96 BIOLOGICAL LECTURES. 



taken up by the ciliary apparatus B, and undergoes destructive 

 metabolism with the newly formed ciliary substance C; IV" = 

 waste products, which are carried into the receptaculum, and 

 lV=free excretory granules, which undergo the same fate. 

 Thus the circle is closed. We find now all the excretory sub- 

 stances assembled in the receptaculum, and it is of interest to 

 know how they get out of this vesicle and into the nephridium. 

 We know that the receptaculum is surrounded by a wall of 

 connective tissue, which is only open at the point of insertion 

 of the crown cells, and there is no perceivable connection with 

 the nephridial cell in the form of a canal or even a break in the 

 wall. It was very difficult for me to imagine a reason for this 

 fact, until lately a very simple explanation occurred to me, 

 which seems quite satisfactory. 



We know that the funnel projects with its crown into the 

 coelomic spaces. In the leeches the body cavity is filled with 

 blood, as the coelomic and the vascular system are in open 

 communication with each other. We easily see that, as a 

 stream of blood is carried into the receptaculum by the ciliary 

 motion, if there was an open communication with the ne- 

 phridial cells it would also be drawn into these and carried to the 

 exterior. This would imply a continuous hemorrhage at every 

 nephridiopore of the animal. This hemorrhage would cer- 

 tainly be highly disadvantageous to the animal, and in order to 

 prevent it the receptaculum is closed. One might oppose to 

 this theory the fact that in numerous other groups we find 

 funnels which are in open communication both with the coelom 

 and the exterior, but in all these cases the body cavity is 

 entirely separated from the vascular system. Thus in the 

 Oligochaeta, Polyckaeta, and Vertebrata, no blood can enter the 

 nephridium or the pro- or mesonephros respectively. The recep- 

 taculum in the leeches acts as a reservoir, or as a sorting 

 mechanism, into which ever fresh quantities of waste products are 

 brought. The solid particles are unable to get out of the recep- 

 taculum, because the cilia of the crown cells form a regular hedge 

 around the only opening of the vesicle. The liquid blood simply 

 overflows and the granules stay within. 



The process is to be compared with the throwing of small 



