Water Vascular system of Sea star

 WATER VASCULAR SYSTEM IN STARFISH

In Echinoderms a peculiar system is met with. It is filled with sea water, it is called water vascular system or Ambulacral system. The entire system is lined with ciliated epithelium. This system is well developed in Asteroidea.

It con­tains

1.      

1.     Madreporite

2.     Stone canal

3.     Ring canal

4.     Radial canals

5.     Tiedemann's bodies

6.     Lat­eral canals

7.     Tube feet.

 

Madreporite:

A calcareous plate-like structure is present on the aboral surface of the central disc of star fish. It is placed near to two arms and these two arms are called 'Bivium.' On surface of the madreporite grooves of furrows are present. Each furrow contains a number of pores. Hence it is called serve plate. These pores led into small canal which open into a stone canal.

Stone Canal:

It is an 'S' shaped canal, it is called the Madreporic canal. It travels towards the oral side on unite with ring canal at one of inter radius. The walls of the stone canal are strengthened by calcareous rings. The inner surface of the stone canal is lined with flagellated cells. The movement of flagella will draw water currents into the stone canal.

 

Ring Canal:

It is penta - radial and is present around the mouth, on the oral side. At each inner radius, it shows Polian vesicles and Tiedemann's bodies.

Pollan vesicle :

In the Asteroidea groups, four muscular like poiian vesicles are present at the 4 inter radi. Some scientists believe that they store water and are considered as reservoirs. Some scientists consider that they produce amoebocytes.

Tiedmann's bodies :

Usually in Asterias 9 Tiedeman's bodies are present. On either side of the polian vesicle two Tiedemann bodies are present. At the stone canal union with ring canal only one Tiedmann body is present. They are lymphoid sac like structures. They are believed to produce amoeboytes.

Radial canals

From the ring canal five radial canals arise and run throughout the entire length of the arm. Each radial canal lies below the ambulacra! the groove of the oral surface of the arm. Each radial canal ends at the tip of the arm as a tentacle. It is olfactory in function.

Lateral canals

 From the radial canal of each arm pairs of lateral canals will arise and they end with tube feet.

Tube feet:

Structure of Tube foot 

The basal part of the tube feet is bulged and is called the ampulla. It continues as a long tube feet which end with a 'Sucker'. The ampulla contains circular and longitudinal muscles. The long tube feet like structure contain only longitudinal muscles. Lateral canal will open into ampulla and is guarded by a valve.

Functions: Water vascular System: his mainly used for locomotion and food collection.

1)    Locomotors function:

The contraction of ampulla extends the tube feet. The tube feet will emerge out of the arm and attaches to the substratum with the help of suckers. The water from the ampulla will never enter the lateral canal because of the presence of valves.

The longitudinal muscles of the tube feet will contract and the animal is pulled forward and the Podia becomes short. The fluid is drawn back into the ampulla. Thus relaxation and contraction of tube feet will bring locomotory movement in Starfish. Usually, the locomotion is carried on by tube feet of one or two arms in one direction only.

2)    Food collection :

 In order to open the shell valves of the adhesion, Starfish utilizes its tube feet.-Then it feeds the soft material of Molluscan animals. Along each arm of the starfish extends an open ambulacral groove, from which arises a series of tube feet or podia. These are arranged in two rows and usually have suckers at their tip. Each tube foot has a coelomic cavity that communicates by a narrow neck with an ampulla lying in the main perivisceral coelom.

Actually the ampulla is a small muscular sac that bulges into the aboral side and opens directly in a canal that passes downward between the ambulacral ossicles and leads into the tube foot or podium. Each ampulla is associated with a tube foot and forms with it a func­tional unit.

Like the body wall, the podium is covered on the outside with a ciliated epithe­lium and internally with peritoneum. Between these two layers, lie connective tissues and longitudinal muscle fibers.

Role of Tube Feet in Locomotion: 

Mechanism of locomotion in Sea star (Tube Foot) 

In general, sea stars or sea star move rather slowly and tend to remain within a restricted area. In Asterias, the entire water vascular system operates as a hydraulic system dur­ing locomotion. In Asterias,  the circu­lating fluid enters each tube foot on the side of the ambulacral margin, and leaves it on the side of the ambulacral axis, while the current enters the ampulla on its aboral side and leaves it orally.

Each tube foot-ampulla unit may be cut off from the rest of the water vas­cular system by a valve, so arranged as to maintain the pressure developed within the unit. The functioning of the unit depends upon differences between the musculature of the ampulla and the tube foot .

In the ampulla the muscles consist mainly of rings of smooth muscles which are set verti­cally and lie parallel to the long axis of the arm. Contraction of these muscles brings about protraction of the tube foot and drives the fluid out of the ampulla into the foot.

The increase in the pressure is wholly translated into the elongation of the foot that subsequently comes in contact with the substratum. The musculature of the tube foot, in contrast to that of the ampulla, consists of longitudinal muscles, which are bounded on the inner side of the coelom. Contraction of these muscles is followed by withdrawal of foot from the substratum, bending and short­ening of the tube foot, and forceful return of the fluid back into the ampulla. All these make possible the highly organized stepping movement by which the sea star  pull themselves.