Editing Sea urchin (section)

=== Circulation and respiration ===
[[File:Diadema setosum (Kenya).JPG|thumb|''[[Diadema setosum]]'']]

The water vascular system leads downwards from the madreporite through the slender stone canal to the ring canal, which encircles the oesophagus. Radial canals lead from here through each ambulacral area to terminate in a small tentacle that passes through the ambulacral plate near the aboral pole. Lateral canals lead from these radial canals, ending in ampullae. From here, two tubes pass through a pair of pores on the plate to terminate in the tube feet.<ref name=Ruppert/>

Sea urchins possess a hemal system with a complex network of vessels in the mesenteries around the gut, but little is known of the functioning of this system.<ref name=Ruppert/> However, the main circulatory fluid fills the general body cavity, or [[coelom]]. This coelomic fluid contains [[phagocyte|phagocytic]] coelomocytes, which move through the vascular and hemal systems and are involved in internal transport and gas exchange. The coelomocytes are an essential part of [[blood clotting]], but also collect waste products and actively remove them from the body through the gills and tube feet.<ref name=IZ />

Most sea urchins possess five pairs of external gills attached to the peristomial membrane around their mouths. These thin-walled projections of the body cavity are the main organs of respiration in those urchins that possess them. Fluid can be pumped through the gills' interiors by muscles associated with the lantern, but this does not provide a continuous flow, and occurs only when the animal is low in oxygen. Tube feet can also act as respiratory organs, and are the primary sites of gas exchange in heart urchins and sand dollars, both of which lack gills. The inside of each tube foot is divided by a septum which reduces diffusion between the incoming and outgoing streams of fluid.<ref name=Ruppert/>