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 (Amiohioxus InnceoJafus), 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 ; 

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

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

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

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

 swellings on the branches of the latter; ?r, hollow 

 vein {vena cava) ; o, intestinal vein. 



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

 •^//: 1^1 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 Ami^hi- 

 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. 



Tlie 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 



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