Editing Echinoderm (section)

===The water vascular system===
{{Main|Water vascular system}}

[[File:FMIB 52615 Diagram of water-vascular system of a starfish ;.jpeg|thumb|Diagram of [[water vascular system]] of a starfish, showing the ring canal, the radial canals, ampullae (small bulbs), and [[tube feet]] ]]

Echinoderms possess a unique water vascular system, a network of fluid-filled canals modified from the [[coelom]] (body cavity) that function in gas exchange, feeding, sensory reception and locomotion. This system varies between different classes of echinoderm but typically opens to the exterior through a sieve-like [[madreporite]] on the aboral (upper) surface of the animal. The madreporite is linked to a slender duct, the stone canal, which extends to a ring canal that encircles the mouth or [[Esophagus#Invertebrates|oesophagus]]. The ring canal branches into a set of radial canals, which in asteroids extend along the arms, and in echinoids adjoin the test in the ambulacral areas. Short lateral canals branch off the radial canals, each one ending in an ampulla. Part of the ampulla can protrude through a pore (or a pair of pores in sea urchins) to the exterior, forming a podium or [[tube feet|tube foot]]. The water vascular system assists with the distribution of nutrients throughout the animal's body; it is most visible in the tube feet which can be extended or contracted by the redistribution of fluid between the foot and the internal ampulla.<ref>{{harvnb|Dorit|Walker|Barnes|1991|pp=780–791}}</ref><ref>{{harvnb|Brusca|Moore|Shuster|2016|pp=980–982}}</ref>

The organisation of the water vascular system is somewhat different in ophiuroids, where the madreporite may be on the oral surface and the podia lack suckers.<ref>{{harvnb|Dorit|Walker|Barnes|1991|pp=784–785}}</ref> In holothuroids, the system is reduced, often with few tube feet other than the specialised feeding tentacles, and the madreporite opens on to the coelom. Some holothuroids like the Apodida lack tube feet and canals along the body; others have longitudinal canals.<ref>{{harvnb|Brusca|Moore|Shuster|2016|p=982}}</ref> The arrangement in crinoids is similar to that in asteroids, but the tube feet lack suckers and are used in a back-and-forth wafting motion to pass food particles captured by the arms towards the central mouth. In the asteroids, the same motion is employed to move the animal across the ground.<ref>{{harvnb|Dorit|Walker|Barnes|1991|pp=790–793}}</ref>