<P> The five radial canals run into the arms and branch several times to supply all of the individual branches and pinnules lining the arms . As in other echinoderms, the radial canals give rise to lateral canals, but there are no ampullae, and clusters of three tube feet branch from the ends of each canal, except around the mouth, where they are found singly . In the absence of ampullae, water pressure is maintained by the ring canal, which is surrounded by contractile muscle fibres . </P> <P> The water vascular system of sea cucumbers has no connection to the outside, and is thus filled with the internal coelomic fluid, rather than sea water . The madreporite is present, but lies within the body cavity, just below the pharynx . The stone canal is relatively short . </P> <P> The ring canal normally has one to four polian vesicles, but in the order Apodida, there may be as many as fifty . The radial canals run through notches in the calcareous plates surrounding the mouth and then run along the ambulacral areas along the length of the body . Lateral canals run to both the tube feet and the large oral tentacles, all of which possess ampullae . The Apodida, which have no tube feet, also have no radial canals, with the canals to the tentacles branching off directly from the ring canal. 991--992 </P>

How do sea stars use their water vascular system to move