422 THE EVOLUTION OF MAN. 



a fine vascular network around the blind-sac of the lirei 

 and then passes, as a liver vein, into a vessel, directed 

 toward the front, which we may call the hollow vein 

 (Fig. 151, n). This last passes again directly to the ventral 

 side of the gill-body, and here directly re-enters the gill- 

 artery, which we took as a starting-point. Like a circular 

 closed aqueduct, this single main vascular tube passes along 

 the intestinal tube through the whole body of the Amphi- 

 oxus, pulsating throughout its entire length both above and 

 below. Within about a minute the colourless blood is thus 

 driven through the whole body of the little creature. When, 

 in pulsating, the upper tube contracts, the lower fills with 

 blood, and vice versa. Above, the current of blood is from 

 front to rear ; below, on the contrary, it is from the rear to 

 the front. The entire long vascular tube, which runs below 

 along the ventral side of the intestinal tube, and which 

 contains venous blood, probably represents the so-called 

 ventral blood-vessel of Worms (Plate IV. Fig. 7, v). On the 

 other hand, the long straight vascular tube, which runs 

 above along the dorsal line of the intestinal tube, between 

 it and the notochord, and which contains arterial blood, is, 

 on the one hand, evidently homologous with the aorta of 

 other Vertebrates, and, on the other hand, with the so-called 

 dorsal blood-vessel of Worms (Plate IV. Fig. 7, t). 



Johannes Muller recognized this important similarity in 

 the formation of the system of blood-vessels of the Lancelet 

 and of Worms. He directed special attention to the analo- 

 gies of the two, and their physiological resemblance, the 

 blood in both being driven by the pulsating contractions of 

 the great vascular tubes throughout their entire length, and 

 not by a centralized heart, as in all other Vertebrates. But 



