Ascaris is a nematod, commonly called roundworm belonging to phylum of Nematoda of superphylum Aschelminthes.
Habit and habitat :
The most common and best known species Ascaris lumbricoides is a gastrointestinal endoparasite of man.
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It is found more frequently in children than in adults. It is a monogenetic parasite i.e., it completes its life cycle on single host.
It has a cosmopolitan distribution, but chiefly found in India, China, Korea, Philippines and Pacific Islands.
Image Source: herramientas.educa.madrid.org
External features :
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Body is elongated cylindrical and gradually tapering at both ends, the anterior end is more slender than the posterio.
Ascaris is dioecious and shows sexual dimorphism. Female ascaris measures 20.0 to 40.0 cm in length and 4.0 to 6.0 mm in diameter with a straight tail. While male is smaller upto 20.0 cm in length and 2.0 to 4.0 mm in diameter with a tail curved ventrally.
The colour of ascaris is light yellow and light pink because of transparent cuticle. Cuticle bears minute transverse striations giving a pseudosegmented appearance to the worm.
All along the length of body four longitudinal streaks or lines – one middorsai, one mid-ventral and two laterals are visible. Dorsal and ventral lines appear pure white, while, lateral lines appear brown.
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Anterior end:
On the anterior end is a small terminal triradiate mouth guarded by three broad lips or labia. Inner or oral margin of each lip has forked fleshy core, consisting of two anterior extensions of labial parenchyma and bears minute denticles.
These denticles help in rasping of tissues. Outer surface of each lip bears minute sensory papillae (L., papilla, nipple).
A single amphid, near papilla on lateral lip is also present. Papillae are perhaps tangoreceptors while amphids are probably chemoreceptors.
Posterior end:
The posterior end is straight in male. In female, a little in front of the tail end lies a mid ventral transverse aperture, or anus guarded by thick lips. Only digestive tube opens to outside through anus.
In male, anus is replaced by a cloaca. It is the common aperture for digestive and genital tubes. Two small needle-like and cuticular penial spicules or copulatory setae are often seen protruding out from the cloacal aperture.
They serve to transfer sperms into female vagina during copulation. The ventral body surface in cloacal region bears 50 pairs of preanal (precloacal) papillae in front of cloacal aperture and 5 pairs of postanal (postcloacal) papillae behind it. These papillae serve to hold the female during copulation.
Excretory pore:
A single excretory pore lies mid-ventrally at a distance of about 2 mm from the anterior end.
Body cavity or pseudocoel
The body cavity of Ascaris is called false coelom or pseudocoelom because –
(i) It is not lined by coelomic epithelium
(ii) It has no relation with reproductive and excretory organs and
(iii) It develops from blastocoel i.e., between mesoderm and endoderm of embryo.
Pseudocoel is filled with an adorous protein-rich fluid, the perieuteric or pseudo- coelomic fluid, which serves for transportation of metabolites and keeps the body distended.
There are five giant stellate mesenchymal cells known as pseudocoelomocytes, occupying fixed positions.