372 THE EVOLUTION OF MAN. 



in the dorsal vessel moves from back to front, while in 

 Vertebrates, on the contrary, it flows in the opposite direc- 

 tion, from front to back. As the heart of the Ascidian 

 constantly alternates between these two opposite directions, 

 it exhibits permanently, to a certain extent, the phylogenetic 

 transition between the older direction of the dorsal blood- 

 current toward the front in Worms, and the newer direc- 

 tion of the same toward the rear in Vertebrates. 



Ah in the more recent Chorda Animals, which gave 

 rise to the Vertebrate tribe, the newer direction became 

 permanent, the two vessels which proceeded from the 

 two ends of the heart-pouch, acquired a constant signifi- 

 cance. The front section of the ventral vessel, since then, 

 has steadily conducted the blood from the heart, acting, 

 consequently, as an artery; the hinder section of the 

 ventral vessel, on the contrary, leads the blood, circulating 

 in the body, back into the heart, and must, therefore, be 

 called a vein. In reference to their relation to the two 

 sections of the intestine, we may speak of the latter, more 

 accurately, as the intestinal vein, and of the former as the 

 gill-artery. The blood contained in both vessels, which 

 alone fills the heart also, is venous blood ; that is, containing 

 much carbonic acid. On the other hand, the blood which 

 flows from the gills into the dorsal vessel is there re- 

 furnished with oxygen ; is arterial blood. The most delicate 

 branches of the arteries and veins pass into each other, 

 within the tissue, through a network of extremely fine 

 neutral hair-vessels or capillaries (Fig. 296). 



If we now turn from the Ascidia to the nearest allied 

 form, the Amphioxus, we are immediately surprised by an 

 apparent retrogi'ession in the development of the vascular 



