water Vascular System In Asteroidea [Echinodermata ]

Water vascular system in Asteroidea echinodermata|Class Asteroidea is one of the largest and most familiar classes of phylum Echinodermata| members are commonly called Sea stars or star fishes|These are marine body is formed of pentagonal central disc and usually 5 radiating arms, water vascular system is an hydraulic pressure system devised specially for the functioning of the tube feet| It consists of madreporite, stonecanal, ring ambulacral vessel, radial ambulacral vessels podial canals and tube feet


madreporite also called sieve plate|It is a hard rounded and sieve like calcareous plate, lying on the abroal surface of the central disc|It is situated in the inter radial position near the bases of two adjacent arms forming the bivium| The surface of madreporite is provided with a number of furrows as many as 250 minute pores| each pore lead into a pore canal|it lead into bag like ampula beneath the madreporite|The ampulla opens into a stone canal|

.Stone Canal

The stone canal is a s shaped canal opening on the oral side into circular water vessel around the mouth| Internally the stone is lined with flagella or cilia,the movement of which draws the sea water into the canal| The ampulla contract cyclically acts as pump assisting fluid flow

.Ring Ambulacral Vessel

It is wide pentagonal ring like vessel lying around the oesophagus above the peristome| Angles of pentagonal lies in radial position|

.Tiedemann`s Bodies

On the inner side of ring ambulacral vessel are attached tiedemann`s bodies |these are small rounded, yellowish, There are only 9 tiedemann bodies in Asterias|The cavity of each Tiedemann body is divided into a number of chamber and contain phagocytes, which remove foreign matter such as bacteria,

.Radial Ambulacral Vessels

The ring ambulacral vessel gives off five radial ambulacral vessels along the ambulacral grooves of the arms| The radial ambulacral vessels run up to the tip of the arms and end as the lumen of the terminal tentacle|

.Podial Canal

The radial ambulacral vessel gives out small side branches called the lateral or podial canals| The latter pass out between the ambulacral ossicles, and open into the tube feet, The opening of the podial canal into the tube feet is guarded by a valve which, if closed, will not allow the contents to flow back from the foot into the canal |

.Tube Feet

There are two double rows of tube feet in each ambulacral groove |Each tube foot is a hollow, elastic, thin, walled closed cylinder|It extend to the ambulacral base which lies between the two adjacent ambulacral ossicle for the passage of the podium |Each tube foot consist of a pull a, podium, and sicker, which extend through the ambulacral grooves |

Internal fluid of water vascular system is essentially sea water but also contain amoeboid coelomocytes, some proteins and high level of potassium salts [Robert barnes 1982]

.Functions Of Ambulacral System

Most of asteriods,the podial canal are of equal length so that the tube feet are arranged in two rows,one side of ambulacral groove |but in some species the podial are arranged in a tightly zig zag rows giving the appearance to the present in 4 rows in each ambulacrum|

At the end of each arm, the end most tube foot is modified into a short immotile structure, called optic cushion, which bears a red spot or eyes, So is sensory in function

In most of asteroids, there are 5 pairs of interradial Tiedemann bodies arising from the wall of ring ambulaeral vessel| in some starfish, there are also present interradial muscular sacs, called the polian vesicles|these are attached to ring ambucral vessel whose function may be osmoregulation pressure regulation in the system|

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