44 THE LEECH 



C. The Excretory System, 



The excretory organs of the leech consist of a series of 

 seventeen pairs of nephridia or segmental organs, placed in 

 the somites from the second to the eighteenth inclusive. They 

 lie 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. 14 and 15). 



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 lie 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. 



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 pigmented 

 connective-tissue capsule, and receives a very abun- 

 dant supply of blood from branches of the lateral 

 vessels, 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 water : examine it with loio and high powers of 

 the microscope. 



The entire nephridium consists of the following 

 parts. 



