from Hany M. Elsheikha writing in Essentials of Veterinary Parasitology:
Classification of nematodes has been traditionally based on the presence or absence of a posterior cuticular chemoreceptor called 'phasmid'. Nematode species with phasmid are known as phasmidea (Secernentea) and nematodes that lack phasmid are called aphasmidea (Adenophera). It is important to realize that the parasite taxonomy is an evolving field and there is no a single scheme that is always acceptable. Class Nematoda encompasses numerous species that infect livestock and companion animals. This chapter focuses only on the most economically important nematode infections in livestock and companion animals. General taxonomy of nematodes considered in this chapter is given to the genus level.
Further reading: Essentials of Veterinary Parasitology