Digestive system of Freshwater Prawn. Palaemon.

Digestive system of the Freshwater Prawn.

The digestive system of prawn consists of Alimentary canal and the Hepatopancreas.

[I] Alimentary canal.

Alimentary canal constitutes three distinct regions.

(i) foregut,

(ii) Midgut and

(iii) Hindgut.

Foregut and hindgut are lined internally by cuticle, called intima, which is shed with the exoskeleton when the animal moults. The midgut is lined by endoderm.

(i) Foregut.

It comprises

(a) mouth,

(b) Buccal cavity,

(c) Oesophagus,

(d) Stomach.

(a) Mouth.

It is a large slitlike aperture situated mid-ventrally below the anterior end of the head between the third and fourth segments.It is externally lined by a labrum, on the sides by incisor processes of the mandibles, and behind by the bilobed labium.

(b) Buccal cavity.

The mouth leads into a short buccal cavity. The molar processes of the mandibles project into two sides to crush the food between them.

(c) Oesophagus.

The buccal cavity leads into short but broad oesophagus running almost vertically upward from the buccal cavity to the floor of the cardiac stomach.

Internally the thick muscular mass of oesophagus is thrown into four prominent longitudinal folds, one anterior, one posterior and two lateral. Of these four folds, the anterior is short.The lateral folds being subdivided into two smaller unequal folds.

(d) Stomach.

Stomach occupied most of the cephalothoracic cavity. It remains buried laterally, ventrally and posterally in the hepatopancreas.The stomach of prawn is thin-walled and double chambered. It is divided into two parts

(i) A large anterior bag-like cardiac stomach, and

(b) A very small posterior pyloric stomach. Both of them are separated by a valve.

1.Cardiac stomach.

The wall of the stomach is guarded by some cuticular plates . Tha cardiac stomach is lining by cuticular lining or intima and these are present in numerous, inconspicuous, longitudinal folds covered with small bristles.The cardiac stomach is embedded by the large triangular plate in the mid-ventral floor.It is also known as a hastate plate because it seems to be like the head of a spear. The upper surface of the hastate plate has a thick growth of delicate setae . They carry the distinct median ridge with gradually sloping sides.

The lateral side of the hastate plate is supported beneath by a longitudinal cuticular supporting rod. The hastate plate of the triangular part is depressed and fringed with setae along its edge on the posterior side.Either along the lateral border of the hastate plate, a narrow lateral plate is run and is supported by a cuticular plate which resembles open drain pipe and known as grooved plate.It forms a cardio-pyrolic aperture or anterior border.On the inner side the lateral groove is bounded by the supporting rod and on the outer side, each lateral groove is bounded by a long cuticular ridged plate. The ridged plate is fringed all along with a row of delicate bristles,in the inner border forming a comb-like structure . The forming of the comb-like structure is called is called named as combed plate. At the anterior end, the two combed plate are fused completely enclosing the hastate plate and on the another end at the posterior end they remain separated by the cardiovascular aperture. Cardio-pyloric aperture is narrow, x-shaped and leads into the pyloric stomach. It is guarded by four valves. Anterior valve is formed by the posterior valve by a semilunar fold of stomach wall and the two lateral valves by the large flap-like posterior ends of guiding ridges.

2. Pyloric stomach.

Pyloric stomach.The pyloric stomach is very small and they are narrow chamber just below the posterior end of the cardiac stomach. The wall present in the pyloric muscle is muscular and thick. The pyloric stomach divides the lumen into the smaller and the larger chamber. The smaller is known as the dorsal chamber and the larger is known as the ventral chamber.The floor is covered by the filter plate, which is made up of two rectangular surfaces and appears V-shaped in cross-section. The ventral chamber of the lateral wall is lined with a thick cuticle and covering with closely set bristles, together with the filter plate.In the pyloric stomach, there is presence an A-shaped plate known as the pyloric filter which allows only liquid food to enter it. The dorsal chamber gives out a small blind caecum dorsally and then leads behind the midgut. The junction of the two is guarded by one median dorsal and two lateral group of elongated setae that project into the midgut.

(ii) Midgut.

It is composed of Intestine.

It is long, narrow, straight and slender tube running back along the median line, between the extensor and flexor muscle up to 6 abdominal segments. The internal lining of the midgut is lined by the epithelial cell at the posterior side distinguished into many longitudinal folds, thus, greatly reduced its lumen.

(iii) Hindgut.

It includes rectum.

Rectum.

It is the shortest portion of the alimentary canal, leading from the midgut to anus.It's swollen muscular part, present on the anterior side is called the rectum. The rectum bears many longitudinal folds. The terminal narrow, tubular part opens to the exterior through the anus, which is a sphincter mid-ventral longitudinal slit-like opening, situated on a raised papilla at the base of the telson.

Hepatopancreas.

The hepatopancreas or so called the liver is a large bilobed, compact, orange coloured granular mass occupying a considerable area of the cephalothoracic cavity. It lies on the floor and sides of the stomach. On the dorsal side, it usually lined with the reproductive system and heart and ventral surface it covered by the nerve cord. It consists of numerous branching tubules completely held together by connective tissue.

Wall of tubules consists of a single layer of columnar epithelial cells, made up of,(i) granular cells, (ii) ferment cells, (iii) hepatic cells with globules of fat, and (iv) replacement of basal cells. The epithelium cells rest on a basement membrane.It secrets and absorption in function. It secretes a proteolytic enzyme, amylolytic enzyme and lipolytic enzyme. These enzymes act upon carbohydrate and converted into amino acid, acts upon carbohydrate and converted into glucose and acts upon fats and converted it into glycerin and fatty acid respectively. All these are absorbed by hepatopancreas. All of these are stored in it like that of the liver of the higher animal.

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.

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