This group contains many serious pathogens of plants and animals, as well as many free-living representatives. Many species are also facultative parasites, able to survive in an animal host if they should be ingested accidentally.
Animal-parasitic species may have direct or indirect life cycles, and infect all classes of vertebrates and many invertebrates. Some species alternate between free-living and parasitic generations.
Parasitic forms may be dioecious, hermaphroditic or parthenogenetic females. The mouth is simple, with 3 or 6 small lips. The esophagus is rhabditiform although some developmental stages may have a filariform esophagus.