147] COMPARATIVE STUDIES ON NEMATODES— HETEERINGTON 43 



4. The fundamental symmetry is bilateral. True radial symmetry is 

 rare but apparent radial symmetry is very noticeable in the cephalic 

 region and is a secondary condition resulting primarily from the 

 sessile tendency. 



5. Disymmetry is tertiary as is asymmetry. 



6. The simplest and perhaps most primitive cephalic organization was 

 and is in some forms today, a round or triquetrous mouth opening 

 directly into the triquetrous esophagus, with small or indistinct 

 oral papillae. Successive complication and evolution of structure 

 may be thought of as taking place in the following order: 



(a) Three lipped forms with no pharyn.x. 



(b) Three lipped forms with developing pharynx. 



(1) Two-lipped forms with no pharynx arising from loss of the 

 dorsal lip. 



(2) Jaws arising from loss of upper or dorsal cephalic sector 

 and migration laterally of the two subventral sectors. 



(c) Multiple lips by division of the primitive number. 



(d) Partial fusion of multiple lips and development of large armed 



pharynges. 



(e) Capsule formed by fusion of lips in connection with a large 



pharynx armed generally by strong onchi. 

 (1) Spear-bearing forms from fusion of lips, elongation of 

 pharynx, and fusion and separation of onchi from the 

 pharyngeal walls to form the buccal stylet moved by special 

 muscles. 



7. Disymmetry is more noticeable among parasitic forms than among 

 free-living forms. 



8. Cephalic organization has kept pace with other specializations and 

 bears a relation to habitat and is indicative in a general way of the 

 evolutionary status of a genus. 



