CIRCULATION 



205 



thick that the deeper lying cells would be starved by such a 

 method of food distribution. Moreover, the wall of the digestive 

 tract is so thick that it would greatly impede the transfer of 

 absorbed food to the body fluid. There is, therefore, necessary 

 a system of channels by which the food may more readily be 

 transferred from the seat of digestion to the place of assimila- 



A 



B 



FIG. 113. The circulatory system of annelids. A, A longitudinal section of 

 a blood-vessel of a small fresh-water annelid (Chaetogaster) showing extremely 

 thin walls. B, Cross section diagram of nereis to show the arrangement of the 

 vessels; D.V., dorsal vessel; Int, intestine; N. nephridium; P.V., parapodial ves- 

 sels; V.I., intestinal vessels and capillaries; V.V., ventral vessel. All vessels 

 black. 



tion. These channels consist of a network of tubes of extremely 

 small calibre, which penetrate to every part of the intestinal 

 wall, immediately outside the intestinal epithelium. Larger 

 vessels lead from this network of capillaries to a much larger 

 vessel which runs longitudinally along the mid-dorsal line of 

 the body. In each segment branches of the dorsal vessel lead 

 out laterally to the muscles, epidermis and all other organs of 



