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, surrounded by hairs r 

 b, anal opening; c, gill-pore (porus branchialis) ; 

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

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

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

 swellings on the branches of the latter; 7), hollow 

 vein (vena cava); o, intestinal vein. 



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 

 gill-body. This is the liver of the Amphi- 

 oxus, the simplest form of liver that we- 

 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 



