256 



THE EVOLUTION OF MAN. 



Fig. 53. 



Fig. 54. 



Fig. 55. 



Fig. 52. — The ideal Primitive Vertebrate type, seen from the left side : 

 mr, medullary tube ; x, chorda ; na, nose ; au, eyes ; g, ear-vesicle ; md, 

 mouth ; Tc, gill-body ; ks, gill-openings ; kg, gill-arches ; ma, stomach ; I, 

 liver ; d, small intestine ; af, anus ; v, intestinal vein ; hz, heart ; a, body- 

 artery ; n, primitive kidney canal ; e, ovary ; h, testicles ; c, body-cavity 

 (visceral cavity) ; ms, muscles ; Ih, leather-skin (corium) ; oh, outer-skin 

 (epidermis) ; f, skin-fold, acting as fin. 



Fig. 53. — Same as above, viewed from the ventral side. 



