INTESTINAL RESPIRATION IN ANNELIDS. 7^7 



pp. 210-218), and to Vejdovsky (55). The network will therefore originally be 

 contractile, as, for example, it is seen to be in ^olosoma, since the muscular coat 

 within which it lies is continually engaged in antiperistaltic action. 



The further history of the intestinal network is the history of the differentiation 

 from it of numerous vessels, which either separate completely or remain as specialised 

 channels in the intestinal wall, and of the concomitant reduction in size and 

 physiological importance of the network itself. 



The Dorsal Vessel. 



Since the antiperistaltic movements of the whole gut- wall necessitate a forward 

 movement of the fluid contained in the irregular system of intercommunicating spaces 

 in the wall, the specialisation of definite channels may be expected to follow. The 

 dorsal vessel is one of such ; and it is the one in which contractility, originally the 

 contractility of the gut-wall, becomes specially developed. 



In the Bnchytrseidse there is, in the greater part of the extent of the body, 

 no diff"erentiated dorsal vessel at all. In ^olosoma hemprichi a small mid-dorsal 

 channel is diff"erentiated in the plexus on the stomach, but not on the intestine ; in 

 A. viride this channel may extend back on to the anterior part of the intestine. In 

 the Naididse the dorsal vessel is distinct throughout the body, incorporated in the 

 alimentary wall along the whole of the intestine. In the Tubificidse it is in general 

 in close contact with the intestinal wall, and is surrounded by chloragogen cells ; 

 in Branchiura sowerbyi, however, it does not lie immediately on the intestine, but has, 

 nevertheless, a closer relation to the gut and chloragogen cells than has the ventral 

 vessel. The relations in the Tubificidse are repeated in the earthworms [Lumbricus 

 and Pheretima). 



But if the intestinal part of the dorsal vessel is thus a specialisation of the alimentary 

 network, its remaining portion will hardly own a different origin. An indication that 

 the oesophageal portion of the vessel arose in a similar way is to be seen in the cellular 

 strands which pass across the lumen of the "heart" of j^olosoma hemprichi; these 

 may represent, as has already been suggested, vestiges of the partitions of an original 

 lacunar system, which at an earlier stage invested the anterior as well as the posterior 

 portion of the alimentary tract. 



The differentiation of the circulatory system, as of the alimentary tube itself, has 

 thus proceeded further in the anterior part of the body than in the posterior. The 

 principle holds throughout ; it is seen in the ventral vessel of JEolosoma as well as in 

 the dorsal ; it is seen in the relations of gut-plexus and dorsal vessel in the Enchytrseidse ; 

 it is seen also in the fact that in most aquatic Oligochseta the dorsal vessel, while 

 incorporated in the alimentary wall in the intestinal region, runs free in the coelom 

 anteriorly ; while in the earthworms also the anatomical separation of the dorsal vessel 

 from the alimentary canal is much greater in the oesophageal region than over 

 the intestine. 



TRANS. ROY. SOC. EDIN., VOL. XLIX. PART III. (NO. 14). 105 



