Q.Describe the external structure of Ascaris lumbricoides.

Ascaris lumbricoides, the giant intestinal roundworm, is a large nematode parasite of the human small intestine and the causative agent of ascariasis, one of the most common helminth (worm) infections worldwide.

Structure. The elongated, cylindrical body is covered by a tough cuticle; the female is longer and thicker with a straight posterior end, while the male is shorter and thinner with a ventrally curved posterior end bearing copulatory spicules. The fertilised eggs passed in faeces have a characteristic thick, knobbly (mammillated) outer coat that protects them and allows long-term survival in soil.

Life cycle and extra-intestinal migration. Infection follows ingestion of embryonated eggs from contaminated food, water or soil. The egg hatches in the small intestine, releasing a larva that — unusually for an intestinal parasite — does not develop directly in the gut. Instead it bores into the gut blood vessels, travels via the portal circulation to the liver, then via the heart to the lungs (the extra-intestinal migratory route). In the lung capillaries it breaks into the alveoli, moults, and migrates up the bronchioles, bronchi, trachea and pharynx, where it is swallowed a second time, returning to the small intestine to mature into an adult worm. The adult female then produces vast numbers of mammillated eggs, passed out in faeces to infect new hosts via faecal contamination of soil. …