Lorenzen described two orders, Enoplida and Trefusiida, in the 1980s based on morphology. With the advent of phylogenetic analysis, a reorganisation has been necessary, moving the Triplonchida here to create three orders and expanding it.
The orders are distinguished mainly by habitat type.
De Ley, P & Blaxter, M 2004, 'A new system for Nematoda: combining morphological characters with molecular trees, and translating clades into ranks and taxa'. in R Cook & DJ Hunt (eds), Nematology Monographs and Perspectives. vol. 2, E.J. Brill, Leiden, pp. 633-653.