The water vascular system is a modified part of coelom and consists of a system of sea

water filled canals having certain corpuscles. It plays most vital role in the locomotion of the animals and comprises madreporite stone canal, ring canal, radial canal, Tiedemann's bodies, lateral canals and tube feet.

Madreporite:

The water-vascular system opens to the outside through small pores in the madreporite. It is rounded, sieve like calcareous plate. The madreporite is located on the aboral surface towards one side of the central disc between two of the arms, leads into stone canal. Where water enters in the water vascular system in the water vascular system.

Stone canal:

It is a S shaped stone canal, which descends toward a ring canal around the mouth. The walls of stone canal are supported by a series of calcareous rings. Through it water flows from madreporite to stone canal in the water vascular system.

Ring canal: It is a wide, pentagonal-ring shaped canal around the oesophagus or mouth, connects the stone canal and radial canal. It carries water from the stone canal to the radial canal in the water vascular system.

Tiedemann's bodies: They are also called racemose vesicles. From the inner side, the ring canal produces five pairs of greatly folded pouches called Tiedemanns bodies. Each pair of these pouches has an inter-radial position. Each of them opens into the ring canal on its inner side. In Asterias sp. there are only nine of them. The 10th is absent and its place is occupied by the stone canal. The actual function of tiedemann's bodies is still unknown.

Radial canal:

Ring canal, from its outer surface gives off a radial canal into the ambulacral groove of each arm that runs throughout the length of the arm. It involves in the water flow through it in the water vascular system.

Lateral canal:

The radial canal gives out two series of small, narrow, transverse branches called lateral canals or podial canals, along the sides of the ambulacral groove in each arm. Each lateral canal connects the radial canal to the tube feet, and its provided with a valve to prevent backward flow of fluid into the radial canal.

Tube feet:

As already mentioned, there are four rows of tube feet in each ambulacral groove. A tube foot is a hollow, thin walled, muscular structure. The inner end of which is a muscular sac or ampulla, lies within the body coelom of the arm, a middle tubular podium and the outer end of which usually bears a disc like sucker. Podia pass to the outside between ossicles in the ambulacral groove. The tube feet are chief locomotory and respiratory organ of Asterias sp.

Fig: Water Vascular System of Asterias

Functions of WVS

The water vascular system is filled with fluid that is similar to sea water, but it contains coelomocytes, a little protein, and a high concentration of potassium ions. The system operates as hydraulic system. It performs following functions: 1. Locomotion: The water vascular system is used mainly for locomotion. During locomotion

the system works as a hydraulic system.

The sea water to enter through the madreporite.

The sea water reaches the tube feet and their ampulla.

When the ampulla contract; the valves at the junction of the lateral canals and tube feet, prevent the flow of water into radial canals, and water is forced into the podia, The podia are elongated and projected out through the ambulacral groove.

The podium comes in contact with the substratum and then the suckers adhere to the substratum by secreting a substance that makes bonds with the surface.

After the suckers adhere to the substratum, the muscles of the podium contract the contracts the podium and forces fluid back into the ampulla. During locomotion the podiums wings forward, grips the substratum, and then moves backwards. In a section of an arm most podia perform the same step, and the animal moves forward. The action of podia is highly coordinated. During locomotion one or two arms act as leading arms, and the podia in all the arms move in the same direction. Another secretion breaks the bonds between suckers and substratum and sucker becomes free. Then the ampulla contracts and the whole process are repeated.

2. Food Capture: The tube feet are used to capture the prey. The suckers of the tube feet are also used to capture food.

3. Climbing and Attachment: Combined action of podial suckers enables star fish to climb vertically over rocks and to be attached to the rocks by the tube feet.

4. If a star fish is turned over, it can right itself by folding. The distal ends of one or two arms twist to bring the tube feet in contact with the substratum. The tube feet grip the substratum, now the arms move back beneath the animal so that the body is folded over. It can also right itself by arching the body and rising on the tips of the arms. It then rolls over.