MOORLAND IDYLLS. 



to lie dormant in the mud without food or 

 drink for many weeks at a time. Under 

 these peculiar circumstances, their air-bladder 

 has gradually developed into a true lung ; 

 and, what is odder still, we possess in various 

 countries distinct specimens at all the inter- 

 mediate stages from air-bladder to lung in 

 proportion as the ponds which they haunt 

 become dry for longer or shorter periods. 

 The bow-fin of the United States, for 

 example, lives in turbid waters which do 

 not quite dry up, but it has acquired the 

 habit of rising to the surface every now 

 and then, and gulping in large mouthfuls 

 of air, which enter its swim-bladder. It 

 does so most frequently when the water is 

 foul, and there has been little rainfall in 

 other words, when there is a scarcity of 

 oxygen. Accordingly, its air-bladder 

 though not yet a true lung is spongy 

 and cellular in structure, being adapted for 

 aerating the blood that passes through it. 

 The mud-fish of Queensland, again, to take 

 226 



