An order of nematodes in which the cephalic region may be adorned with three or six labia or the labia may be replaced by a corona radiata. The variable stoma may be well developed or rudimentary but is never collapsed or inconspicuous. The capacious stoma consists primarily of cheilostome; only the base of the cup is surrounded by esophageal tissue. The stoma even when reduced is represented by the cheilostomal vestibule. The esophagus in larval forms is rhabditiform, and the posterior bulb is valved. The adult esophagus is cylindrical to clavate. In the fourth juvenile stage and the adult the excretory system consists of paired lateral collecting tubules, an excretory cell, and paired subventral glands. Other immatures have a single excretory cell. The females have one or two gonads, and the uterus is heavily muscled. The males have unusual caudal alae, referred to as a bursa copulatrix. The bursa differs from other caudal alae in that it contains muscles and rays. Males have paired and equal spicules. All strongylid nematodes are parasitic.