Editing Crinoid (section)

===Water vascular system===
Like other echinoderms, crinoids possess a [[water vascular system]] that maintains [[hydraulic]] pressure in the tube feet. This is not connected to external sea water via a [[madreporite]], as in other echinoderms, but only connected through a large number of pores to the [[coelom]] (body cavity). The main fluid reservoir is the muscular-walled ring canal which is connected to the coelom by stone canals lined with calcareous material. The coelom is divided into a number of interconnecting spaces by [[Mesentery (zoology)|mesenteries]]. It surrounds the viscera in the disc and has branches within the stalk and arms, with smaller branches extending into the pinnules. It is the contraction of the ring canal that extends the tube feet. Three narrow branches of the coelom enter each arm, two on the oral side and one aborally, and pinnules. The action of cilia cause there to be a slow flow of fluid (1mm per second) in these canals, outward in the oral branches and inward in the aboral ones, and this is the main means of transport of nutrients and waste products. There is no heart and separate circulatory system but at the base of the disc there is a large blood vessel known as the axial organ, containing some slender blind-ended tubes of unknown function, which extends into the stalk.<ref name=Ruppert/>

These various fluid-filled spaces, in addition to transporting nutrients around the body, also function as both a respiratory and an excretory system. Oxygen is absorbed primarily through the tube feet, which are the most thin-walled parts of the body, with further gas exchange taking place over the large surface area of the arms. There are no specialised organs for excretion while waste is collected by [[phagocyte|phagocytic]] coelomocytes.<ref name=Ruppert/>