III.] THE EARTHWORM. 26l 



the relations of the mesenteric septa and nephridia, 

 as seen in the undisturbed condition (cf. Sect. E.) 

 and next examine in order 



a. The circular commissural vessels ; one on either side 

 for each segment. They lie close under the body- 

 wall (to which they give branches) midway between 

 adjacent mesenteric septa, and connect the supra- 

 intestinal and sub-neural trunks. 



b. The excretory plexus. The posterior of the two 

 vessels from which this is derived springs from 

 the sub-neural trunk immediately adjacent to the 

 nephridium itself. The anterior one arises 

 from an adjacent circular commissural vessel, 

 and may perforate the mesenteric septum ventro- 

 laterally. 



c. Remove a portion of the body-wall of this region, 

 and gently scrape away as much as possible of the 

 yellow-brown tunic of the intestine. There will 

 thus be brought into view the lateral intestinal 

 vessels; two in number on either side in each 

 segment. They arise from the supra-intestinal 

 trunk and break up into an alimentary plexus 

 within the walls of the digestive canal. A series 

 of small vessels will be seen to pass up towards 

 these from the sub-intestinal trunk. 



The blood-vessels described above can all be made 

 out by careful dissection. No mention has been 

 made of the delicate lateral-neural commissures, or 

 the smaller branches of the majority of the great 



