An order of nematodes in which, generally, the stoma is funnel shaped and lightly cuticularized; however, in some families the stoma is spacious and heavily cuticularized, and is armed with protrusible teeth. The amphids vary from simple spirals to circular forms. Usually the second and third circlets of cephalic sensilla are combined, but in some taxa the third circlet of four distinct setae is separate. The normal pattern of distribution is often disrupted by numerous cervical setae. The near-cylindrical esophagus is sometimes swollen posteriorly. The cuticle may be smooth or may have annuli or ornamentation. When the annuli are distinct, the somatic setae may be long and in four to eight longitudinal rows. The female gonads are outstretched and either single or paired.