328 THE EVOLUTION OF MAN. 



become a respiratory organ. I. ; have already pointed out 

 how characteristic this adaptation is of Vertebrates and 

 Mantle Animals (Tunicata, p. 87). The 

 phylogenetic origin of the gill-openings in- 

 dicates the beginning of a new epoch in 

 the tribal history of Vertebrates. 



The most important process we meet 

 with in the .further ontogenetic development 

 of the intestinal canal in the human embryo, 

 is the origin of the gill-openings. At the 

 head of the human embryo, the wall of the 

 throat very early unites with the outer wall 

 of the body, and four openings then form 

 on the right and left sides of the neck, 

 behind the mouth, and these lead directly 

 from without into the throat-cavity. These 

 openings are the gill-openings, and the par- 

 titions separating them are the gill-arches 

 (Figs. 116-118, vol. i. p. 356; Plates I. 

 and V., Fig. 15, ks). These embryonic for- 

 mations are very interesting ; for they show 



FIG. 282. Lancelot (Amphioxus lanceolatus) , double 

 the natural size, seen from the left side (the longi- 

 tudinal axis is perpendicular, the mouth end above, 

 the tail end below (as in Plate XI. Fig. 15) : a, month- 

 opening, surrounded by bristles ; b, anal opening ; c, 

 gill-pore ( porus branchialis) ; d, gill-body ; e, stomach ; 

 /, liver; g, small intestine; h, gill-cavity; i, notochord, 

 below which is the aorta ; A;, aortal arch ; I, main stem 

 of the gill-artery; m, swellings on the branches of 

 the latter ; n, hollow vein ; o, intestinal vein. 



that all the higher Vertebrates when in a very young state, 



