56 THE LEECH. 



which can be made out without difficulty by dissection, are 

 as follows : 



1. The Sinuses, or non-contractile parts of the ccelom. 



a. The dorsal sinus is a longitudinal tube running 



along the dorsal surface of the whole length of 

 the alimentary canal. It communicates pos- 

 teriorly with the ventral sinus by fine vessels 

 which pass between the intestine and the terminal 

 diverticula of the crop. It has already been seen 

 (p. 52). 



Dissect away the alimentary tract completely, from the 

 oesophagus backwards, taking great care not to damage the 

 underlying organs. The further dissection is greatly facili- 

 tated by putting the leech into strong spirit for ten minutes 

 or so. 



b. The ventral sinus is a median tube running along 



the body beneath the alimentary canal. Within 

 it the ventral nerve-chain lies. It communicates 

 with a series of peri-nephrostomial sinuses, 

 lying on the dorsal surfaces of the testes ; and 

 also with the capillary system on the walls of 

 the crop. 



2. The contractile parts of the ccelom. 



a. The lateral * vessels ' are a pair of longitudinal 



tubes running in a sinuous course along the 

 sides of the body. They are connected together 

 by transverse vessels which pass across beneath 

 the ventral sinus. They send branches to the 

 intestine, the nephridia, and the reproductive 

 organs. 



b. The cutaneous plexuses are very abundant, and 



connect the sinuses and lateral tubes in various 

 places, putting them in free communication with 

 one another. The microscopical characters will 

 be described later on. 



