THE ANATOMY. VASCULAR SYSTEM 69 



This blood-vessel resembles the dorsal vessel in being also directly connected with 

 the ventral vessel; among the Tubificidae such connexions occur, and in Branchiura 

 there is a pair of hearts of this description coexisting in the same segment with a pair 

 of hearts ai'ising from the dorsal vessel ; in the earthworms it is frequently the case, 

 as I mention more in detail further on, that the same pair of hearts arise both from 

 the supra-intestinal and the dorsal vessel. Another point in which this vessel resembles 

 the dorsal is that it is occasionally double ; in Megascolex coeruleus this is the case, 

 as has been described by both Bourne and myself. The supra-intestinal vessel is by 

 no means present in all of the higher Oligochaeta ; it is present in most of the genera 

 belonging to the two families Megascolicidae and Eudrilidae. It may perhaps be 

 a question to which of the two dorsally-placed trunks the single vessel of, for example, 

 the Naidomorpha corresponds. I have suggested that the intimate relations of the 

 supra-intestinal to the alimentary canal indicate that the posterior part of the dorsal 

 vessel is its homologue and that the anterior part of the dorsal vessel is a new structure 

 in the higher Oligochaeta — the relations between the two being somewhat analogous 

 to those which subsist between the vena cava posterior and the posterior cardinals in 

 the Vertebrata. This view seems to be supported by the relations obtaining in the genus 

 Phreodrilus. In that worm there are in the anterior region of the body two trunks 

 upon the dorsal side of the alimentary tract which I homologize respectively with the 

 dorsal and supra-intestinal of other Oligochaeta. These two vessels can be distinguished, 

 not only by their position but also by their minute structure ; the vessel which is closest 

 to the wall of the gut, lying in fact upon it, has thin walls and is full of blood after 

 death ; the other has thicker walls and is not so full of blood after death ; moreover 

 the latter has not the coating of chloragogen cells which occur in the former ; it is 

 coated with flattened cells without pigment ; this vessel, which I believe is the dorsal 

 vessel of the higher Oligochaeta, terminates at about the fifteenth segment ; behind 

 this point there is only the supra-intestinal. The question, therefore, arises whether 

 the vessel, which I have termed dorsal, is really the equivalent of the similarly 

 named vessel in the Naids, etc. 



The Ventral vessel. — This vessel is present along the whole body in all Oligochaeta. 

 It is invariably a single tube never showing any signs of duplication as does the 

 dorsal vessel. The ventral vessel also differs from the dorsal vessel in that it is never 

 contractile^. Another fact of importance is that it is the first blood-vessel to be 

 developed ; this is at any rate the case with Rhynchelmis and Lwmhncus (several 

 species) in which alone the origin of the vessel has been carefully studied. The ventral 



1 Phreoryctes is said to be an exception to the statement. 



