234 PROCEEDINGS OF THE AMERICAN ACADEMY. 



teristic cell-structure, which serves to distinguish them from the other 

 cells of the nervous system. This structure will be described later, after 

 a description and comparison of the courses of the fibres in the two 

 species has been given. 



The processes of the different giant cells, after a more or less irregu- 

 lar course, enter the Leydig's fibre, and immediately upon their union 

 with it the individual processes lose their respective sheaths. The con- 

 tents of the process, fusing with the contents of the Leydig's fibre, lose 

 their identity. Such a connection of fibre and cells has been demon- 

 strated in dozens of my preparations. Several times a single fortunate 

 section has shown the connection of cell and fibre, but far more often 

 the connection was established only by following through several succes- 

 sive sections. A large number of absolute demonstrations in various parts 

 of the worm leave no doubt that the Leydig's fibres are continually receiving 

 the processes of certain large and peculiar cells, occurring in very restricted 

 number in the midst of the smaller cells of the nervous system (Plate 4, 

 Figs. 22, 23). 



The Leydig's fibre is single and median in position through the whole 

 thoracic region of both worms ; but in the sub-cesophageal ganglion it 

 •divides into two symmetrical arms, which diverge and apparently end in 

 a pair of symmetrically placed giant cells. I have not been able to 

 determine with absolute certainty the nature of the anterior termination 

 ■of these arms, for I have not ascertained whether the anterior end of 

 Leydig's fibre always terminates in giant cells, or whether it may extend 

 forward of the most anterior cell-processes with which I find it connected. 

 My reason for believing that it terminates in a pair of giant cells is, that 

 in the same section in which the most anterior part of the paired portion 

 of the fibre is met with, or in sections very near to it, there is also found 

 the anterior jJtur of giant cells. Sometimes these are clearly connected 

 with the corresponding branches of the fibre, but the evidence on this 

 point is not always satisfactory, and in some cases it seems possible that 

 the branches of the fibre may end independently of the first giant cells. 



In one important respect the results here presented regarding these 

 cell-processes and Leydig's fibre differ from those of Friedlaender, viz. 

 that no branching or anastomosing of the cell processes or of Leydig's 

 fibre has been observed, and I believe that no such branching occurs. 



e. Comparison of the Course of Leydig's Fibres in the two Worms. 



In Clymene producta the conditions in the first twenty segments have 

 been studied. Although the worm probably consists of more than 



