EARTHWORM 89 



The Lateral Neural Vessels receive blood from the anterior end of 

 the body and distribute it to the nerve cord throughout its length. 



A pair of short longitudinal vessels, the extra-esophageal, 

 is present in the anterior end of the body, one on either side of the 

 esophagus. They arise by small branches in the pharyngeal region 

 and extend posteriorly through segment xii. The blood collected 

 anteriorly is carried back to segment xii, where connection is made 

 with segmental vessels leading to the dorsal vessel. 



(2) Segmental Vessels. In each segment of the body, pos- 

 terior to the aortic loops, small vessels connect with the larger 

 longitudinal vessels. These segmental branches carry blood to 

 and from the many tissues and organs of the body. The princi- 

 pal segmental vessels are as follows : 



(a) The Dorso-subneural vessels which, branching from the 

 subneural vessel, run dorsally along the septa and empty into 

 the dorsal vessel. Enroute they collect blood from the body wall 

 and the nephridia. 



(b) The Dorso-intestinal vessels which collect blood from the 

 tissues of the intestine and carry it to the dorsal vessel. 



(c) The Ventro-intestinal vessels which distribute to the tissues 

 of the intestine the blood received from the ventral vessel. 



(d) The Ventro-parietal vessels which distribute to the body wall, 

 nephridia, and reproductive organs the blood received from the 

 ventral vessel. 



Course of the Circulation in the Earthworm. The gen- 

 eral course of the blood flow through the body is as follows. Start- 

 ing at the posterior end of the worm and continuing anteriorly to 

 the aortic loops, blood is received into the dorsal vessel from the 

 various segmental vessels (dorso-subneural and dorso-intestinal). 

 The blood is forced anteriorly by rhythmic waves of contraction, 

 or peristalsis, in the muscular walls of the dorsal vessel, which 

 begin at the posterior end of the vessel and move anteriorly in a 

 regular manner. At the anterior end of the body a large propor- 

 tion of the blood flows into the aortic loops which contract syn- 

 chronously with the dorsal vessel and force the blood into the 

 ventral vessel. Valves are present in the aortic loops, which 

 prevent a back-flow into the dorsal vessel. The main flow in the 

 ventral vessel is posterior, although a small amount in the region 

 of the aortic loops is forced to the extreme anterior end of the 

 body. The ventral vessel is not contractile, so the flow of blood 



