Circulatory system of Asterias.

Circulatory system of Asterias:

The circulatory system present in Asterias is represented by a haemal system which enclosed within peri haemal system. In Asterias, a true blood vascular system is wanting in the echinoderms group.

[I] Haemal system:

The haemal system consists of intercommunicating sinuses or channel heaving no coelomic epithelium and is devoid of embryologically from the blastocoel. The haemal sinuses or channel is filled with the coelomic fluid containing coelomocytes and is enclosed in the coelomic space of the peri haemal system. It is greatly reduced in sea star and is of open types like the haemocoel of Arthropoda and mollusca.The main peri haemal system or channel are as follows.

1. Oral haemal ring:

It is connected with the aboral haemal ring through axial, gland. It is a circular haemal sinus, located around the mouth just below the ring canal of the water vascular system.It is also a ring of lacunar tissue or the fine channel which run in the septum dividing the hypo neural sinus.

2. Ring haemal sinuses or Strands:

These are radially from the oral haemal ring and one extends into each arm, along the floor of the ambulacral groove just below the radial canal of the water vascular system. The radial haemal or strands also give off branches into the podia.

3. Axial complex:

In Asterias, the peri haemal and haemal system are intimately connected by a complicated structure called axial complex. It comprises following three parts the thin-walled, tubular coelomic cavity called axial sinus containing the stone canal of water vascular system and axial glands. Both the water vascular and axial glands are closely attached with the all of the axial sinus by the mesenteries.

4. Axial gland:

This is the principle part of the haemal system often referred as 'heart ' or 'brown gland'. They are elongated, fusiform, brownish or purple coloured spongy body and remains enclosed within the axial sinuses of the peri haemal system. The axial gland is externally lined by coelomic epithelium often referred as the heart, ovoid gland dorsal organ, septal organ , brown gland. The axial gland is connected with the oral and aboral haemal sinuses at the oral and the aboral ends respectively.It is composed of connective tissue outlining numerous spaces containing irregularly arranged cells of the nature of coelomocytes. They are also related to the circulation of blood in peri haemal and haemal channels.

The axial gland becomes thin and terminates in the septum at the oral end that subdivided the hypo neural ring sinus. The axial gland present in the aboral end has an aboral extension or terminal process called head process which is lodged in a separate, closed contractile sac called dorsal sac .The dorsal sac located below the madreporite, close to the ampulla of the stone canal, but has no communication with ampulla. A pair of tufts also arises from the haemal sinuses in the wall of the cardiac stomach and opens into the axial near its aboral end. Digested food from the stomach passes into the haemal circulation through the gastric gland.

5. Aboral haemal ring:

The aboral ring canal lying beneath the aboral surface of the central disc. From the aboral haemal ring or canal extends five pairs of genital haemal strands to the gonads.

Function:

The haemal system acts as a pathway for the distribution of food substance carries by the contractile activity of the dorsal sac. The axial gland acts as a genital stolon , producing sex cells, which reach the gonads through the aboral haemal ring and its branches.

[II] Perihaemal system:

Like the water vascular system,they are derived from the coelom and is composed of tubular coelomic sinuses such as axial sinus, aboral ring sinus, genital sinuses, radial peri haemal sinuses, marginal sinuses and peribronchial sinuses.

It composed the following parts:

1. Axial sinuses.
2. Aboral sinuses.
3. Oral ring sinuses.
4. Marginal sinuses.
5. Peribranchial sinuses.
6. Radial Perihaemal sinuses.
7. Genital sinuses.

1. Axial sinuses:

It is thin-walled,vertical, wide tubular coelomic cavity enclosing the stone canal of water vascular system and the axial gland The three forming a well developed axial complex. Axial gland forms a small dorsal sac to enclose the dorsal process of the axial gland and located just below the madreporite.

2. Aboral sinuses:

It is a tubular, pentagonal channel or sinus around the intestine, lying just inside the aboral wall of the central disc, and enclosed the aboral ring sinuses.

3. Oral ring sinuses:

The oral ring sinus is a large tubular sinus which is divided into an inner narrow and the outer wide range by a circular septum called haemal strand.It opens into the inner division of a circular channel, the oral, peri buccal, peri haemal, or hypo neural ring sinuses which run around the mouth in the oral end.

4. Marginal sinuses:

These are 10 in number , two in each arm, one on each side below the marginal nerve cord, opening into the radial peri haemal sinuses of their respective arm.They run longitudinally on each side just aboral to the marginal nerve cord. The final lateral channel connected to the marginal sinuses channels with radial peri haemal sinuses.

5. Peribranchial sinuses:

These are circular spaces around the basal parts of papulae or gills are called peribranchial sinuses.

6. Radial peri haemal canals or sinuses:

Each radial canals enclosed a radial haemal strand running into an arm. Like oral ring sinus, they longitudinally divided into a to by a vertical partition or septum, continuous with a haemal strand and give out fine channels into the tube feet.

7. Genital sinuses:

It gives off five pairs of genital branches, one pair in each arm which enclose the gonads and the genital haemal strands.The genital sinuses surround the gonad.

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/

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