142 THE COMMON FROG. [chap. 



utility. But how can we understand the primitive un- 

 broken series of arches in fishes ? Their utiHty was 

 yet to come ! 



The frog when adult has, besides its skin, no 

 breathing organs but the lungs. As has been said 

 before, other members of the frog's class retain gills 

 and aquatic respiration during the whole of life, as for 

 example Menobranchus. 



Every one kind, however, whether provided perma- 

 nently with gills or not, develops lungs, and it might 

 easily be imagined that similarly every gilled-creature 

 which has lungs is also a Batrachian. 



This, however, would be a mistake. 



The Mud-fish or Lepidosiren, already referred to 

 more than once, is furnished with both gills and lungs 

 throughout the whole of life. On this account it has 

 been reckoned by some naturalists to be a fish and 

 not a Batrachian. Its fish-nature, however, has now 

 been thoroughly established, and thus the probability 

 of the existence of lungs within the class of fishes is 

 also established. 



But what is a lung } 



A lung is a sac-like structure capable of being 

 distended with air, supplied with venous blood direct 

 from the heart, and sending arterial blood directly to 

 it. Generally the whole of the blood from the lungs 

 goes back to the heart directly, but in one Batrachian 

 — the celebrated Proteus — a portion of the blood from 

 the lungs finds its way not into the heart but into 

 vessels of the general circulation. When there is an 

 air-sac which does not both receive blood directly 

 from and return it directly to the heart — i.e. when 



