86 PRACTICAL ZOOLOGY. 



The young fishes of all kinds have heterocercal tails, but 

 as they grow older, the higher fishes develop homocercal 

 tails. The earliest fishes had heterocercal tails, as have 

 many of the lower fishes at the present day. 



Again, the early fishes had cartilaginous instead of bony 

 skeletons. All young fishes have cartilaginous skeletons, 

 but the higher fishes develop bone in place of cartilage as 

 they grow older. 



It will be seen then that the stages of development of 

 the higher fishes repeat the order of the geologic appear- 

 ance of fishes, and review the classification of fishes from 

 the lowest to the highest. What do these facts signify ? 



Use Jordan's " Manual of the Vertebrates " (latest edi- 

 tion), or, better, Jordan and Gilbert's " Synopsis of the 

 Fishes of North America," for finding the names of fishes 

 as "Gray's Manual " is used for "analyzing" plants. 



THE FROG. 



In the spring frogs gather in ponds and streams to 

 breed. Later, they may be found in meadows and fields 

 near water. "When seeking them in such places walk 

 rapidly along so as to frighten and make them jump so 

 they can be seen. When studying frogs it is well to get 

 a good supply and keep them alive in a box, which should 

 be shut securely, leaving a few holes. This box may be 

 kept in a cellar or in a watering-trough. Frogs will live 

 a long time without feeding. In capturing frogs observe 

 how they dive into the mud to escape. 



