INTEtSTlNAI. liESPJKATlON IN ANNELIDS. 767 



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

 contractile, as, for example, it is seen to be in yEoIosoma, 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. 



71ie 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 Enchytraiidae there is, in the greater part of the extent of the body, 

 no differentiated dorsal vessel at all. In j-Eolosoma liemiyrichi a small mid-dorsal 

 channel is differentiated 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 Naididae the dorsal vessel is distinct throughout the body, incorporated in the 

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

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

 in Branchiura soiverhyi, 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 Tubificida; are repeated in the earthworms {Luinbncus 

 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 difierent 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 yEulosoma hempnchi; 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 ])art of the body than in the posterior. The 



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



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



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 cadom 



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



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



the intestine. 



TRANS. ROY. SOU. EDIN., VOL. XLIX. PART 111. (NO. 14). 105 



