428 Proceedings of the Royal Physical Society. 



from Megascolex, the peculiar disposition of the dorsal vessel 

 was almost exactly repeated ; in five of the anterior segments 

 of the body the dorsal vessel is separated into two distinct 

 tubes, which lie widely apart in the cavity of the segments 

 which they occupy, but reunite before perforating the 

 mesenteries ; the transverse trunks or " hearts," as they have 

 been termed, which unite the dorsal with the ventral blood 

 vessel, arise in every case from the unpaired portion of the 

 dorsal vessel, close to the posterior mesentery of their seg- 

 ment; after the tenth segment the dorsal vessel becomes 

 single. The accompanying figure will illustrate this point 

 (PL XXII., Fig. 1). In both these instances cited it is only the 

 most anterior section of the dorsal vessel which has retained 

 its original paired condition. This, however, is not always 

 the case; there are other earthworms which retain much 

 more completely than either of these two, the original paired 

 dorsal vessel. The collection of earthworms from New 

 Zealand already referred to prove to belong to three distinct 

 species, probably referable to Perrier's genus Acanthodrilus ; ^ 

 in two of these the dorsal vessel is formed of two separate ticbes, 

 which run close together from end to end of the body, only 

 uniting upon the pharynx. 



There is some difference in detail: in one species the 

 structure of the dorsal vessel in the anterior region of the 

 body is precisely similar to that which has been already 

 described in Megascolex ; the two vessels unite at the point 

 where they traverse the mesenteries, but in the interior of 

 each segment become very widely separated. Contrary to 

 what is found in Megascolex, the dorsal vessel has the same 

 structure throughout its whole extent; in each segment of 

 the body the two dorsal vessels are completely separate, but 

 unite at their passage through the mesenteries ; the vessels 

 being naturally injected with their own blood, it was easy 

 to ascertain that this was really the case, which was also 

 proved by transverse sections; it is rather remarkable to 

 find that in another species closely allied to the foregoing, 

 only differing in very trivial characters, but still distinct, the 

 dorsal vessel does not exhibit the same embryonic character. 



^ Archives d. Museum, t. vii. 



I 



