Alimentary canal

ALIMENTARY CANAL 
  • It consists of three distinct regions.
  • Foregut and hindgut are lined internally by cuticle, called intima which is shed along with the exoskeleton when the animal moults. The midgut is lined internally by endoderm.

Forgut

    • It is large and slit-like aperture lying mid-ventrally below the anterior end of the head. It is bounded in front by fleshy labrum laterally by the plate like incisor processes of mandibles and posteriorly by the bilobed labium
  • Buccal cavity
    • The mouth leads into a short buccal cavity. It is antero posteriorly compressed and has a thick cuticular lining which is irregularly folded. The molar processes of mandibles lie opposite to 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 the stomach. Internally the thick muscular wall of oesophagus is thrown into four prominent longitudinal folds, one anterior and two lateral.
    • The stomach occupies most of the cephalothoracic cavity. It remains buried laterally, ventrally and posteriorly in 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 (ii) a much smaller posterior pyloric stomach.
    • Cardiac 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 few cuticular plates which remain 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 trinagualr plate is embedded in the middle of the floor of cardiac stomach. It is called the hastate plate. 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 cardiac stomach – pyloric aperture.
    • Pyloric stomach
      • The pyloric stomach is a small and narrow chamber laying 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 continuous by a narrow vertical passage. The floor of the ventral chamber is raided into a ventral median longitudinal ridge, which divides it in to two lateral compartments. The floor is covered by a filter plate. The filter plate has the form of invested half open notebook filter plate bears a series of alternating longitudinal ridges and grooves. The ridges of the filter plate form an efficient strainer of filter. This pyloric filtering apparatus allows only liquid to pass through it. The paired opening of the hepatopancreatic ducts lie behind the filtering apparatus just below the junction of the dorsal chamber of the pyloric stomach and midgut, these openings are guarded by a group of elongated seate arising from the posterior end of median ridges of the filter plate. The dorsal chamber gives out a small blind caecum dorsally and then leads behind into the midgut.
      • Intestine is lined by a simple columnar epithelium consisting of absorptive cells and the mucus secreting cells.
      • The sharks and relatives (Chondrichthytes and few other fishes (sturgeons, lung fishes and latimeria) have a spiral valve inside the intestine that improves the digestion and absorption efficieincy

Midgut

  • It is a long, narrow and straight tube running back along the medianline, between the extensor and flexor muscles, upto the sixth abdominal segment. Its lumen is wide at the anterior end but reduced posteriorly due to the presence of longitudinal folds.

Hindgut

  • It is the shortest portion of the alimentary canal, leading from the midgut to the anus. Its anterior swollen muscular part, called the intestinal bulb or rectum bears many internal longitudinal folds. The terminal narrow, tubular part of the hindgut opens to the exterior through the anus, which is a sphinctered longitudinal slit-like opening, at the base of the telson.
Last modified: Sunday, 18 September 2011, 5:07 AM