28 ILLINOIS BIOLOGICAL MONOGRAPHS [132 



and Dorylaimus, exhibit the same general structure but some of the 

 species among the numerous ones in the latter genus show distinctly the 

 lip-like nature The fusion is complete enough to produce a circular 

 mouth opening but the peripheral outline of the crown of lips viewed en 

 face is scalloped showing the external outline of six lips (Dorylaimus labia- 

 tus de Man). There seems to be in these spear-bearing forms, derived from 

 others possessing armed pharynges, an interesting correlation between 

 extent of lip confluence and pharyngeal specialization. The mononchs 

 with wider but less complex pharynges, from the standpoint of genesis, 

 possess less confluent lips. In this connection one may ask whether the 

 forms cited in an earlier portion of this section as possessing no trace of 

 lips or pharynges have never acquired a pharynx or lips, or whether these 

 structures have been lost by devolution, or whether they may indicate a 

 greater stage of evolution. It cannot be said definitely at present which 

 view is the better one to accept but in consideration of the primitive 

 nature of the gonads in some of those forms it is quite possible that the 

 weight of evidence favors the view of primitiveness. 



Another interesting oral and cephalic structure and one which might 

 be interpreted as the forerunner of jaws is to be found in Synonchium 

 obtusum Cobb, which was described in connection with pharyngeal modifi- 

 cations. The six double lips have fused completely showing only a con- 

 tinuous crenate rim, as it were, surrounding the large flaring triquetrous 

 mouth and pharynx (Fig. 16). Incidentally this nematode possesses pure 

 tri-radial symmetry as far as the level of the amphids which shift the total 

 symmetry of the head to bilaterality. By the transition from lips to jaws 

 bilaterality becomes the only symmetry in the head region. The apparent 

 change from the bi-radial pharynx to jaws comes through the loss of the 

 dorsal sector of the pharynx accompanied by a lateral shifting of the other 

 two sections. SelacMnema ferox Cobb illustrates this change very beauti- 

 fully because there remains a vestigial dorsal sector which, however, is. 

 greatly overshadowed by the two powerful submedian chitinous jaws. 

 Another species yet undescribed by Cobb shows no remnant of this dorsal 

 sector. Chieronchus vorax Cobb by a similar elimination of the dor- 

 sal sector has two jaws or mandibles. Pseudoncltus rolundicephalus Cobb 

 again supports the formation at least of bilateral mouth parts from the 

 loss of the dorsal pharyngeal sector. So far there is no evidence which 

 supports the view that submedian sectors ever fuse giving rise to jaws 

 which work in apposition dorso-ventrally. 



Perfect disymmetry in a dextro-sinistral sense (Text fig. C) is found in 

 the three extraordinary genera, Diploscapter, Wilsonema and Heth, of 

 which D. coronatus (Maupas), W. capitalum, Cobb, and H. jtili Cobb are 

 respective examples. The first possesses four strong outwardly directed 

 hooks, two dorsal and two ventral and between them laterally are two 



