III.] THE EARTHWORM. 259 



These structures are sometimes absent, and 

 they are not confined exclusively to the nephridial 

 plexus. 



F. The circulatory system. 



Examination of this system is materially facilitated by 

 killing the animal as directed at the outset, and allowing it 

 to remain undisturbed for six or eight hours, in order that 

 the muscular walls of the larger vessels, no less than those 

 of the body itself, may be rigid and well-set prior to dis- 

 section. 



1. Pin down, at opposite ends, the body of a worm 

 treated as above, and open it up along the dorsal 

 middle line. Reflect the two flaps of the body wall, 

 and note — 



a. The supra-intestinal vessel ; a large median dorsal 

 vessel, running along the whole length of the roof 

 of the alimentary canal, to which it distributes 

 branches. 



Large circumoesophageal vessels will be seen aris- 

 ing from this in segments 6—12. Their detailed 

 connections are described at § 2 ^. 



b. Dissect away some few of the mesenteric septa of 

 one side, and displace a portion of the alimentary 

 tube thus liberated. Note, overlying the nervous 

 axis, the median longitudinal supra-neural vessel. 



The smaller lateral-neural vessels may sometimes 

 be observed at this stage. (Cf. Sect. G. e. y.) 



2. Take a second worm and pin it down on its side, 

 being careful not to injure the anterior twenty seg- 

 ments. Make an incision with the small scissors 

 along its dorsal middle line, raising the body wall 



17 — 2 



