ascaris Nematode worms (Nematoda), all of which are parasites, some of mammals, others of birds, and some of invertebrates. The group includes Toxocara canis and Ascaris lumbricoides (the dog and human ascarids respectively). In both these species, eggs are voided and begin to develop in faeces. If ingested by another host, the eggs hatch in the small intestine, releasing small worms that penetrate the gut wall, enter the blood stream, and are carried to the lungs. There they break into alveoli (see ALVEOLUS), move along air passageways to the oesophagus, and are swallowed, returning as adults to the intestine, where they lay eggs.

Ascaris (ass-kă-ris) n. a genus of parasitic nematode worms. A. lumbricoides the largest of the human intestinal nematodes. Larvae hatch out in the intestine and then migrate via the hepatic portal vein, liver, heart, lungs, windpipe, and pharynx, before returning to the intestine where they later develop into adult worms (see also ascariasis).