They are characterized by their elongated, cylindrical bodies and unsegmented structure.

Some species are known as eyeworms and infect the orbital cavity of animal hosts.

The Camallanida are sometimes included herein as a suborder, and the Drilonematida are sometimes placed here as a superfamily.

There are doubts about the internal systematics of the Spirurida, and some groups placed herein might belong to other spirurian or even secernentean lineages.

This Secernentea roundworm- related article is a stub.