52 STOEY OF THE AMPHIBIANS 



by which peculiarity frog-forms may be arranged into 

 groups or classified. 



You can see that there are no ribs — ^just mere 

 "transverse processes." Only one family of frogs 

 has any hint of true ribs. In the skeleton of sala- 

 manders there are pieces of ribs which in csecilians 

 are longer. , 



If we turn the skeleton of our frog over, we shall 

 find that while there are no ribs to meet it, there is a 

 very respectable breast-bone {^sternum) to which the 

 fore limbs are well anchored. This is the first real 

 breast-bone in Nature, though the fishes have hinted 

 at it. The tailed amphibians have it in gristle only. 



Skull 



A noticeable amount of open space is seen in the 

 top view of the skull of our frog. Opposite is an en-; 

 larged figure of the head (Fig. 24). Much of it in' 

 life is gristly, and in the lowest tailed forms it is so 

 much more so that Professor Huxley has said that 

 here it is little better than that of jthe lamprey — a 

 low form of the fishes. In the long-ago, however, 

 the monsters of the class had more bones and more 

 ione in the roof of the head, as you may see from 

 Figs. 31 and 32, page 63. 



Nebvous System 



The amphibians — especially the frogs — show 

 many peculiarities of the nervous system not found 

 so strikingly in the mammals. They retain in their 

 bodies hints of their low ancestry ; and indications 



