Nematodes are one of the most diverse groups of parasites. Many people will (jokingly) say that all nematodes look alike. Fortunately, the hard external cuticle and a variety of hardened internal structures take on a form that is characteristic for each species, and seems to be relatively invariant within a species. Morphological diversity among nematodes is most evident in external features, such as type of lips, cuticular ornamentations near the cephalic and caudal extremities, the structure of the spicules and associated structures, and the structure of the esophagus.