30 ILLINOIS BIOLOGICAL MONOGRAPHS [134 



today, because there are in the free living species organs such as amphids 

 and ocelli, and because there are with or without these parts, the cephalic 

 bristles and sensory papillae which in a great number of forms fall into a 

 bUateral arrangement, despite radial ordering of other structural elements. 

 In an otherwise radially symmetrical head, one often finds only four ce- 

 phalic bristles instead of six, as if the median lateral ones had been lost and 

 in cases of duplication of bristles or papillae on the lips, the lateral median 

 lips are the ones which lag behind the others in this respect. 



This peculiarity is well illustrated by Cobb as existing among the mon- 

 onchs in relation to the labial and cephalic papillae. The arrangement of 

 papillae in these forms follows the law for the arrangement of tactile ce- 

 phalic setae of nemas in general, namely: "... When six are present one 

 is found on each of the two lateral lines and one on each of the four sub- 

 median lines; when more than six are present, the increase occurs first on the 

 submedian lines, the commonest number being ten, — one on each lateral 

 line and two on each of the four submedian lines; when the number is in 

 excess of ten, the increase is again more commonly found on the submedian 

 lines." Obviously structures following this order of arrangement shift 

 apparent radial symmetry into bilaterality again. Radial symmetry with 

 few exceptions is actually attainable only if lips alone are concerned, as has 

 already been shown. 



The pharyngeal region is frequently non-radially symmetrical, rarely 

 is this not true, when it becomes armed with onchi (Text fig. F) because the 

 dorsal ones usually have a tendency to surpass in size the other onchi. The 

 small, smooth, prismatic or cylindrical, and unspecialized pharynx readily 

 falls in lines with any symmetry which the lips impose upon it. Disym- 

 metry either dorso-ventral or dextro-sinistral exists, as we have seen (Text 

 figs. C and D), in a few free living forms. Its origin is explicable in a few 

 cases as the result of loss of the dorsal lip and pharyngeal sectors. In such 

 cases the cephalic symmetry shifts undeniably into the fundamental 

 bilaterality. Asymmetry occurs least of the other types. In summary it 

 appears then that true radial symmetry is not as general a condition among 

 nematodes as a superficial examination would lead one to expect. Radial 

 symmetry, however, is common, and a striking feature if sensory organs, 

 pharyngeal onchi, and other armatures are neglected or considered second- 

 ary in importance to the basic plan of the head region. The apparent order 

 of symmetrical succession in the nematode body beginning with the primi- 

 tive worm is most probably the following, applied, of course, only to the ce- 

 phalic region: 



1. Primitive bilateraUty 



2. Radial symmetry (Text figs. A and B) 



3. Disymmetry (Text figs. C and D) 



4. Asymmetry (Text fig. E) 



