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| A detailed study of the digestive system of Crustacea |
By:
Snigdha |
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DIGESTIVE SYSTEM
The digestive system is composed of the alimentary canal and the digestive gland, called the hepatopancreas or the liver
ALIMENTARY CANAL | HEPATOPANCREAS
ALIMENTARY CANAL
The alimentary canal consists of three distinct regions— (1) the fore-gut, comprising the mouth, buccal cavity, oesophagus and stomach; (2) the mid-gut and (3) the hind-gut.
FORE GUT
The fore-gut and the hind-gut are lined internally by cuticle,. called intima, which is shed with the exoskeleton when the animal moults. The mid-gut is lined internally by endoderm. The mouth is a large, slit-like aperture lying 1 Fore-gut mid-ventrally below the anterior end of the 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.The mouth leads in a short buccal cavity. It is anteroposteriorly compressed and has a thick chitinous lining which is. irregularly folded. The molar processes of mandibles lie opposite each other in the buccal cavity to crush the food between them. The short, wide oesophagus runs vertically upwards from the buccal cavity to the floor of stomach. Internally the thick muscular wall of oesophagus is thrown into four prominent longitudinal folds, one anterior. one posterior and two lateral. The cuticular lining of oesophagus bears short bristles.
The stomach occupies most of the cephalothoracic cavity. It remains buried laterally, ventrally and posteriorly into the hepatopancreas. The stomach, of prawn is thin-walled and double-chambered, consisting of two parts: (I) a larger anterior bag-like cardiac stomach; and (2) a much smaller posterior pyloric stomach. -
The inner cuticular lining or intima of the cardiac stomach presents numerous, inconspicuous, longitudinal folds covered by minute bristles. The wall of the stomach is supported by some cuticular plates embedded in its internal lining. Forming the anterior wall of the oesophageal opening is a circular plate; and behind it on the roof of the stomach is a lanceolate plate. A large triangular plate is embedded in the middle of the floor of cardiac stomach. It is called the hastate plate; because it looks like the head of a spear. The upper surface of the hastate plate is armed with a thick growth of delicate setae and carries a distinct median ridge with gradually sloping sides. The posterior triangular part of the plate is depressed. fringed with setae along its posterior border and touches the cardio-pyloric aperture, Each lateral side of the hastate plate is supported beneath by a longitudinal cuticular supporting rod. A narrow lateral groove runs along either lateral border of the hastate plate. The cuticular plate covering floor of each lateral groove is like an open drain-pipe and is called the groove plate. Each lateral groove is bounded on its inner side by the supporting rod and on the outer side by a long cuticular ridged plate. The inner border of each ridged plate is fringed all along with a row of delicate bristles, forming a comb-like structure, so that it is also named as a combed plate. The bristles bridge over the lateral groove and partially overlap the lateral margin of the hastate plate, where they constantly keep moving in a living prawn. The two combed plates are united anteriorly, thus completely enclosing the hastate plate except that their incurved posterior ends remain separated by the cardio-pyloric aperture. Outside the combed plates, on either side, the lateral wall of the cardiac stomach is folded inwards to form a prominent lateral longitudinal fold The two folds are very low anteriorly but gradually increase in height posteriorly and also bend inwards to form the sides of the cardio-pyloric aperture. These folds are also known as the guiding ridges; because they guide the food towards the cardio-pyloric aperture. The cardio-pyloric aperture is narrow, X-shaped and is guarded by four valves. The anterior valve is formed by the depressed posterior part of the hastate plate the posterior valve by a semi-lunar fold of the stomach wall, and the two lateral valves by the large flap-like posterior cads of the guiding ridges. This aperture leads into the pyloric stomach. The pyloric stomach is a small and narrow chamber lying below the posterior end of the cardiac stomach. Its lateral walls are thick, muscular and prominently folded inwards, so that the cavity of the stomach is imperfectly divided into a big ventral chamber and a small dorsal chamber, continued by a narrow vertical passage. The floor of the chamber is raised into a ventral median longitudinal ridge, so as to divide B its cavity into two lateral compartments. The floor is covered by a triangular - shaped filter plate, each side of which is rectangular in outline and bearing a series of alternating longitudinal ridges and grooves. The ridges bear rows of bristles, which form felt-like covering over the grooves. The side-walls of the ventral chamber are also covered with closely-set bristles, which together with the filter plate form an efficient strainer or filter. This pyloric filtering apparatus allows only liquid food to pass through it. The paired openings of the ducts of the hepatopancreas lie at the posterior end of the ventral chamber, just behind the filtering, apparatus. TIie dorsal chamber gives out a small, blind caecum dorsally and then leads behind into the mid-gut.
MID-GUT
The mid-gut is a long, narrow and straight tube running back along the median line up to the 6th abdominal segment . Its internal lining is an epithelium, which in the posterior part is thrown into many longitudinal folds thus greatly reducing its lumen.
HIND GUT
It is the shorest portion of the alimentary canal, leading from the mid-gut to the anus. Its anterior swollen muscular pert, called the intestinal bulb or rectum, bears many internal longitudinal folds. The termigal narrow, tubular part of the hind.gut opens to the exterior through the anus, which is a sphinctered. longitudinal slit-like opening, situated on a raised papilla at the base of the telson.
HEPATOPANCREAS
It is a large, bilobed, dense, orange-colored glandular mass,which nearly fills up the cephalothoracic cavity. Ii surrounds the stomach from its lateral, ventral and posterior sides. Developmentally, it arises from the mesenteron of the embryo as a pair of hepatic caeca, which repeatedly branch in a racemose manner to form the massive gland of the adult. It consists of numerous branching tubules completely held together by connective tissue. The tubules rejoin to form larger and larger canals, finally forming the two large heepatopancreatic ducts, which open into the ventral chamberof the pyloric stomach just behind the pyloric filter plate. The ‘hepatopancreas combines in itself the functions of pancreas, small intestine and liver of the higher animals. Like the pancreas, it secretes digestive ferments, which can digest carbohydrates, proteins and fats. Like the mid-gut, it absorbs the digested food material. Like the liver, it serves as an important storage organ for glycogen, fat and calcium. Some intracellular digestion also seems to take place here.
http://www.indipets.com/arthropoda/arachinda.htm |
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