TWO AQUATIC OLIGOCHiETES. 



13 



The middle layer is that of the mnscle fibres, whose arrange- 

 ments are shown diagrammatically in fig. 4. They branch and 

 reticnlate in various ways, and terminate in the circular muscle 

 layer of the body wall, after separating into a number of fibril- 

 bundles at the two ends. The peritoneal cells that cover up the 

 muscle layer on either side are exceedingly flattened, but are 

 otherwise similar in appearance to those of the body wall. 



In the posterior septa of the body there occur, here and 

 there, cells with compact cytoplasm, less flattened and rather 

 spindle-shaped, with a thin cell wall and containing each a large 

 nucleolated nucleus. These cells are different from the amœbo- 

 cytes and the peritoneal cells, and are probably neoblasts, which 

 take an active part in regeneration. 



Near the posterior end of the body, the septa become more 

 or less incomplete and still more posteriorly they are represented 

 by undifferentiated cell- strands. 



ip— 



-ip 



Fig. s. 



L. gotoi. Cross section through segment VI to show the position of the septal sacs, x 150. f) 

 chl — chloragogues, dv — dorsal vessel, ip — lateral line, nc — ventral nerve cord, siv — supraintestin- 

 al vessel, ss— septal sac. 



