CIRCULATOEY SYSTEM 43 



The arrangement and relations of these vessels and sinuses 

 are best determined by the microscopical examination of 

 sections, and will be described later on : the broad features, 

 most of which can be made out without difficulty by dissec- 

 tion, are as follows. 



1. The Sinuses, or parts of the coelom. 



a. The dorsal sinus is a longitudinal tube running 



along the dorsal surface of the whole length of 

 the alimentary canal. It communicates posteriorly 

 with the ventral sinus by fine vessels which pass 

 between the intestine and the terminal diverticula 

 of the crop. It has already been seen. 



Dissect away the alimentary tract completely, from the 

 (esophagus backivards, 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 communi- 

 cates 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 Blood-vessels. 



a. The lateral blood-vessels are a pair of longitudinal 



vessels running in a sinuous manner 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 networks form very abundant 



plexuses, which connect the sinuses and blood- 

 vessels in various places, putting them in free 

 communication with one another. Their micro- 

 scopical characters will be described later on. 



