CLASS AMPHIBIA. 297 



the tail, when adult, and the body becomes much short- 

 ened. In the tailless types, particularly, the changes in 

 the form of the body are related to the development of 

 legs. As the strong legs develop, the bones of the legs 

 become better grown and the connection with the inner 

 skeleton becomes more firm. In connection with these 

 legs also, the muscles of the body itself, those that act 

 on the vertebrae, become relatively less important and 

 the great muscles are the ones that move the legs. This 

 places the bulk of flesh at a different point and the shape 

 of the animal changes correspondingly. In most of the 

 long- tailed forms the legs are comparatively weak. 



There is a breast-bone in the amphibians, but the ribs 

 are very little developed. The bones about the head are 

 more closely fused than in the fishes. 



307. Respiration and Circulation. Reference has al- 

 ready been made to the fact that the early form of breath- 

 ing is by means of gills. They are much like the gills of 

 fishes in position and number. The circulation in the 

 tadpole must of course take the blood to these gills to 

 be aerated, and hence the course of the circulation at first 

 is quite like that in fishes. As the lungs grow and gradu- 

 ally take the place of the gills, certain blood vessels 

 going to the lungs increase and those going to the gills 

 change their form in such a way as to give quite a dif- 

 ferent appearance to the adult vessels in this region 

 (Fig. 117). The heart undergoes some changes to corre- 

 spond to this. In the fishes the heart handles impure 

 blood only, the pure blood going from the gills to the 

 system. In the adult frog the pure blood goes back at 

 once from the lungs to the heart, in addition to the im- 

 pure blood that returns from the system. The pure and 



