FOUND LINKS. 



animals, resemble in the mud-fishes at least rudi- 

 mentary limbs. Then the nostrils, thirdly, open into- 

 the mouth a character agreeing with frogs and all 

 higher vertebrates, but possessed by one other fish- 

 group only- the low hag-fishes, which are poor rela- 

 tions of the lampreys. These characters, then, are 

 the characters of frogs, and not of fishes. But a far 

 more interesting likeness to the frogs and higher ver- 

 tebrates yet remains for notice. The " air-bladder" 

 of the mud-fish and of the " Jeevine " alters wonder- 

 fully, both in form and function, from its nature in 

 other fishes. It becomes divided in two, and it opens 

 into their throat by a windpipe, at the top of which 

 is a "glottis/' corresponding to part of our own- 

 organ of voice. Furthermore, it is divided internally 

 into cells in a word, the air-bladder of the mud-fish 

 and its neighbour has become a lung. But this won- 

 derful transformation is not quite ended with the 

 recital of the altered structure of the air-bladder in 

 these fishes. A lung is an organ which not merely 

 receives blood in an impure state, but which, as in 

 ourselves, returns that blood pure to the heart for re- 

 circulation through the body. If, therefore, the 

 ' f lung " of the fish is to be accounted a true " lung,"" 

 we should be able to show that it performs the func- 

 tions and discharges the duties of an organ of 

 breathing. 



Now the life of these fishes exhibits exactly the 

 peculiarities which demand the exercise of an air- 

 breathing organ like a lung. The mud-fishes inhabit 



