EXCRETORY SYSTEM. 57 



C. The Excretory System. 



The excretory organs of the leech consist of a series of 

 seventeen pairs of nephridia or segmental organs, lying in 

 the somites from the second to the eighteenth inclusive. They 

 he at the sides of and below the alimentary canal, and open 

 to the exterior by paired apertures on the ventral surface. 



1. Structure of a nephridium (figs. 23 and 24). 



Each nephridium is a cellular rod, thicker in the 

 middle than at the ends, and bent on itself so as 

 to form a loop, the two limbs of which are anterior 

 and posterior respectively. The looped portion lies 

 at the side of the alimentary canal, extending up 

 into the dorsal part of the body ; the ends he in the 

 ventral region, that of the anterior limb being in 

 most cases prolonged inwards almost to the median 

 plane, and that of the posterior limb being rolled up 

 on itself. From the anterior limb a duct runs back 

 to a vesicle with muscular walls, which opens to the 

 exterior on the ventral surface of the body. 



The cellular rod is traversed by a complicated 

 system of ducts, which perforate the individual cells, 

 and receive minute ductules ramifying in the sub- 

 stance of the cells. 



The whole nephridium is invested by a pig- 

 mented connective-tissue capsule, and receives a 

 very abundant supply of blood from branches of the 

 lateral tubes, the efferent stream passing inwards 

 through the peri-nephrostomial sinus to the ventral 

 sinus. 

 Dissect carefully one of the nephridia about the middle of 



the body, noting its several parts, and their relations to one 



another and to other organs. 



Remove another nephridium from the body, and dissect it 



on a slide in a drop of water ; examine it with low and high 



powers of the microscope. 



Snip off with fine scissors the dorsal half of one of the 



testes, with the testis-lobe of the nephridium ; mount it on a 



