III.] THE EARTHWORM. 261 



3. Still working from the side, carefully cut (with scissors) 

 into any three or four somitic constrictions of the 

 intestinal region, and without dissecting further ma- 

 nipulate the parts with forceps under water. Note 

 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 ne- 

 phridium 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 seg- 

 ment. 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 



