125] COMPAPATIVE STUDIES ON NEMATODES— HETHERINGTON 21 



genera in that category will have to be removed from it as not being 

 related, such as Trichosomotdes crassicauda Bell, which, according to 

 Rauther, shows a triquetrous esophagus, at least for a considerable part 

 of the length of that organ. 



The triradial nature of the esophagus is such a distinctive feature of 

 the phylum Nematoda and is so nearly universal throughout the group, 

 that it may be accepted as one of the most stable factors in nematode 

 organization. For this reason, it may be considered a primitive feature; 

 certainly, if not primitive, it is one of the earliest features to have been 

 established in the evolving ancestor. When this triquetrous organ, which 

 underlies all the superficial structures of the cephalic region, is used as the 

 basis of determining the symmetry of the head, the only possible symmetrical 

 divisions involving all structures would be two in number; namely, one of 

 bilaterality, and one of triradiality, the latter of which by division of sectors 

 might readily pass into conditions of multiple symmetry among the 

 more superficial structures like the lips. Exceptions to triradiality would, 

 of course, occur in nematodes possessing cephalic branches to the lateral 

 excretory canals, amphids and ocelli. Normally radiality merges pro- 

 gressively into bilaterality as the region of the nerve ring is approached in 

 an antero-posterior direction, suggesting rings or horizontal planes of 

 symmetry appearing at different levels of the cephalic region. The more 

 anterior structures are more truly arranged radially symmetrically while 

 those later succeeding levels as has been said pass into bilateral groupings. 

 When, however, the more superficial structures of the nematode head 

 and pharyngeal region are examined, these fundamental di- and tri-radial 

 symmetries give place to curious mixtures of symmetrical patterns in one 

 and the same nematode, involving plans based on multiples of two and 

 three radii. Lips, papillae, sensory hairs, cephalic bristles, teeth, and 

 cuticular processes are compounded in a variety of ways; for example, 

 Oxyuris obvelata Rudolphi bears three lips arranged in correspondence 

 with the three sectors of the esophagus but the six papillae are grouped in a 

 dextral and sinistral row of three each (Fig. 1). Protosplrura muris 

 Gmelin carries a right and left row each of three lip-like divisions and four 

 papillae, one at the base of each terminal division of each row (Fig. 10); 

 again, the elaborately constructed Mononchus gerlachei de Man, a marine 

 nematode, possesses six radially arranged lips each bearing, centrally 

 placed, a single papilla and at a lower level each carrying two papillae save 

 the two central lateral lips which have again only a single papilla each. 

 Immediately below the lips on the walls of the vestibule are twelve rounded 

 projections of unknown significance. Beneath these there is a chitinous 

 skeletal structure hexagonal in optical section merging into the triangular 

 lumen of the pharynx which itself passes into the ever present triquetrous 

 esophagus (Fig. 3). 



