Digestive system of asterias.

Digestive systems of the Asterias.

The digestive system of Asterias consists of the alimentary canal, food and feeding mechanism and digestion, absorption and egestion.

[I] Alimentary canal:

Sea star possesses a complete digestive tract situated in the central disc. It lies along the oral-aboral axis and is quite short due to flattened of the body along that axis. In Asterias, the alimentary canal is a short straight, tubular, extending vertically along the oral-aboral axis in the central disc. Between the mouth and anus, it can be differentiated into an oesophagus, stomach, and intestine.Histologically, the alimentary canal resembles the body wall.

It comprises the following parts:

1. Mouth:

It is a pentagonal aperture surrounded by a delicate perioral membrane called peristome, situated on the oral side of the central disc. It is guarded by 5 groups of tubercles called mouth papillae or oral spines. It is provided with sphincter muscles and radial fibres and capable of great expansion and retraction. The mouth leads upwards into the oesophagus.

2. Oesophagus:

The mouth leads into a very short, wide and verticle tube called oesophagus. It opens aborally in the stomach.

3. Stomach:

It is s broad sac and occupying in the interior of the central disc. It is typically divided into a horizontal constriction into a voluminous oral part, the cardiac stomach and the flattened aboral part, the pyrrolic stomach. The cardiac muscle has a numerous muscular, highly folded wall bulged out to form five lobes, one opposite each arm.

a. Cardiac stomach:

It is a large sac having five lobes along the five radii of retractor muscles which arise from the lateral sides of the ambulacral ridge. It is thin wall muscular and highly folded. The cardiac stomach is connected to the ambulacral ridge of each arm by ligaments of muscles and connective tissues called mesenteries or gastric ligaments which serve to anchor the cardiac stomach in place. During the feeding process, the cardiac stomach can be contracted of the muscle of the body wall.

b. Pyloric stomach:

It is a small, flat pentagonal sac opening aborally into the intestine. The pentagonal angle lie along the radii and each receives a duct, called pyloric duct, from the corresponding pyloric caeca or digestive glands. It communicates with the intestine.

4. Intestine:

It is a short, narrow, five-sided tube that runs straight upwards to open out at the anus. It gives off two or three little hollow diverticula called intestinal or rectal cancer placed inter-radially. The intestinal caeca are brown in colour and each bears several short, irregularly- shaped diverticula. However, they are considered as excretory organs, because they secrete a brown fluid.

Anus:

It is a small, rounded aperture, situated somewhat eccentrically on aboral surfaces of the central disc.

[II] Food feeding and mechanism:

Asterias is a voracious carnivorous animal which preys on sedentary marine animals like clams, oysters mussels, snail, crabs, worms, barnacles, worms and other small fishes. Sometimes it also feeds on small fishes and injured and dead animals. Asterias ingest and digest food in such a manner that prey is captured and held in position by the arm and tube feet. Asterias conveniently creeps over the claim, archies its body over it and firmly attached the tube feet two shell valve in such a way that the ventral margin of the claim comes to lie in front of its mouth. It then tries to pull apart the two valves held tightly together by the powerful adductor muscle.A few tube feet at the tips of arms are also attached to the substratum to aid are also attached to the substratum in this process.

[III] Digesion, absorption and egestion.

The mode of feeding or ingestion in Asterias is of most unusual type. When the cardiac stomach is everted over the captured prey, secretion of a stomach and pyloric caeca are poured over it. It shallows the small-sized animals directly through the mouth. The prey is held by the tube feet and cardiac stomach is everted and wrapped round it. Enzymes protease, amylase and lipases, digest the proteins, starches and fats respectively. Digestion takes place outside the body. Digestion materials are then carried into alimentary canal by retraction of the cardiac stomach.Digestion is semi-digested materials, if any, is completed in stomach and pyloric caeca. Some small animals are, however, ingested as such and digested only in stomach and pyloric caeca.

To feed shelled molluscs starfish adopts another fan stating technique that it creep over the claim and holding it with tube feet, orients to the mouth. It now arches its body, assuming a characteristic humped or umbrella-like posture. The more proximal tube feet firmly grip both the valve of the bivalve's shell, while the more distal ones are attached to the substratum.

Digested food is absorbed mainly by pyloric caeca and distributed to various parts of the body by the coelomic fluid. Excess food is stored in the storage cell of the pyloric caeca.

As the absorption of food is more or less complicated in pyloric caeca, it does not require a long intestine. Hence, the intestine is very short. As animal ingests partially digested food, it has little-undigested material, which is mainly eliminated through the mouth itself. Little, if any, egestion take place through the anus.

Reference:

Aggarwal Sarita. A Text Book of Biology. New Delhi.: Madhuban Educational Books, 2011.

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.

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