18 



LECTURES ON BIOLOGY 



for whole half-days, hunting its prey and moving as freely as in 

 the water. In order to be able to do so it carries under the 

 tightly closed gill-covers a good supply of water so that even on 

 dry land the gills are kept continually moist. 



This adaptation to a terrestrial life has made still further 

 progress in the Lung-fishes (Dipnoi), and particularly in an 

 Australian species, the Barramunda (Neoceratodus forsteri). 



FIG. 3. THE AFRICAN MUD-JUMPER (Periophthalmus koelreuteri). 



These fishes do not only possess the normal respiratory apparatus, 

 the gills, but their swim-bladders, too, have been transformed 

 into lungs, so that they are able to breathe both in the water and 

 on land. The two pairs of extremities of Neoceratodus are 

 uniformly developed, one might almost say, like legs, but they 

 are probably too weak to support the big body on land, for the 

 Barramunda reaches a length of 6 ft. It is therefore question- 



