INTESTINAL RESPIRATION IN ANNELIDS. 775 



is no longer in direct association with the intestine it still retains, in the direction of 

 its contractions, a mark of its origin. 



The course of the blood between intestinal network and dorsal vessel has been 

 variously stated. Thus Beddard (3, p. 74) states that in the Naidomorpha and Tubi- 

 ficidse " the (intestinal) network is fed from the dorsal vessel, and the blood returns 

 into the ventral vessel " ; adding, " the network at any rate has connections with both 

 vessels" (this is denied, however, on p. 240 as regards the connection of the network 

 with the ventral vessel in the T'ubificidse, except Ilyodrilus). Vejdovsky, on the other 

 hand (54, p. 115), speaking of the Oligochseta in general, says : " There are usually one 

 or two pairs of such thicker anastomoses (between intestinal network and dorsal vessel) 

 in each segment, which conduct the blood from the network to the dorsal vessel " ; and 

 (p. 113) the dorsal vessel "is to be regarded as the organ for the collection and pro- 

 pulsion of the blood which enters it from the intestinal network of vessels." Bouene 

 also thought that the blood went from the intestine to the dorsal vessel and not vice 

 versa [cf. Beddard, Ioc. cit, p. 76). 



In the absence of floating corpuscles in the blood, actual observation of the direction 

 of the flow is difficult. Vejdovsky's view is however the one which harmonises better 

 with the considerations here put forward. The fluid in the network is moved forwards 

 by the antiperistaltic contractions of the intestinal walls ; and since the specialised 

 median dorsal channel is first developed as a forward continuation of the path taken by 

 the fluid, which travels in the same direction in both, the fluid must enter the dorsal 

 vessel from the network, and not vice versa ; the ventral vessel, having been specialised 

 and early separated from the intestine for the purpose of providing a return channel, 

 must deliver the blood again to the network. 



The Origin of the Vascular System. 



In the present section I have attempted to sketch the evolution of the various com- 

 ponents of the vascular system of the Oligochaeta, the idea on which the sketch is based 

 being that that evolution has proceeded by progressive diff"erentiation of the intestinal 

 plexus, with the formation and separation from the intestinal wall of distinct blood- 

 vessels. Such an evolution has been clearly traced for the dorsal, supraintestinal, and 

 subintestinal vessels ; and there are at least indications of a similar origin for the 

 ventral vessel and the lateral commissures. We come then to the conception of a 

 simple series of intercommunicating vacuole-like spaces in the wall of the alimentary 

 canal throughout its extent, as the first representative of the circulatory system. The 

 final question is. How did these fluid-containing spaces originate ? 



The suggestion that most readily presents itself is that the spaces originated by 

 transudation of liquids from the lumen of the gut ; in using the word " transudation," 

 however, it is not intended to exclude an active participation of the intestinal epithelium 

 in the process. The alimentary matters introduced by the mouth, it is to be supposed, 



TRANS. EOY. SOC. EDIN., VOL. XLIX. PART III. (NO. 14). 106 



