CHAPTER XXXII 



ANIMAL RESPIRATION— VERTEBRATES THAT BREATHE 



IN WATER 



'TWIXT BRAIN 

 FORE BRAIN 



CEREBRAL 

 HEMISPHERE 



HIND BRAIN 



One of the essential characters of Vertebrate or Backboned 

 animals is the possession, during part or all of life, of slits in the 

 side of the throat (visceral clefts) by which that part of the diges- 

 tive tube which immediately succeeds the mouth-cavity [i.e. 

 pharynx) communicates 



. , 1 . ,r MID BRAIN\ 



With the exterior (ng. ^ — . eye 



511). These slits pri- 

 marily have to do with 

 breathing, and for that 

 reason may be called 

 gill-clefts, since in many 

 cases gills, which are 

 organs specially con- 

 cerned with aquatic 

 breathing, are found 

 as outgrowths of their 

 sides. By studying the 

 development of a fish 

 it is possible to follow 

 the stages in the for- 



VISCERAL ARCHES 



Fig. 511. — Front part of Chick Embryo. Enlarged 



mation of gill -clefts. 



The sides of the pharynx grow out into a series of pouches which 

 first of all come into contact with the skin and then fuse with it. 

 Later on, by the absorption of tissue, the actual openings or clefts 

 come into existence. 



When, however, we enquire how gill-clefts came to be devel- 

 oped in the remote and so far unknown ancestors of vertebrates, 

 it is impossible to do more than speculate in a general sort of way 

 upon the matter. We know, as a matter of fact, that almost any 



