Alimentary canal starts with the mouth at the anterior end and ends with the anus at the posterior end. Digestive system consists of mouth, pharynx, oesophagus, intestine and rectum.
Mouth: It is a triradiate aperture located at the anterior end of the body. Mouth is guarded by three lips or labia
Pharynx: Mouth opens into pharynx which is short, cylindrical, thick walled and muscular. The wall of the pharynx consists of syncytial epithelium traversed by radial muscle fibres and glands. Pharynx is externally bounded by membrane and internally lined by cuticle which is in continuation with the body wall. The lumen of the pharynx is also triradiate. Groups of radial muscle fibres extend between inner cuticular lining and the outer bounding membrane. The contractions of these muscle fibres dilate the lumen. Also between the outer end of each ray of lumen and the outer bounding membrane the connective tissue fibres called as marginal fibres are present. These fibres help in maintaining the triradiate form of the lumen.
Intestine: Pharynx is followed by intestine which extends throughout the length of the body. At the junction of the pharynx and the intestine a small valve is present which stops the food from going back into the pharynx. Intestine consists of single layer of columnar cells lined externally by basement membrane and a thin layer of cuticle. The inner free margins are extended into hair-like projections called as microvilli which help the surface area of absorption. There are no muscle fibres in the intestine.
Rectum: Rectum is the last part of the digestive system. It is short and dorso-ventrally flattened. The wall of the rectum consists of long columnar epithelial cells and is also lined by the cuticle which is in continuation with the body wall. In males the rectum opens into cloaca which also receives ejaculatory duct. Whereas in females, rectum opens out through anus. The anus is guarded by lips and is also provided with special muscle.
The food of Ascaris consists of blood and partially digested food occurring in the fluid form in the host’s gut. The food is sucked by the rhythmic pumping action of pharynx. Digestion is completely extracellular in intestine. The digestion is done by the enzymes like proteases, amylase and lipase. Excess food is stored as glycogen and fat in syncytial epidermis. Digested food is absorbed by intestinal cells.
Due to the absence of the circular muscles in the body wall, the change in the length of the body length is restricted. But the fibres of the cuticle enable a limited change in the length of the body. These fibres are themselves inelastic but their arrangement in spirals and mesh allows this limited change in the body length.
The alternate contraction of the dorso-lateral and ventro-lateral muscles of the anterior end helps in the undulating movements. This undulating movement helps the parasite to counteract the peristaltic movement of the host intestine.
Respiration is anaerobic or anoxybiotic as the oxygen content in the host’s intestine is usually low. In the process of anaerobic respiration, glycogen undergoes glycolysis to yield carbon dioxide, fatty acids and energy. The main fatty acids which are produced include valerianic butyric and caproic acids. These are excreted through cuticle and these impart a characteristic smell like that of a canned pineapple.
The excretory system of Ascaris is simple as there are no flame cells. The excretory system is H shaped formed by huge excretory Renette cell during the juvenile stage. In the cytoplasm of this cell, tunnel-like structures are present and these act as excretory canals. It consists of two lateral longitudinal excretory canals called as right and left longitudinal excretory canals. They are connected anteriorly by a transverse canalicular network.
Each longitudinal canal extends posteriorly along entire length of the body and is closed at both ends. The left canal is slightly wider than the right canal. The lumen of the canals does not have cilia. From the left side of transverse canalicular network, a short terminal excretory duct extends towards the excretory pore situated a little the anterior end.
The excretory product of Ascaris is mainly urea and so it is also known as ureotelic animal. The excretory canals collect the excretory products from different parts of the body and these excretory products are eliminated through the excretory pore. The pressure of the pseudocoleomic fluid helps in ultrafiltration process. Some ammonia and urea are also passed out through the anus.
The nervous system of Ascaris consists of the following parts,
Central nervous system– It consists of rich ganglionated nerve ring. The ganglia present on the nerve ring are a single dorsal ganglion, a pair of sub-dorsal ganglia, a pair of lateral ganglia and a pair of large ventral ganglia.
Peripheral nerves– From the nerve ring, 8 nerves run anteriorly to supply to the anterior parts of the body. Out of these, 6 supply to labial papillae. The remaining 2 nerves are known as amphidial nerves as these supply to the amphids. A dorsal nerve cord runs through dorsal epidermal chord.
A ventral ganglionated nerve cord runs through ventral epidermal chord. It terminates at the posterior end after forming anal ganglion. Also 2 lateral nerve chords are present. Above and below the lateral nerve chords, 2 dorso-lateral nerve chords and 2 ventro lateral nerve chords are present. Several commissures are arranged asymmetrically along the entire length.
Rectal nervous system– The posterior end of the male roundworms has lateral nerve cords to supply to pre-anal papillae, ventral nerve cord to supply to post-anal papillae. Dorsal, ventral and lateral nerve cords are interconnected. But such complex rectal nervous system is absent in female roundworms.
The sense organs of Ascaris are the simple extensions supplied by nerves. The following are the sense organs of these roundworms.
Papillae: They are in the form of small villi situated on different body parts.
a) Labial papillae– They are formed by sensory cells surrounded by many supporting cells. These are gustatoreceptors and present on three lips surrounding mouth.
b) Cervical papillae– These are a pair of papillae present behind the lips. These act as tactile organs.
c) Anal papillae– These papillae are present at the posterior end of male worms which help in copulation.
Amphid: On each latero-ventral lip, a single amphid is present near a single papilla. These are gustatory sensory or chemo-receptors
Phasmids: These are chemo-receptors unicellular glands, opening one on either side of tail.
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