264 THE EVOLUTION OF MANV 



Not less interesting and significant is the circumstance 

 that the later respiratory organs of Mammals, Birds, and 

 Reptiles develop from the front, or respiratory portion of the 

 intestinal canal. A bladder-like fold develops at an early 

 period from the throat of the embryo, and soon takes the 

 form of two large sacs, which are afterwards filled with 

 air. These sacs are the two air-breathing lungs which take 

 the place of the water-breathing gills. But this bladder- 

 like fold, from which the lungs arise, is simply the well- 

 known air-filled bladder which is called the swimming- 

 bladder in Fishes, and serves throughout life as a hydro- 

 static organ, a swimming-apparatus lightening the specific 

 gravity of the Fish. Human lungs are a modification of 

 the swimming-bladder of Fishes. 



The vascular system of Vertebrates stands in the closest 

 morphological and physiological relation to the intestinal 

 canal, its most important parts being developed from the 

 intestinal-fibrous layer. It consists of two distinct parts, 

 which are, however, immediately dependent on each other, 



FIG. 60. Ideal Primitive Vertebrate, ventral view: na, nose; o, 

 eyes ; g, ear ; md, month ; fc, gill-body ; ks, gill-openings ; kg, vascular 

 gill-arches ; hz, heart ; v, intestinal vein ; ma, stomach ; I, liver ; d, small 

 intestine ; of, anus ; n, primitive kidneys ; e, ovary ; h, testicles ; c, body- 

 cavity ; ms, muscles ; /, skin-fold, acting as float. 



