56 THE COMMON FROG. [chap. 



-1 



We have now reviewed the closest as well as the 

 more remote allies of our Frog, and have seen how 

 the Frog is a species of a group {Aiioiird) which is 

 one of three existing and widely diverging orders, 

 supplemented by* an extinct ordinal group of the 

 carboniferous period — the four orders (i. Anoura, 

 2. Urodela, 3. Ophioinorpha, and 4. LabyriiitJwdonta^ 

 being embraced in a higher unity termed a '' Class," 

 which is the Frog's class, as " Anoura " is his order. 

 This class is with propriety spoken of as the Frogs 

 class, since the Frog is the species from which its 

 scientific derivation BatracHIA is derived. This 

 class may now be considered as a whole. 



The Batrachians (of all three existing orders) are in 

 the main aquatic animals, inasmuch a-s the greater 

 number, even when adult, frequent, at least at 

 intervals, ponds and streams, or delight in humid 

 localities. Water also is necessary for the larval 

 stages of almost all ; and absolutely all, at one period 

 of life, possess gills, while some (as we have seen) 

 retain gills during their whole existence, and are 

 permanently and constantly inhabitants of water. 



The extinct forms [Labyrinthodonta) were, no doubt, 

 also aquatic, as, besides their general relation to 

 other Batrachians, traces or indications of the hard 

 parts which supported the branchiae of some Laby- 

 rinthodonts appear to have been actually found. 



It is somewhat singular that, in spite of this predo- 

 minating aquatic habit, all Batrachians, both living 

 and fossil, appear to inhabit, and to have inhabited, 

 fresh-water only. No Batrachian of any period is yet 

 known to have been marine. This is the more re- 



