TTTBIFEX. 357 



segment 9 at least, and occasionally segment 8, also 

 enclose a portion of the sac. The development of this sac 

 both anteriorly and posteriorly to the segment in which it 

 first appears may be due to simple pressure exerted by the 

 developing spermatozoa. 



It has already been said that the sac arises as a simple 

 outgrowth of the septum between segments 10 and 11. As 

 it increases in size, however, its structure becomes more 

 complicated. The cavity becomes divided up into a series 

 of much smaller spaces by the growth inwards of its walls. 

 These small spaces remain in communication with one 

 another, and are filled with spermatozoa in all stages of 

 development. It is difficult to believe that the tremendous 

 number of sperm morulae present in the sac have all been 

 derived from the small and inconspicuous testes. Some 

 writers have suggested that the peritoneal epithelium 

 lining these coelomic spaces of the sperm-sac is capable 

 of forming germinal tissue. If all the spermatozoa were 

 derived from the spermatogonia of the testes which are 

 transferred, as they are formed, to the sperm-sac, one 

 would expect to find the youngest sperm morulae nearest 

 the segment in which the testes lie. But this is not the 

 case, for it is common to find spermatogonia and fully 

 developed spermatozoa lying side by side in any part of 

 the sperm-sac, and there is no suggestion of regularity in 

 their arrangement. This irregular arrangement is what 

 one would expect if the epithelium of any part of the 

 sperm-sac were capable of forming germinal tissue. 



Throughout the life of the worm the ciliated funnel 

 continues to hang freely into the cavity of segment 10, 

 and never becomes enclosed in the sperm-sac as is the case 

 in Lumbricus. It docs not seem quite clear, perhaps, at 

 first sight, how the mature spermatozoa from the sperm- 

 sac reach the funnel. If, however, we bear in mind the 



