The Monogenea is the second major class of "flukes". The monogeneans are ectoparasites with direct life cycles. There are two orders that are distinguished on the basis of attachment organ structure, which exhibits remarkable diversity but is not surprising considering the importance of attachment for any ectoparasite.
The acanthocephalans, or thorny-headed worms, also have a very uniform life cycle, but it involves three larval stages (unlike the nematode's four) and all three occur in a single intermediate host. These parasites are "gutless wonders", like the tapeworms, but can be easily distinguished from tapeworms by the presence of a body cavity. All acanthocephalans are parasitic.
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. Like the nematodes, adult morphology of acanthocephalans is highly uniform, but most features that distinguish the higher taxa are easily observed.