An order of nematodes in which the number of labia varies from a full complement of six to three or two or none. The tubular stoma may be composed of five or more sections called rhabdions. The three-part esophagus always ends in a muscular bulb that is invariably valved. The excretory tube is cuticularly lined, and paired lateral collecting tubes generally run posteriorly from the excretory cell; some taxa have anterior tubules also. Females have one or two ovaries; when only one is present the vulva shifts posteriorly. The cells of the intestine may be uninucleate, binucleate, or tetranucleate, and the hypodermal cells may also be multinucleate. Caudal alae, when present, contain papillae.