20 ILLINOIS BIOLOGICAL MONOGRAPHS [124 



life develops only very slowly in the course of the second half of the larval 

 life." 



There are no known living forms which possess all the primitive charac- 

 ters set forth either by Steiner or Seurat. The latter author makes no 

 statements regarding primitive spatial orientation and further regards the 

 three-lipped form — with one dorsal and two ventral lips — as the probable 

 early type while the former author postulates a simple digestive tract 

 devoid of diverticula. Among the free living nematodes members of the 

 genus Rhabditis have conserved some of the primitive characters in the 

 structure of the digestive tube and genital organs but have gone far afield 

 in the acquisition of a radial symmetry of the mouth, in the reduction of the 

 male genital system to a simple tube and also in the structure of the lateral 

 lines. On the other hand, among all the parasitic nematodes, those 

 guarding the most numerous primitive features are the members of the 

 oxj'urid group. They possess primitive musculature, and show primitive 

 structure of the lateral bands, the excretory apparatus and the digestive 

 tube. Contrary to these ancestral features are the extreme modifications 

 of the ovejector in the female and of the spicular organs and truncated 

 tail of the male. The larvae are, however, rather undiflferentiated and 

 afiford some of the data upon which the primitive nature of the group is 

 based. 



Cephalic Structure in Free-living Nematodes 



Symmetrical type of the esophagus 



After the preceding discussion of the primitive nematode, its bilateral 

 nature and orientation, the following sections will be limited to a considera- 

 tion of the structure and symmetrical content of the cephalic region (1) of 

 free-living species, and (2) of parasitic species, in an eSort to determine the 

 primitive condition and the successive changes which evolution has im- 

 posed upon the early type. 



One element of the anterior region which is ever a possessor of triradial 

 symmetry in all the members of the Myosyringata Ward (1917) is the 

 esophagus. In cross section, this organ exhibits a triquetrous lumen, "sech- 

 seckig" as Schneider (1866) calls it, with three alternating obtuse angles 

 directed apex lumen-ward, the other three, acute angles, apex outward. 

 Of the three portions into which the muscular tube is divided, one-third is 

 always dorsal and the remaining two-thirds are subventral, so that one of 

 the obtuse angles mentioned is always directed ventrally. The few ex- 

 ceptions existing to this type of esophagus have been placed in the group 

 Trichosyringata Ward, a group characterized by the possession of a 

 capillary esophagus. The morphology of such an esophagus has not been 

 carefully worked out so that as yet statements regarding its symmetry 

 and structure are not on a substantial basis. It may be that some of the 



