TUBIFEX. 361 



cilia are not all equally active at the same time, but that 

 some cease entirely to vibrate for a time, and then resume 

 their action again. Outside the ciliated epithelium is a 

 single layer of very much flattened peritoneal cells, which 

 is continuous with the peritoneum, completely investing 

 the rest of the vas deferens. This layer is extremely 

 delicate, but is rendered more conspicuous by the 

 somewhat swollen, oval nuclei which the cells contain 

 (PI. IV, fig. 17, p.). 



The ciliated funnel of a mature worm has a very 

 characteristic appearance. It is no longer cup-shaped, 

 but the edge is so sharply recurved towards the vas 

 deferens that the funnel becomes much more shallow and 

 is almost turned inside out (PI. IV, fig. 17). The surface 

 of the funnel which bears the cilia is thus exposed as fully 

 as possible. There is not the least doubt that this 

 interesting change in the shape of the funnel is intimately 

 connected with the transference of mature spermatozoa 

 from the sperm sac to the vas deferens. But it is difficult 

 to say whether this change in shape is completed before 

 the spermatozoa actually reach the funnel, in order to 

 expose as much surface as possible for their reception, or 

 whether it is due entirely to the pressure exerted by the 

 enormous number of spermatozoa which congregate upon 

 it. In a mature worm the whole of the ciliated surface 

 of the funnel is covered with a dense mass of sperma- 

 tozoa, not arranged irregularly, but with their heads 

 entangled amongst the cilia of the funnel, and their tails 

 lying parallel to the cilia (PI. IV, fig. 17, sm. t.). At 

 first sight, the funnel appears to be provided with 

 enormously long cilia, but a closer examination of the 

 stained preparations reveals the true state of things. 

 Just outside the boundary of the cells composing the 

 funnel is a narrow zone which is clearly differentiated 



