234 ELEMENTS OF SCIENCE 



Such structures are termed gills, and in them the blood 

 exchanges its carbonic acid for oxygen, as it does in the 

 lungs of the cat and other beasts. It is not that the 

 water is dissolved into oxygen and hydrogen. The 

 oxygen absorbed is gained from air which is mixed up * 

 in the water. When this air has been expelled by 

 boiling the water, no animal with gills can any longer 

 live in it. Adult frogs and toads have no gills but lungs, 

 by which they breathe, as is the case with almost all 

 their tailed relatives, the efts, and as all the higher 

 animals do. 



The immense group of fishes all breathe by gills, 



FIG. 48. 



THE EFT AMBLYSTOMA. 



though a few kinds also possess an apparatus for breath- 

 ing air which is more or less comparable with a lung. 



Beasts, birds, reptiles, frogs, and their allies, with 

 fishes, make up the great group (class) of vertebrate, or 

 back-boned, animals. 



We can here only refer to two other great sub- 

 kingdoms, referring the reader for all else to works 

 devoted to zoology. 



The first of these is far richer in the number of kinds 

 it contains than are all the other classes of animals 

 taken together. It is the sub-kingdom Arthropoda, 



* See ante, p. 153, 



