From an evolutionary and morphological perspective, the Nematodes (also called Nemathelminthes or roundworms) are more complex organisms than the platyhelminthes. One of the major evolutionary advancements that they exhibit is a body cavity. As was mentioned earlier, the flatworm organs are embedded in parenchyma; the roundworm organs, on the other hand, are found in a body cavity known as a pseudocoelum.
Many worm species that belong to the nematodes are free-living. However, even many of the parasitic nematodes have some things in common with their free-living cousins: soil. Nematodes, like insects, undergo a series of "molts" which often occur in the soil as they grow and develop. Even some of our human roundworms have an obligatory phase in the soil and can molt there or in the human body. Another thing that many of the parasitic roundworms have in common is a migratory phase, often coinciding with their molting. You will see this when you look at the life cycles. This course covers only the intestinal nematodes, the majority of which can be diagnosed by their eggs in the feces. There are also nematodes that can live in the blood and tissue, which are not covered here.
For completeness, it is worth mentioning that there is a third group of worms besides the platyhelminthes and the nemathelminthes known as the Acanthocephala (spiny or thorn-headed worms), which can, on rare occasions, parasitize humans. They are mainly parasites of wild and occasionally domestic animals. Humans can accidentally be host to some of these, but because of their rarity, they will not be covered here.
