Sea Urchins
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.
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.
Read more about this topic: Water Vascular System
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