Digestive system of Unio: Freshwater Mussel.

Digestive system of the Unio. Freshwater Mussel.

The digestive system of Unio comprises of Alimentary canal, and a pair of digestive gland.

[I] Alimentary canal.The alimentary canal is a long, coiled tube and consists of the following parts.

(i) Mouth.

(ii) Oesophagus.

(iii) Stomach.

(iv) Intestine and

(v) Rectum.

(i) Mouth.

The mouth is a transverse slit, lying in the anterior end of the body ventral to the anterior adductor muscle. On each side of the mouth is a pair of oval, triangular, flattened fleshy flaps, and ciliated called the labial palps, one in front and one behind the mouth. The labial palps are joined to their fellow of the side and form upper and the lower lips.The two labial palps of each side so called lips enclose the ciliated oral groove which leads to the mouth.There is no jaws and radula.

(ii) Oesophagus.

The mouth leads behind and dorsally into a short tubular passage, called oesophagus. The inner wall is ciliated.

(iii) Stomach.

The oesophagus widens to form a sac-like, thick-walled stomach heaving a ciliated lining. The stomach lies dorsal to the visceral mass and it is surrounded by a large digestive gland or liver which opens into the stomach by many ducts. The stomach is divided into to reason-

• A dorsal portion of into which open the oesophagus and the ducts of digestive gland,
• And a ventral tubular style sac, containing a gelatinous rod, called the crystalline style.

The rod is made up of concentric layers of mucoprotein secreted by the ciliated epithelium of the style sac and contains digestive enzymes, amylase and glycogen as. The crystalline style is a transparent, solid, gelatinous and flexible rod-like structure being secreted by the cells of the stomach itself. The crystalline style has a matrix of protein, it contains mucus and a carbohydrate-splitting amylase and glycogen as; the amylase is condensed over the protein molecules. The style rotates due to cilia in the stomach by which its free anterior end erodes and liberates amylase so that partial digestion of starch takes place extracellularly in the stomach. Rotation of the style also aids in mixing the contents of the stomach.

The dorsal part of the stomach has folded wall around the opening of the ducts of digestive gland.These folds are helped in the sorting mechanism. They conduct the useless substance to the intestine. They also help in conducting fine particles to partially digested food particles to the ducts of digestive gland.

(iv) Intestine.

The intestine arises from the posterior end of the stomach. It runs ventrally into the foot coiling its way through the visceral mass, where it is closely surrounded by the gonad. Near the stomach, the intestine turns back into the rectum.

(v) Rectum..

The post terminal part of the intestine is called rectum.It wall is produced internally into a longitudinal, mid-ventral fold called typhlosole. The rectum passes backwards through the pericardium , it transverse through the ventricle and opens by an anus above the posterior adductor muscle into the exhalant siphon serving as the cloaca.

Digestive gland.

It is also called the digestive diverticulum or liver. The liver is only the digestive gland which surrounds the stomach from the lateral and posterior sides. It is large and paired structure of dark brown or green colour. It opens to the dorsal part of the stomach by many ducts. Owen (1955) has reported that in lamellibranch molluscs, such as Unio, the ducts of the digestive gland displays two tracts, (i) the

(i) the non-ciliated inhalant track for carrying food particles into the gland and (ii) and the ciliated

(ii) and the ciliated excellent track for sending the wastes back into the stomach. This gland not only secretes digestive enzymes, in fact, the fine particles enter into the digestive ducts to reach its cells here they are ingested and intercellular digested.

Food and Feeding.

Unio is a planktonic feeder and the food of Unio comprises of minute plants, Protozoa, diatoms, other microorganism and the organic debris, brought in by the circulating water current. Unio display filter-feeding or ciliary finding and ctenidia have assumed the function of obtaining food. The respiratory current brings in particles of food into the mantle cavity. On entering the mantle cavity the current of water becomes slow and heavier particles sink down and pass to the posterior region. Smaller particles pass with the current over the gill filaments of ctenidia. The different cilia of gill filaments perform various functions. The lateral cilia cause the food-laden current to enter the mantle cavity, the lateral frontal cilia defect the fine food particles onto the face of the filaments and they prevent large particles from clogging the ctenidia. Then the frontal cilia collect and pass the particles up or down the surface of ctenidia into the food grooves.The ctenidia produce mucus in which the food particles become entangled to form string-like masses which pass along the dorsal and ventral food grooves towards the mouth.The cilia of the labial palps direct the food laden mucus along the ciliated oral grooves into the mouth. The labial palps have the function of sorting and conveying food to the mouth, they can also reject some food particles and deflect them towards the outgoing current.Through the mouth, the food passes the stomach. When the good amount of heavier particles are accumulated on the labial palps, these fling to sweep them to the bottom of the mantle cavity. A sudden thrust of water outward through the incurrent siphon carries the rejected food particles and the sediment out of the mantle cavity.

Digestion Absorption and Egestion.

Within the stomach, the food particles are subjected to sorting, maceration, digestion and partial absorption. Digestion is both intracellular and the extracellular. The crystalline style performs the function of a stirring rod and a windlass. The crystalline style is made of protein and mucus, its material is mixed ith food in the stomach and it produces an amylolytic enzyme for digestion of carbohydrate. Amoeboid wandering leucocytes ingest food and also digest it, and they also transport digested food to all parts of the body. Absorption of digested food takes place in the stomach and also from the digestive gland.The digestive glands produce enzymes which bring about digestion in the stomach. The cells of the digestive gland take up solid particles of food and intracellular digestion of proteins and perhaps further digestion of carbohydrate take place by means of intercellular enzymes.

The amoeboid cells, present throughout the digestive tract, are said to engulf and digest food particles, a method that recalls the part played by the amoebocytes of sponges.

In the intestine, the food content is moulded as faecal pellets that are conveyed into the rectum and finally voided through the anus into the cloaca, to be carried away with the outgoing water current.

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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