v.] THE COMMON FROG. 59 



frog to other Batrachians thus more clearly seen, but 

 also the relations and affinities borne by the several 

 orders of Batrachians and by the whole class to the 

 other orders or other classes of the Vertebrate sub- 

 kingdom. 



Accordingly, we have now to make an acquaintance 

 with more than those obvious and external characters 

 which are found in the Frog, and to penetrate into its 

 inner anatomy, surveying successively its bony frame- 

 work and the various parts and organs which subserve 

 the several actions necessary to its continued existence. 



At the same time the more noteworthy resemblances 

 presented by the Frog to other creatures will be 

 pointed out. Thus we shall become acquainted with 

 the relations existing first between the Frog and 

 other members of its order ; secondly, between the 

 megibers of its order [Anoiira) and its class fellows — 

 i.e. other Batrachians ; thirdly, we shall, comprehend 

 the degree of relationship existing between the Batra- 

 chia and the other classes of the Vertebrate sub- 

 kingdom ; and fourthly, we shall come to recognise 

 certain singular resemblances which exist between the 

 various groups of Batrachians (the Frog's order of 

 course forming one), and some of the orders into 

 which other vertebrate classes — especially the class 

 of Reptiles — have been divided. 



The skeleton of the Frog, both external and in- 

 ternal, naturally comes first as the support and 

 foundation of the other structures. The internal 

 skeleton (or endo-skeletoii) will include the bones of 

 the head, i.e. the skull, back-bone (already referred 

 to), and the bones of the limbs. The external skele- 

 ton {exo-skeleton) will consist of the skin only. 



