574 



CHAPTER VI. 

 CLASS OF BATRACHIA. 



515. ALTHOUGH the number of species included in this class 

 is small, the peculiarities which they present are of such a na- 

 ture that they seem fully entitled to rank as a group distinct 

 from the Reptiles, with which, however, they are still arranged 

 by some Zoologists. They seem, in point of fact, to occupy a 

 completely intermediate position between the Reptiles and Fishes, 

 as their respiration is aerial in the mature state, and aquatic 

 when young ; and even when adult, most of them still exhibit 

 traces of the branchial arches from which the gills were sus- 

 pended, springing, as in Fishes, from the hyoid bone. In some 

 cases, the gills are even retained after the development of lungs 

 has enabled the creatures to breathe air ; and the question whe- 

 ther one of these should be placed with the Batrachia or the 

 Fishes, is still a matter of dispute amongst Naturalists. 



516. Small as the number of its members is, however, this 

 class presents even a greater variety of form and organisation 

 than that of Reptiles ; the higher forms possess perfectly de- 

 veloped limbs, and many of them are quite lizard-like in their 

 forms ; whilst amongst the lower groups the limbs frequently be- 

 come more or less rudimentary, and sometimes disappear alto- 

 gether. The structure of the skeleton is still more variable ; for 

 whilst in some species we find the bones of the skull, vertebral 

 column, and limbs perfectly ossified, in others even the vertebral 

 column is reduced to what is called a dorsal chord, consisting of 

 a thread of gelatinous matter enclosed in a fibrous sheath. In 

 these the skull also is merely cartilaginous, and perfectly con- 

 tinuous with the dorsal chord. In the forms with a perfectly 



