An order of nematodes in which the labia are seldom well developed; however, a hexaradiate symmetry is distinct. The external circle of labial sensilla may appear setose, but they are always short, never long or hairlike. The stoma may be slender and elongate, or spacious, or any gradation between. The stoma may be armed or unarmed; the armature may be movable teeth, fossores, or a pseudostylet. The corpus is always muscled and distinct from the postcorpus, which is divisible into an isthmus and glandular posterior bulb. The metacorpus is almost always valved. The female reproductive system may have one or two ovaries, and males may or may not have caudal alae; however, a gubernaculum is always present. The male tail commonly has nine pairs of caudal papillae; three are preanal and six are caudal.