Water vascular system

In the past, "aquiferous system" was also used to refer to many unrelated invertebrate structures,[7][8] but today, it is restricted to water channels in sponges[9] and the hydrostatic skeleton of some mollusks like Polinices.

[10] In sea stars, water enters the system through a sieve-like structure on the upper surface of the animal, called the madreporite.

This overlies a small sac, or ampulla, connected to a duct termed the stone canal, which is, as its name implies, commonly lined with calcareous material.

In some species, however, there are alternately long and short lateral canals, giving the appearance of two rows on each side of the groove, for four in total.

In between each radial canal, in many sea star species, there lies a muscular sac called a polian vesicle.

Instead, a simple valve at the upper end of the foot helps to control water pressure in the tube feet, along with contraction of the associated canals.

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.

[11]: 972–973 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 radial canal ends in a small water-filled tentacle which protrudes through the uppermost plate of the ambulacral region.