258 



SKELETONS OF THE LOWER ANIMALS 



The absence of lime in the bones of the fish makes them 

 extremely flexible, and fits the body for the bending nec- 

 essary to swimming rnotion. Furthermore, the support 

 which the M'ater furnishes to the body makes such hard- 

 ness unnecessary as a supporting structm-e. Another fea- 

 ture of the fish skeleton is the great number and small size 

 of the bones. (See Fig. 107.) 



The frog skeleton. — In the amphibians, of which the 

 frog is a type, we find a marked difference in the composi- 

 tion and number of 

 the bones, while the 

 arrangement of the 

 separate bones much 

 more nearly follows 

 the order in our own 

 bodies. The explana- 

 tion of this change is 

 found in the adapta- 

 tion necessary to fit 

 the animal for a land 

 existence. As soon as 

 the animal is removed 

 from the support of 

 water, the flexible, cartilage bones are unable to support 

 the body, and we should expect, therefore, to find the bones 

 of the frog more rigid than those of the fish. The new 

 method of motion requires the modification of fins into 

 legs and arms, and the front legs of the frog are supposed 

 to have developed as modifications of what we call the 

 pectoral fins of the fish. In the same way, the hind legs 

 of the frog are modifications of the pelvic fins of the fish. 

 Another reason for the rigid character of the frog skeleton 



Fig. 108 — Skeleton of a frog. 



