Water Vascular or Ambulacral System of Asterias.

Water Vascular or Ambulacral System.

The water vascular system is responsible for circulating water through inside the body and performs so many vital functions. Physiologically, it throws out the water product outside the body. The water vascular system is a modified part of coelom and it consists of the certain corpuscles. It plays the most vital role in the locomotion of the animal and comprises madreporite, stone canal, ring canal, radial canal, Tiedeman's bodies, lateral canals, and tube feet.It is also the combination of several canals which functions together and it is also called canal system It consists of following parts.

(i) Madreporite:

It is thick rounded calcareous plate occurring on the aboral surface of the central disc in interradial position. Its surface bears a number of radiating, narrow, straight or wavy grooves or furrows. Each furrow contains many minute pores at its bottom.

There may be about (200-250) pores and pore-canals. Each pore leads into a very short, fine, tubular pore canal which possesses inward in the substance of the madreporite. Each pore leads to pore canal which unites to form larger collecting canal which is known as the stone canal.

(ii) Stone Canal:

The wall of the stone canal is supported by a series of calcareous rings. The madreporite open into an S-shaped structure known as the stone canal.The stone canal extends downwards (orally) and opens into a ring canal, around the mouth.The lumen of the stone canal is lined by very tall flagellated cells.In embryonic stages and in young Asterias, the stone canal remains a simple tube but in adult Asterias, the lumen of stone canal possesses a prominent ridge with two spirally rolled lamellae which by branching become more complicated in structure. During its course, the stone canal is ensheathed by a wide, thin-walled tubular coelomic sac, called axial sinus. It opens in ring canals.

(iii) Ring canal:

It is wide and pentagonal canal forming a ring around the oesophagus, it directs in all five central axes. The ring canal or water ring is located to the inner side of the peristomial ring of ossicles and directly above (aboral) to the hypo neural sinus.

(iv) Tiedemann's bodies:

These are glandular sac-like inter-radially, yellowish colour or rounded glandular bodies called racemose or Tiedemann's bodies, from the inner margins opening into the ring on the inner side. There are 9 Tiedemann's bodies attach with the ring canal except one side, all have two tiedemann's bodies each shell of the radiating tubular bears flagella.The Tiedemann's bodies rest upon the peristomial ring of ossicles.The actual function of Tiedemann's bodies unknown , however, it serves as filter devices and some rays as it is lymphatic gland probably manufacturing the phagocytic and amoebocyte which are released into water vascular system.

(v) Polian vesicles:The ring canal gives off on its inner side in the inter-radial position one, two four, little, pear-shaped, thin walled, contractile bladders or reservoirs with necks called Polian vesicles. They are supported to regulate pressure inside the ambulacral system and to manufacture amoeboid cells of the ambulacral system.Polian vesicles, however, wanting in families Asteridae which includes Asterias and Echinasteridae.

(vi) Radial canal;

Along the outer surface of each radius, the ring canal gives out a radial canal into each arm that runs throughout the length of the arm and terminates as the lumen of terminal tentacles. In the arm, the radial water canal runs immediately to the oral side of the ambulacral muscles.

(vii) Lateral canals:In each arm, the radial canal gives out two series of short, narrow, transverse branches called lateral or podium canals. Each lateral canal is attached to the base of a tube feet and is provided with a valve to prevent backwards flow of fluid into the radial canal.

(vii) Tube feet:

As already mentioned, there are four rows of tube feet in each ambulacral groove. There is two double role of tube feet in each of arms, alternating placed along and short lateral canal A tube feet or podium is a hollow, elastic, thin-walled, closed cylinder or sac-like structure having an upper sac-like ampulla, a middle tubular podium and a lower disc-like sucker.

Each tube is distinguished into three region.i,e.

1. Ampulla.
2. Podium.
3. Sucker.

1. Ampulla.

It is rounded sac-like ampulla situated above the adambulacral ossicles and projecting into coelom which is a gap between the adjacent ambulacral ossicles for the passage of the podium.

2. Podium.

It is thin tubular structure extending through the ambulacral groove.

3. Sucker.

A cup like a sucker at the lower end of the podium.

Function of Tube feet:

1. helps in locomotion by providing hydraulic pressure.
2. serve for the respiratory exchanges of gases.
3. helps in capturing and handling the food.

Reference:

Bhamrah, H.S., and Kavita, Juneja. A Text Book of Invertebrates, New Delhi: Anmol Publications Pvt Ltd, 2011.

Jordan E.L. and P. S., Verma. Invertebrate Zoology, New Delhi,: S. Chand and Company Pvt. Ltd., 2011.

Kotpal, R. L., Modern Text Book of Zoology: Invertebrates, New Delhi, India: Rastogi Publications,2011.

http://www.biologydiscussion.com/

https://en.wikipedia.org/wiki/Water_vascular_system