Some older references place the Acanthocephala and Nematoda in the same phylum, Nemathelminthes, based on the presence of a pseudocoele. We deal with them as separate phyla, and as you study them you will see that they have little in common.
The nematodes, also called roundworms, have a fairly uniform life cycle involving four larval stages before the adult, but exhibit tremendous diversity in where the adults and larvae occur. There are direct and indirect life cycles, and nematodes of some sort have evolved to exploit just about every tissue or cavity in the vertebrate body (and various invertebrates as well). Adult morphology is much more uniform than the platyhelminths, but the major taxa of nematodes still possess easily recognizable features. Some taxa of nematodes are exclusively parasitic, some have both free-living and parasitic representatives, and others are exclusively free-living. Some nematodes are parasites of plants (we do not include any of those in this course) and others are parasites of animals.