As mentioned previously, some parasitic roundworms have very interesting life cycles in which the larva or embryonated egg is ingested, but rather than developing directly into an adult in the intestines, the larva penetrates the intestinal wall, migrates via the circulation to the diaphragm or lungs, is then coughed up and swallowed and returns to the intestine where it finally develops into an adult. These migratory larvae are typically quite small, but occasionally, an adult can migrate accidentally and coughed up or vomited; this can be seen sometimes in
Ascaris infections. This image shows how large an adult worm is, so it is amazing that something so large could move through the body:
(31)Unlike many flatworms, the roundworms have separate sexes, which can sometimes be very morphologically different from one another.
To the right are life cycles of Ascaris and Hookworm; they are similar in that they both have a migratory phase in the human and that some development of the larvae must occur in the soil. Differences include that the person gets Ascaris by swallowing embryonated eggs from contaminated food or hands, whereas the larva of Hookworm directly penetrates the human skin. Both are associated with areas where defecation into the soil occurs.
