46 STORY OP THE AMPHIBIANS 



Heaeing 



All amphibians when adult have ears, but the 

 tailed forms and some frogs have very poor ones 

 which are devoid of any drum cavity. In most of 

 them, however, there is an internal ear of some sort 

 opening into the mouth — sometimes by one hole in 

 the roof — sometimes by two, always behind those of 

 the nostrils. The number and position of thpse holes 

 aid in describing groups. In the lowest forms and in 

 the tadpoles, the ear is a mere sac in a cavity of bone. 

 In this sac a sort of chalky body (or bodies) is found 

 as is the case in all higher ears. The cavity is simply 

 covered with skin. No amphibian has an outside 

 opening to the ear, but the higher frogs and toads 

 have a drum cavity and a tough drum-membrane 

 over it, which is flush with th^ surface. The size 

 and shape of this membrane is f ery distinctive. In 

 the genus Rana (bullfrog, green-frog, wood-frog, 

 etc.) it is set in a sort of gristly ring and is very 

 noticeable. Sometimes the drumhead seems itself to 

 be a gristly plate. 



There can be no doubt about frogs hearing well. 

 "While writing this book, the author stepped out to 

 listen to some frogs in a pond one-fourth mile away, 

 but he unfortunately let the door slam a little too 

 hard, whereat the concert ceased. Similar experi- 

 ences occur in trying to creepi upon them. 



There is much doubt, however, about their hear- 

 ing high-pitched tones, or distinguishing changes. 

 A frog " changes his tune " very slightly ; and while 



