). Also seen on the model are several of the gonads (13), which are occupy a proximal position in each arm.