26 



ILLINOIS BIOLOGICAL MONOGRAPHS 



[130 



of the three lips of the primitive form into two parts (Text figs. A and B). 

 Division, tho incomplete, of these six lips gives rise to twelve lipped forms 

 such as Anaxonchiiim litorium Cobb; complete division, to twelve small lips 

 as lotodoriis puncHdaius Cobb, and perhaps to the twelve highly modified 

 lips or labial setae in Pomponema mirabile Cobb. Division of lips would 

 then indicate a more evolved nature than the three lipped condition. This 

 fact seems to be well borne out because many lipped forms occur among 

 genera which have rather elaborate pharynges (Mononchus), and which 

 have specialized in other lines too, like choice of habitat, loss of structures, 

 as caudal glands and of one gonad by suppression. The Mononchs are 

 to a large extent land inhabiting predatory nematodes rather more ad- 

 vanced in this last respect than their fresh and salt water relatives. The six 

 lipped condition permits the oral aperture to open widely allowing the 

 mononch greater certainty in seizing its prey and macerating it against the 

 pharyngeal onchi. 



Text Fig. A. Diagram of nematode head en face showing a simple radial symmetry with 

 three lips, d, dorsal; v, ventral. 



Text Fig. B. Diagram showing six lips derived by division of the primitive three. The 

 super6cial symmetry is multiradial; the deeper symmetry tri-radial on account 

 of the esophagus. 1, 2 and 3 indicate rings of symmetry previously mentioned 

 in the te.xt: anteriormost, the ring of papillae; next, posteriorly, a ring of 

 long cephalic bristles; and third, a ring of lesser bristles. 



While specialization may go in one direction, namely, division of lips, 

 still further specialization even in advance of division results by the union 

 or amalgamation of lips. Degrees of confluence or amalgamation may be 

 easily seen after a survey of a large number of species. For instance, one 

 may begin with a form possessing three distinct lips (Rhabditis) and these 

 may then become confluent as in Monhystriiim transitans Cobb. The 

 mouth opening resulting in such a case is triangular or triquetrous. Mon- 

 hystera stenosoma de Man seems to indicate this even better than the 

 preceding species. By far the most common number of lips is six, and 



