231 



musculature (fig. 3 & 4), it is very difficult to tracé this cellular layer. 

 lu fig. 4 a tliia layer with a few very faint nuclei is visible to 

 the left of the lumeu of the duet Nd , but to the right hardly 

 any tracé of the coating can be seeu , as is also the case in fig. 3. 

 Whether this must be ascribed to distension in this particular 

 case or to a very considerable flatteniug of the cellular coating 

 in general, cannot be decided with certainty. It will appear all 

 the more plausible that the absence of any distinct epithelial 

 coating to these apparent solutions of continuity between circu- 

 lar and longitudinal muscular layer must have contributed to let 

 these ventro-dorsal nephridial ducts pass uuobserved up to now. 

 They may have been frequently seen in sections, but have no 

 doubt often been mistaken for defects in the sections. 



Fig. 4 will show this at a glance. And ouly by the preser- 

 vatiou and comparison of the unbroken series of sections can 

 their exact relatiou both to the external pore and to the in- 

 ternal reservoir be definitely made out. 



If the epithelial liuing of the part of the nephridial duet be- 

 tween the nephridiopore and the wide muscular duet is thus 

 rather difficult to make out , still I think there can be no doubt, 

 as for as I can gather from my preparations , that this portion 

 of the nephridial system is uot intracellular as are the coiled 

 tubes that He in the coelom, but intercellular. 



It should be noticed that the nephridial ducts Nd, both the 

 shorter vertro-lateral (fig. 5) and the longer dorso-lateral ones, are 

 situated about in the same vertical plane in which an important 

 metameric nervestem emerging from the veutral nervecord enters 

 the longitudinal muscular layer at some distance mediad of the 

 first seta and subdivides before having penetrated the whole thick- 

 ness of the longitudinal muscles into a smaller stem (n ', fig. 6) 

 which turns ventralwards , and a more considerable one (n , fig. 6) 

 which takes its course dorsalwards between the longitudinal and 

 circular muscular layers. 



This nervestem in more than one section is in the closest con- 

 tiguity with the nephridial duet. 



