CHAPTER X. 



SKELETAL AND INTEGUMENTARY STRUCTURES 

 IN VERTEBRATES. 



The frog belongs to the group of Vertebrata whose characters 

 are given below. The Vertebrata contrast with any Inverte- 

 brate type by the possession of an elaborate internal skeleton, 

 which is in the lowest forms entirely, or nearly entirely, cartila- 

 ginous, and ossified in the higher types. The characters derived 

 from the study of the skeleton are exceedingly useful in classi- 

 fying vertebrates. In the following pages, therefore, an account 

 will be given of the skeleton in three types : in the frog, the 

 fowl, and the rabbit. 



The Vertebrata can be also, to a large extent, differentiated 

 by certain external features. The Mammalia, for example, are 

 the only vertebrates which possess hair ; feathers are unknown 

 outside the class of Birds, while the scales of Serpents and 

 Lizards are totally different from the similarly named structures 

 of Fishes. As the teeth are really, as will be shown later, 

 epidermic structures, they will be included in a survey of the 

 principal external (integumental) characters of the vertebrates. 



Vertebral Column. 



The vertebral column of the Frog consists of a number of 

 separate vertebrs — very few as compared with the other 

 vertebrate types. The frog has altogether only ten vertebrae, 

 exclusive of the long urostyle, which is never broken up into 

 separate vertebrae, but appears to be the equivalent of three. 



The first vertebra is called the atlas, and it is followed by 



