CIRCULATION IN THE ANNELIDA. 



253 



dominal vessel as performing also the same 

 function of carrying the blood forwards towards 

 the head, and the two lateral vessels as convey- 

 ing it backwards, thus completing the circuit. 

 This is illustrated by the diagram (Fig. 347) ; 



where a is the anterior and p the posterior 

 extremity of the animal, the dorsal vessel occu- 

 pying the middle straight line between the two 

 lateral vessels, and the direction of the stream 

 in each being indicated by the adjacent arrows. 

 The blood in the abdominal vessel following 

 the same course as that in the dorsal vessel, 

 the same diagram represents also these vessels 

 seen from below. Fig. 348 is a lower view 

 of the Erpobdella, showing the numerous rami- 

 fications of the abdominal vessel ; the lesser 

 branches encircling the nervous ganglia, and 

 accompanying the principal nervous filaments 

 which proceed from them ; while the lateral 



