THE ANCESTRY OF THE VERTEBRATES 81 



The vascular system, in which a colourless blood 

 circulates, is very simple ; there is no differentiated 

 heart, all the larger vessels being feebly pulsatile ; 

 and we can only say that it resembles the Vertebrate 

 vascular system in being closed off from the coelom 

 and in the fact that the blood passes forwards in the 

 ventral vessel and backwards in the dorsal. 



We now pass to the condition of the body-cavity 

 and musculature. The first thing that strikes one 

 on looking at a Lancelet is that the body is cut up 

 transversely into a series of narrow zig-zag strips 

 (Fig. 17). If these strips were straight instead of 

 zig-zag, we should say that the body was formed of 

 a series of rings, as in the Earthworm, and we should 

 suspect that the animal was metamerically segmented. 

 Now if we cursorily examine a fish or any other 

 Vertebrate, we might not immediately perceive, what 

 is in fact the case, that all Vertebrates, too, are 

 essentially metamerically segmented animals. Yet, 

 on reflection, the fact is plain. The vertebral column 

 or spine of the Vertebrate is formed of a series of 

 repeated homologous parts, the vertebrae. Corre- 

 sponding with these vertebrae there issue from the 

 central nerve-tube a series of repeated homologous 

 spinal nerves, just as from the segmented nerve 

 ganglia of the Earthworm, or any other Appendicu- 

 late animal, there arise nerves which pass to the 

 corresponding segments of the body. But, as in the 



S. A. K. 6 



