THE INTESTINAL CANAL IN PRIMITIVE VERTEBRATES. 263 



respiration of Vertebrates. All lower Vertebrates, the 

 Amphioxus, Lampreys, and Fishes, continually take in 

 water through the mouth, and let it pass out through 



FIG. 59. The ideal Primitive Vertebrate, seen from the left side : na, 

 nose ; au, eye ; g, ear ; md, mouth ; Ics, gill-openings ; x, chorda ; mr, 

 spinal tube ; kg, gill-vessels ; k, gill-intestine ; hz, heart ; ms, muscles ; 

 tno, stomach ; v, intestinal vein ; c, body-cavity ; a, aorta ; Z, liver ; d, small 

 intestine ; e, ovary ; h, testes ; n, kidney canal ; of, anus ; Ih, leather skin ; 

 oh, outer skin (epidermis) ; /, skin-fold, acting as fin. 



the lateral openings of the neck. The water that passes 

 through the mouth serves for breathing. The oxygen 

 contained in it is inhaled by the blood-channels which 

 extend along the " gill-arches " (kg), situated between 

 the gill-openings. These very characteristic gill-openings 

 and gill-arches are found in the human embryo, and in 

 the embryos of all higher Vertebrates, at an early period 

 of their development, in that form in which they are 

 retained throughout life by the lower Vertebrates. In 

 Mammals, Birds, and Reptiles they never act as true organs 

 of respiration, but gradually develop into very different 

 organs. The fact that they originally actually exist in the 

 same form as in Fishes, is, however, one of the most interest- 

 ing proofs of the descent of these three higher classes of 

 Vertebrates from the Fishes. 



