552 FRANK E. BEDDARD. 



we are dealing here with a perihsemal coelomic space not found 

 in connection with supporting mesenteries of the blood-vessels; 

 further forward in the segment the two lateral membranes 

 come to be attached to the intersegmental septum where this 

 forms the floor of the perihsemal chamber. The supra-oeso- 

 phageal vessel is for the most part not enveloped in a peri- 

 haemal space, but it is connected in parts by the membranes 

 with the walls of the oesophagus ; these gradually shift their 

 areas of attachment to the oesophagus until they are nearly- 

 continuous with the two membranes which enclose the sub- 

 oesophageal vessels. 



The formation of perihsemal spaces is an interesting, but 

 not a perfectly new fact in the anatomy of Oligochseta. 



I believe that I was the first to call attention to the fact 

 that, iu an earthworm (Deinodrilus) belonging to quite a 

 different family, the dorsal vessel is for the greater part of its 

 course enclosed in a '^ pericardium.^^ An almost identical 

 structure has been figured by Professor Spencer in Mega- 

 scolides australis. It is noteworthy that in both these 

 cases the " pericardium " contains abundant perivisceral cor- 

 puscles which stain deeply, and appear to be merely the 

 younger forms among the corpuscles of the perivisceral fluid. 

 I suggested the possibility of these corpuscles being principally 

 formed in that space. 



Similar corpuscles occur in the space which surrounds the 

 supra-oesophageal vessel in Hyperiodrilus and Helio- 

 drilus, and I have found them in Libyodrilus. I did not, 

 however, observe any proliferation of these cells from the lining 

 membrane of the perihsemal space in Libyodrilus; there 

 were simply masses of these cells (fig. 17, corp.) lying here and 

 there within the space. 



Fig. 21 represents the course of the sub-oesophageal vessels 

 in Segments 9 — 11. 



They generally lie at some distance from the oesophageal 

 wall, and are connected at frequent intervals by branches with 

 the peri- oesophageal plexus. Towards the anterior part of 

 Segment 9 the two trunks are connected by a branch which 



