420 



THE EVOLUTION OF MAN. 



which the Amphioxus takes up in the water it breathes — • 

 Infusoria, Diatomacese, parts of decayed plants, and animal 



Fig. 151. — Lancelet {Amphioxus lanceolatus) , twice 

 the natural size ; seen from the left side (the longitu- 

 dinal axis stands upright; the mouth end is turned 

 upwards, the tail end downwards, as in Plate XI. 

 Fig. 15) : a, mouth-opening, sui-rounded by hairs ; 

 6, anal opening; c, gill-pore (x>orus hranchialis) ; 

 d, gill-body ; e, stomach ; /, liver ; g, small intestine ; 

 h, gill-cavity ; i, notochord (below this the aorta) ; 

 Tc, aorta-arch ; I, main trunk of the gill-artery ; m, 

 swellings on the branches of the latter; w, hollow 

 vein {vena cava) ; o, intestinal vein. 



S7/- 



■^: 



,V,t 



^s- 



bodies, etc. — pass back from the gill-body 

 into the * digestive section of the intes- 

 tinal canal, and are there taken up as 

 food and assimilated. From a rather wider 

 section, corresponding to the stomach 

 (Fig. 151, e), proceeds an oblong, pouch- 

 like blind-sac (/), which passes directly 

 forward, and ends on the right side of the 

 •giU-body. This is the liver of the Amphi- 

 oxus, the simplest form of liver that w^e 

 know of in any Vertebrate. In Man also, 

 as we shall see, the liver develops as a 

 pouch-shaped blind-sac, which protrudes 

 from the intestinal canal behind the 

 stomach. 



The structure of the system of blood- 

 vessels in our little animal is not less re- 

 markable than that of the intestine. For 

 while all other Vertebrates have a compressed, thick, purse- 

 shaped heart, which develops at the throat from the lower 



