1371 COMPARATIVE STUDIES ON NEMATODES— HETHERINGTON 33 



extent of parasitism in connection with cephalic structure, and such is 

 indeed true as will come out in following discussions. 

 Pharyngeal modifications 



Pharynges among the parasitic round worms are not as diverse in 

 form nor as complicated in structure as many of those in the free living 

 nematodes. There are no intergradations from a simple conoid pharynx, 

 through the various changes of size and armature to the fusion of parts 

 and formation of spears which one finds among the nonparasitic genera. 

 Indeed the pharynges are few and are prominent only among the Stron- 

 gyles where one finds them exceptionally well developed. The spear- 

 bearing forms exist only among the phy toparasitic nematodes as Heterodera 

 and Tylenchus but, as these round worms are to a large extent free 

 living, they cannot be adequately classed with the more fully parasitic 

 animal forms. 



The simplest pharynx is really little less than a circular vestibule in 

 many species; for example, Protospirura muris (Fig. 10) illustrates this 

 quite weU as a short cylindrical or faintly prismatic passage leading from 

 the lips into the esophagus. Other instances of such a simple pharynx 

 may be found in such nematodes as Eustrongylides ignotus Jagerskiold and 

 related species. The most elaborate pharynges are, however, found among 

 the Strongyles where they are often large and capacious, almost smooth, 

 or else armed by teeth and cutting plates. These structures are prominent 

 among the characteristic features of the tribes Strongyleae, Bunostomeae, 

 Ransomeae, Cylicostomeae, and a few genera of undetermined tribal 

 relations. But since this capsular formation is so intimately connected 

 with oral structure, its nature will be described in the following paragraphs 

 in connection with modifications of the lip region. 

 Oral structure and symmetry 



When the oral armatures of the parasitic nematodes of the alimentary 

 tract are examined, one finds as a common thing various modifications of 

 lips unarmed or armed with teeth or cuticular thickenings. Three lips are 

 present in the most primitive of parasites, namely genera of the Oxyuridae, 

 Heterakidae and Ascaridae, of which the members of the last family show 

 a great number of modifications. In Heterakis papulosa Bloch the three 

 lips are small and equal. A similar equality and tri-radiality exist in 

 Falcaustra siamensis Baylis; the lips are somewhat flattened antero-pos- 

 teriorly except for two prominent papilla bearing projections on each 

 (Fig. 17). Crossopltorus collaris Hemprich and Ehrenberg, probably an 

 Oxyurid, is tri-symmetrical in every respect save the minor difference 

 between the arrangement of the labial papillae which are two to each lip, 

 but the dorsal lip bears them both at the same level near the lateral borders 

 while the subventral lips carry one papilla a little to the ventral side of 

 the middle of the outer lip surfaces; the other papilla being much smaller 



