This is actually a subdivision of the coelom or body cavity, making up peritoneal-lined tubules. Originally used for food gathering in sea lily-like fossil echinoderms, it is now also used for movement in forms such as sea stars. The water-vascular system is filled with seawater, and can assist with respiration and excretion by involving so much surface area. There are then radial canals extending out to the arms or rays. As the ampulla relaxes, the tube foot shortens, and a suction disk at the base adheres to the surface.
Tube feet line ambulacral grooves .
This phylum design of perforated dermal endoskeleton limits the group to only a few designs and life styles. Water-vascular system and tube feet, unique to echinoderms, could only exist with the support of the endoskeleton. The ossicles of the endoskeleton form units that end up enlarging in geometrical patterns during growth of the organism. Presumably there was selective pressure for a bottom-dwelling lifestyle, where the radial nature can allow the echinoderm to respond in any direction to food, changing conditions, etc. Five arms blend into central disc.