133] COMPARATIVE STUDIES ON NEMATODES— HETHERINGTON 29 



serrated flaps or lips, one right and one left. Cobb interprets in his illustra- 

 tion of this species the two flaps as well as the four hooks, each as a lip. If 

 this is true, the nematode possesses six lips; the two laterals having flattened 

 out and become flaplike; the two dorsals and two ventrals having fused and 

 formed a single dorsal and a ventral double hooked structure. The second 

 species above is distinguished by dorsal and ventral double "combs" and 

 two lateral columns, a dextral and a sinistral one, tipped each by a finer 

 process. Here the disymmetry is most prominent dorso-ventrally (Text 

 fig. D). The last species of the three evinces again lateral disymmetry. 



D 



Text Fig. C. Diagram of nematode head en face showing disymmetry with respect to 



a dextro-sinistral plane. 

 Text Fig. D. Diagram showing disymmetry with respect to a dorso-ventral plane. Both 



this tj'pe and the foregoing are forms of bilateral symmetry. 



Considerations of symmetry in the head region 

 After the considerations of esophageal, pharyngeal, and oral structure 

 in the preceding paragraphs, the following ones will be devoted to an ex- 

 amination of the symmetry of the nematode head as a whole, considering 

 the parts played by these structures in determining this symmetry. In 

 view of the fact that the outstanding symmetry of the nematode body as a 

 whole is bilateral, the same relation must have applied to the cephalic 

 region in the primitive form, a fact which seems to be borne out by what 

 is known of nematode embryology, and by features which nematologists 

 have come to accept as primitive. The primitive mouth was ventral and 

 the esophagus arose from three rows of cells, the dorsal one of which was 

 the equivalent of the other two. These features alone would establish 

 bilaterality in the embryo and in the primitive adult. 



By an equalization of the three esophageal sectors, the triquetrous and 

 the trisymmetrical nature of this organ became apparent. The primitive 

 nematode had three lips corresponding to the symmetry of the esophageal 

 sectors giving rise ultimately to a purely trisymmetrical structure. Only 

 very few radially symmetrical cephalic regions exist among nematodes 



