50 ILLINOIS BIOLOGICAL MONOGRAPHS [170 



At that time the main group lies in an indefinite mass over the esophagus, 

 just anterior to the end of the nerve cord, and is connected with the cord 

 by two large commissures passing around the esophagus. Under the eso- 

 phagus, at the very end of the cord, a smaller group of ganglion cells is 

 more easily distinguishable. 



In further development the dorsal group becomes completely isolated 

 from other tissues, remaining connected only with the nerve cord by the 

 commissures and in some cases also with the anterior hypoderm by scatter- 

 ed, half disintegrated fibres (Fig. 153). No marked change occurs in the 

 ventral group. At the sides of this group fibres pass anteriad to the anterior 

 hypoderm cells. The fibres usually become more or less definitely separated 

 into two ventral and two lateral tracts. 



The ventral cord arises and develops as described for Gordius rohustus 

 (Figs. 135, 146, 147, 158). The cellular elements do not all remain under 

 the fibre tracts, pushing up into them as they did in Gordius rohustus, but 

 grow up over the sides, partly enclosing the fibres and leaving them in 

 contact with the parenchyma over only about one third of the circum- 

 ference, in later stages even less (Fig. 141). On account of this over- 

 growth of the cells the cross fibres do not present a radiating appearanc^ 

 but cross each other at various angles within the cord. There are, how 

 ever, two longitudinal rows of heavier fibres originating from the two 

 primary cell rows and passing to the dorsal side of the cord, dividing the 

 longitudinal fibres into three main tracts (Fig. 159). 



The structure of the large cells in the two primary rows is more easily 

 demonstrated in this species than in the previous one. The cells are bipolar, 

 giving off one fibre to the dorsal wall of the cord and another to the longi- 

 tudinal fibre tract (Fig. 139). The body of the cell is rounded or flask- 

 shaped and the two fibres are given off at one side. Some smaller cells 

 were also found to be bipolar and of similar structure, but in case of the 

 majority of the smaller cells it is impossible to make out the exact structure. 



No cell bodies can be distinguished in the connection between the nerve 

 cord and hypoderm after the two have separated, but fibres can be traced 

 thru it from the cord to the hypoderm. At the point where the fibres from 

 the cord enter the hypoderm longitudinal fibres are frequently found, and 

 these fibres in some cases separate to form a longitudinal canal, the sub- 

 neural canal of European workers. 



In the male the nerve cord ends at the posterior end as it does in the 

 previous species. The cloacal ganglion consists of a sHght thickening of 

 the cord beginning somewhat anterior to the musculature of the sperm 

 ducts and extending back to the point of bifurcation of the cord (Fig. 164). 

 It is intimately connected with the musculature of the vasa deferentia. 



The cloacal ganglion in the female is similar to that found in Gordius 

 rohustus but presents some modifications on account of the posterior exten- 



