This group (also known as the Aphasmidea in some classifications) contains many free-living nematodes, in addition to a few important parasitic groups on invertebrates and vertebrates.
Life cycles of those species parasitic as adults in vertebrates vary. Some are direct cycles in which an embryonated egg is eaten. Others have indirect life cycles with as many as two intermediate hosts. Some species parasitize insects but have free-living adults.
The only unifying morphological features of the group are obscure ones, (often difficult to see even on species possessing them): the lack of phasmids or deirids, and a poorly developed excretory system. Many species have a body divided into an anterior stichosome (composed of numerous donut-shaped cells surrounding an esophagus) and a hind-body that contains the reproductive structures.