THE EARTHWORM 



159 



These muscles produce waves of contraction, which, arising at 

 the posterior end, force the blood forward. In the posterior 

 half of the body small branches pass from this tube into the 

 intestine, ei, supplying its walls, and the blood then enters 

 a rather large vessel in the typhlosole, from which it passes 

 back by short tubes, ai, into the dorsal vessel. The greater 

 part of the blood in the dorsal vessel flows forward to the seg- 

 ments 6-11, where five large circular vessels arise from it, 

 ht, which pass around the sides of the body to enter a sub- 

 intestinal vessel, w, also extending lengthwise and lying be- 



Uv 



FIG. 77. DIAGRAM SHOWING THE CHIEF BLOOD 

 VESSELS OF THE EARTHWORM 



an, the anterior end; 



po, the posterior end of the body; 



dv, dorsal vessels; 



cv, circular vessels; 



ei, efferent intestinal; 



(Bourne and Benham.) 



ai, afferent intestinal; 

 ht, hearts; 



snv, subneural vessel; 

 vv, ventral vessel. 



neath the intestine. These circular vessels are called hearts, 

 since they contract, and force the blood downward into the 

 ventral vessel. When reaching the ventral vessel, part of the 

 blood flows forward, in front of the hearts, and part of it back- 

 ward. From this ventral vessel branches arise which pass out 

 into the body wall and into other organs supplying the body 

 generally with blood. After passing through the organs of 

 the body wall, etc., the blood is collected into another set of 

 vessels which pass into a third longitudinal vessel lying under 

 the nerve chord, the subneural (Gr. neuron = nerve), snv 

 Through this it flows toward the posterior end. In the intestinal 

 region there arises from the subneural vessel, in each segment, 

 a circular vessel, cv, which passes up around the body to empty 



