Digestive System of Ascaris lumbricoides | Wandofknowledge
[I] Alimentary canal
Alimentary canal in Ascaris lumbricoides is a straight and complete tube extending from mouth at one end to anus on other end of body. It comprises a short pharynx or oesophagus representing the foregut, a long intestine or mid-gut, and a short rectum or hind gut.
It is a tri-radiate aperture, situated at the anterior tip guarded by three lips or labia.
Mouth opens into short, characteristic, cylindrical, thick-walled and muscular pharynx. Wall of pharynx, consists of a syncytial epithelium traversed by radial muscle fibres and containing glands. It is bounded externally by a membrane. Internally it is lined by cuticle which, at the margin of mouth, is continuous with the cuticle of body wall. Lumen of pharynx is tri-radiate. Rays are in the form of deep grooves which demarcate the pharyngeal wall into three sectors – one dorsal and two sub-ventral. Dorsal sector contains a pinnately branched pharyngeal or oesophageal gland, while each sub-ventral sector contains a similar palmately branched gland. Numerous groups of radial muscle fibres extend between inner cuticular lining and outer bounding membrane. Contraction of these muscles dilates the lumen. Between outer end of each ray of lumen and outer bounding membrane extend the connective tissue fibres, known as marginal fibres, which help in preserving the tri-radiate form of lumen.
Pharynx is followed by intestine which extends almost along the entire body length. A small valve is present at the junction of pharynx and intestine which stops the food from going back into pharynx. Intestine is dorso-ventrally flattened and its wall consists of a single layer of tall columnar cells, lined externally by a basement membrane and a thin layer of cuticle. Inner free margin of each cell is produced into several hair-like projections, the microvilli, forming a sort of brush border, to increase the absorptive surface area. The microvilli, are formed by a bacillary layer. Muscle fibres are wanting in intestine.
Intestine is followed by a short rectum which too is dorso-ventrally flattened. Wall of rectum consists of tall columnar cells and is lined internally by cuticle and externally by muscle tissue. In male, rectum opens into cloaca which also receives the ejaculatory duct. While in female, rectum opens out through anus. Which is a transverse slit guarded by anterior and posterior lips and is provided with a special muscle, the depressor ani. Anus or cloaca lies at a distance of about 2 mm. from tail end.
[II] Food, Feeding and Digestion
Food of Ascaris lumbricoides consists of blood and fully or partially digested food occurring in fluid form in the host’s gut. It is sucked by the rhythmic pumping action of pharynx. Digestion is completely extracellular in intestine and is aided by the enzymes proteases, amylase, and lipase. Digested food is absorbed by the intestinal cells and distributed by the pseudocoelomic fluid. Excess of food is stored mainly as glycogen and a little fat in syncytial epidermis. Some intestinal cells also engulf small solid particles by phagocytosis, and digest them intracellularly. Defaecation of undigested food whenever it occurs, is facilitated by the depressor ani muscle which raises the dorsal wall of rectum and posterior lip of anus or cloaca.
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