129] COMPARATIVE STUDIES ON NEMATODES— BETBEIUNGTO.X 25 



is triquetrous, it would appear that the most primitive form of mouth 

 would not be a circular orifice but rather a triquetrous or triangular open- 

 ing agreeing in symmetry with the esophagus; however, either view might 

 be taken and some considerations which will follow later will support the 

 former view that the primitive mouth opening was round. In Thoracosloma 

 setosum v. Linstow a triangular mouth occurs but in consideration of certain 

 specializations in the head region, it might better be interpreted as a 

 fusion of three lips. 



The next structurally simplest form and the one at present conceded 

 most primitive is that found among members of the genera Enoplus and 

 Rhabditis. The most primitive type of free living nematode in the esti- 

 mation of nematologists is found in this last genus, where the mouth is sur- 

 rounded by three lips, which are in fact definite and well formed. One of 

 these is dorsal and the other two are subventral corresponding again with 

 the divisions of the esophagus. The lips may be entire or there may be 

 signs of division as in Rhabditis pellio Biitschli where each lip is divided 

 incompletely into two portions by a shallow groove running longitudinally 

 along its mid-region. Similarly Rhabditis lambdiensis Maupas possesses 

 three lips distinctly bilobed, each bearing a pair of prominent setiform 

 papillae, all equal and exhibiting as a whole perfect radial symmetry of the 

 head region. 



Quite in contrast to the division of lips may be mentioned the curious 

 labial variations found arising from outgrowths of the lips in the genera 

 Teratocephalus and Cephalobus. In the species Teratocephalus crassidens 

 de Man the six lips surrounding the shallow pharynx have cuticular wings 

 on the edges, partially fused near the bases of these lips, such that there 

 results a corolla-like structure with a continuous edge. The apices of the 

 four sublateral lips bear each a fine bristle (Figs. 2 and 4). On the other 

 hand Cephalobus ciliaius von Linstow has arising from each of its three 

 lips a thick column which bifurcates and gives forth two rather long 

 processes whose edges are beset with bristles at regular intervals (Fig 6). 

 In a position alternating with the lips, the cuticula near the periphery has 

 become elongated into a stout horn-like process. The significance of such 

 diverse outgrowths of lips as represented in the two genera above is not 

 yet known. The radial symmetry of the heads in these two species is only 

 superficial because just below the lip region on the lateral fields lie the am- 

 phids, one dextral, and one sinistral. They perforce shift the symmetry to 

 bilaterality. 



Evidence gathered from an examination of numerous free living nema- 

 todes seems conclusively to show that forms having a small number of 

 lips acquired these labial organs by the subsequent division of the primitive 

 three lips. Six lips are a very common number among many genera and 

 appear as suggested by Rhabditis to have arisen by the division of each 



