<P> The madreporite of sea urchins is located within one of the plates surrounding the anus on the upper surface of the animal . The stone canal descends from the madreporite to the ring canal, which lies around the oesophagus, and includes a number of polian vesicles . Because sea urchins have no arms, the five radial canals simply run along the inside of the solid skeletal "test", arching upwards towards the anus . </P> <P> The ampullae branching off from either side of the radial canals give rise to ten rows of tube feet, which penetrate through holes in the test to the outside . As in sea stars, the ampullae are arranged alternately, but in most (though not all) cases they split into two as they pass through the test before merging again on the outer side . The tube feet of sea urchins are often highly modified for different purposes . The radial canal ends in a small water - filled tentacle which protrudes through the uppermost plate of the ambulacral region . </P> <P> Uniquely among echinoderms, crinoids have no madreporite . Instead, the oral surface is dotted with numerous minute ciliated funnels that run into the main body cavity . The ring canal has several small stone canals, located between the arms of the animal, but these open into the body cavity, and thus are only indirectly connected to the outside . </P> <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>

Where does water enter and exit a starfish