Alimentary canal:
It consists of three distinct regions:
(i) fore-gut, comprising mouth, buccal cavity, oesophagus and stomach
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(ii) Mid gut including intestine and
(iii) Hindgut or rectum.
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1. Mouth:
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It is a large slit-like aperture lying midventrally below the anterior end of head. It is bounded in front by the shield like fleshy labrum, laterally by the plate like incisor processes of mandibles and behind by the bilobed labium.
2. Buccal cavity:
Mouth opens into a short buccal cavity. It is lined by thick irregular folds of cuticle. The molar processes of mandibles lie opposite each other in the buccal cavity to crush the food between them.
3. Oesophagus:
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It connects buccal cavity to the floor of cardiac stomach. Internally the thick muscular wall of oesophogus is thrown into four prominent longitudinal folds, one anterior, one posterior and two laterals.
1. Stomach:
It is thin walled and double chambered structure consisting of
(i) A large anterior cardiac stomach and
(ii) A much smaller posterior pyloric stomach.
(a) Cardiac stomach:
The inner lining or intima of cardiac stomach has longitudinal folds covered by minute bristles. The wall of stomach is supported by the following cuticular plates :
(i) Circular plate:
It supports the anterior wall of the stomach near the esophageal opening.
(ii) Lanceolate plate:
It supports the roof of cardiac stomach.
(iii) Hastate plate:
Hastate plate makes the mid-ventral floor of cardiac stomach. It looks iike the head of spear. Its upper surface has thick growth of delicate setae and has a median ridge with gradually sloping sides.
The posterior triangular part of hastate plate is depressed and fringed with setae along its edge.
A narrow lateral groove makes the lateral borders of hastate plate and is supported by grooved plates. Inner side of each lateral groove is guarded by the supporting rod, bears a row of delicate bristles.
While outer side is bounded by ridged plate. These bristles always keep moving in a living prawn.
The posterior part of hastate plate forms cardiopyloric aperture. The lateral longitudinal folds of hastate plate at the sides of cardiopyloric aperture act as guiding ridges and guide the food towards the cardio-pyloric aperture.
Further cardiopyloric aperture is gaurded by four valves, hastate plate makes anterior valve, and semilunar fold of stomach wall makes posterior valve. While guiding ridges of hastate plate form lateral valves.
(b) Pyloric stomach:
Pyloric stomach has thick muscular walls and its lumen is divided into two chambers, by two folds of lateral walls of stomach.
Dorsal chamber is small while the ventral chamber is large and both the chambers are connected by a narrow vertical canal.
The floor of ventral chamber is covered by a filter plate made up of two V-shaped rectangular surfaces. Each rectangular surface bears a series of alternating ridges and grooves.
The ridges bear rows of bristles, which together with bristles of side walls forms pyloric filtering apparatus.
This pyloric filtering apparatus allows only liquid food to pass through it. Dorsal chamber remains as a small blind caecum and leads behind into the mid-gut.
The junction of the two is guarded by one median dorsal and two lateral groups of elongated setae. These strain the food, when it goes to mid-gut but prevents its regurgitation.
Hepatopancreas:
It surrounds the cardiac stomach from outside and opens into pyloric filtering apparatus by hepatopancreatic ducts.
Hepatopencreas combines in itself the function of pancreas, small intestine and liver of higher animals.
It secretes digestive enzymes and absorbs the digested food material and as liver, stores it in the form of glycogen, fat and calcium. Some intracellular digestion also seems to take place in hepatopancreas.
2. Mid-gut:
Mid-gut represented by intestine, is long narrow and straight tube running back upto the 6th abdominal segment.
Its lumen is wide at the anterior end but reduced posteriorly due to the presence of longitudinal folds.
3. Hind gut:
Mid gut leads into swollen muscular part, the rectum, bears many internal longitudinal folds. It opens outside through anus which is a sphinctered midventral longitudinal slit-like opening situated on a raised papilla at the base of telson.