VASCULAR SYSTEM OF THE EARTHWORM 145 



Vascular Circulation- -The circulation of the blood is brought 

 about by peristalsis or the consecutive contraction of the circular 

 muscles in the walls of the blood vessels. This wave of con- 

 traction proceeds in the dorsal vessel from the posterior end 

 toward the anterior, the blood being forced ahead of the wave 

 of contraction, as one might force water from a rubber tube. 

 The details of the path followed in the circulation of the blood 

 are not fully known, but there is abundant evidence that the 

 essential features in the following account are correct (cf. 

 Figs. 55 and 57). 



The blood passes into the pharyngeal vessels anterior to the 

 aortic loops; also through the aortic loops into the ventral 

 vessel, where the flow is from the anterior toward the posterior 

 end. From the ventral vessel branches are given off to the 

 digestive tract, the nephridia, and the body wall. In the di- 

 gestive tract these vessels branch repeatedly until they result in 

 fine capillaries running throughout the vascular area of the di- 

 gestive system. Similar capillaries connect with these and 

 conduct the blood, -now loaded with products of digestion, into 

 larger vessels which, in turn, open into the dorsal vessel. It is 

 possible that the waste matters (urea), contained in the blood 

 thus directed into the digestive system, are disposed of through 

 the agency of the chlorogogue cells. The blood vessels which 

 enter the nephridia likewise break up into capillaries where the 

 blood probably gives up its urea. The purified blood then passes 

 into the parietal vessels, or into the body wall, and is ultimately 

 conducted back to the dorsal vessel. In the same way, the 

 vessels which enter the body wall ultimately end in capillaries 

 distributed throughout the general surface of the body. Here 

 the blood loses its C02, and takes in oxygen, and is then carried 

 through progressively larger vessels back to the lateral-neural 

 vessels, from which it passes directly to the sub-neural vessel, 

 and then, by way of the parietals, back to the dorsal vessel. 

 Within each of the parietal vessels, near its point of union with 

 the dorsal vessel, is a conspicuous valve which may be seen in 

 the living worm. This valve is so placed that blood can flow 

 from the parietal into the dorsal vessel after a peristaltic wave 



