Some members of this phylum are parasitic, and these have given the phylum a lot of bad press, since they are the animals usually covered. Ascaris lumbricoides is one of the best known of the parasitic nematodes. Males and females live in the intestines of humans where they graze on intestinal contents.
Eggs pass out with the feces and, if they contaminate food, are introduced to another host.
The larvae hatch out in the intestine of the new host and then burrow through the walls to be carried by the bloodstream to the lungs. At the lungs they burrow through the alveoli and crawl up the trachea and down the esophagus. Larvae usually burrow out of the lungs at night and are unknowingly swallowed by the host. Occasionally larvae get lost and crawl up the esophagus and exit at the nose. In some areas of the world Ascaris is so common that a child is not considered to be part of the tribe until a larva is sneezed out and found in the bed.