Structures of Water Vascular System in Asterias

The following points highlight the eight different structures of water vascular system in Asterias. The structures are: 1. Madreporite 2. Stone Canal 3. Ring Canal 4. Polian Vesicles 5. Tiedmann’s Bodies 6. Radial Canals 7. Lateral or Podial Canals 8. Tube-Feet (Podia) and Ampulla.

Structure # 1. Madreporite:

The water vascular system starts with the madreporite and gives off a system of ves­sels traversing the body. The madeporite is a round calcareous plate (Fig. 21.7B) and has an inter-radial disposition on the aboral sur­face. The madreporite contains furrows which have numerous pores at the bottom. Each pore leads into a pore canal.

The number of pores and pore canals may be about two hundreds. The pore canals unite to form collecting canals which open into a small sac-like ampulla, called madreporic ampulla. The sea water that operates the hydraulic system enters and leaves the water vascular system through the madreporite.

Structure # 2. Stone Canal:

The madreporic ampulla proceeds down­wards as an ‘S’-shaped cylindrical madreporic or stone canal. The wall of this canal is supported by a number of calcareous rings, hence the name stone canal. From the wall of the stone canal projects a ridge which bifurcates into two lamellae. The lamellae become spirally rolled to occupy a consider­able portion of the lumen of the stone canal.

In some species of starfishes, the lumen of the stone canal becomes very much compli­cated due to extensive development of the lamellae. Most of the pore canals from the madreporite open to the stone canal and the rest open to the axial sinus. The stone canal serves as a pump that drives the circulation of sea water.

Structure # 3. Ring Canal:

The stone canal opens below into a con­siderably wide pentagonal ring-like canal situated around the mouth.

Structure # 4. Polian Vesicles:

In certain starfishes, there occur on the outer-side of the ring canal pear-shaped sacs, called polian vesicles, which are connected with the ring canal inter-radially. The polian vesicles hang in the perivisceral coelom.

The usual number of polian vesicles is ten, two in each inter-radius. But the numbers vary in different starfishes. In Asterias sp. polian vesicles are wanting as such. The vesicles function as expansion chamber for storage of fluid of water vascular system.

Structure # 5. Tiedmann’s Bodies:

The neck of each polian vesicle is pro­vided with a pair of small spherical yellow­ish glandular bodies attached to the inner wall of the ring canal, called Tiedmann’s bodies. As the polian vesicles are absent in Asterias sp., the ring canal gives off inter- radially nine such Tiedmann’s bodies. The interrdius which bears stone canal has only one Tiedmann’s body.

The significance of these bodies is not properly known, probably helps to filter fluid from the water vascular system into the body cavity. Hyman (1955) regarded this organ as lymphatic glands and probably manufacture the amoebocytes of the water vascular system.

Kowalevsky (1889) first observed that if a sea-star is allowed to live in sea-water containing In­dian ink or some vital dye, the colour is accumulated in the epithelial cells lining their lumina. But the absorptive function of the epithelial cells is not yet confirmed.

Structure # 6. Radial Canals:

The ring canal gives off five radial canals along the ambulacral grooves of the arms. The radial canals run up to the tip of the arms and end as the lumen of the terminal tentacle.

Structure # 7. Lateral or Podial Canals:

The radial canal gives out many paired small side branches, called the lateral or podial canals. Each lateral canal is attached to the base of the tube-foot and contains a valve which prevents the back flow of water from the tube-foot to the radial canal. The valve controls the flow of fluid from lateral canal to ampulla and podium (tube-foot).

Structure # 8. Tube-Feet (Podia) and Ampulla:

Each lateral canal, after reaching the ambulacral pore, divides at right angles into two branches. One of the branches is contin­ued used as the lumen of the podium (tube-foot) and the other as the cavity of the ampulla (Figs. 21.7C, 21.8). The ampullae are muscu­lar, rounded, sac-like structures situated at the anterior side of the podia.

Usually one ampulla is present in each tube-foot. In cer­tain starfishes, the ampulla may be bilobed (Astropecten irregularis), with a constriction at the middle. In Asterias sp., the ampullae are simple and undivided. Each podium is a hollow, elastic, tube-like structure which bears at its tip a flattened portion forming a sucker for attachment.

The tube-feet project out- ward on the body surface and lie in the ambulacral groove. Calcareous bodies are often present in the connective tissue layer of the tube-feet particularly at the sucker-like terminations.

The mechanism of podial locomotion operates by the antagonistic musculature of the ampulla and tube-foot and a contained volume of watery fluid. The ampulla con­tains smooth circular muscle fibres remain­ing in position vertically, also called ampullary muscles (Fig. 21.8). The tube-foot or podium consists of retractor or longitudi­nal muscles.

On the outer-side of the retrac­tor muscles there are collagenous connec­tive tissue sheath which is covered by cuti­cle. On the inner side the muscles (longitu­dinal muscles) are bounded by ciliated epi­thelium towards the lumen of podium.

The postrual muscles which lie below the ambulacral ossicle provide the orientation of the different muscles of the podium (Fig. 21.8). The levator muscles are situated near the sucker.