152 THE COMMON FROG. [chap. 



The class Batrachia agrees both with fishes and 

 reptiles in having the blood cold, more than one aortic 

 arch, and (except in crocodiles) in not having the 

 distinct ventricles. 



The class agrees with fishes, reptiles, and birds, in 

 having no complete diaphragm, and no corpus cal- 

 losum ^ in the brain, and no single aorta arching over 

 the left bronchus. 



We have now arrived at the end of those considera- 

 tions seemingly best suited to enable us to answer the 

 initial question, " What is a Frog .'' " The requisite 

 definition might, of course, have been given much 

 earlier, but these inquiries have seemed necessary to 

 enable the ceader to understand the technical terms 

 of such definition — to give them, in his eyes, a real 

 meaning. 



The Frog is a tailless, lung-breathing, branchiate 

 vertebrate, with four limbs typically differentiated, 

 undergoing a complete metamorphosis, and provided 

 with teeth along the margins of the upper jaw. 



If further exposition be necessary, it may perhaps 

 now be most conveniently effected by means of a 

 series of statements respecting the anatomical diff"er- 

 ences existing between the frog and other groups of 

 vertebrate animals. 



The froGfs differ from toads in that the former have 

 marginal upper teeth. 



They differ from other Batrachians in that there are 

 four limbs, while the tail is absent. 



* As to this structure see Lesson in "Elementary Anatomy," 

 pp. 367, 375- 



