HOW FISHES BREATHE. 25 



In some bony fishes, and in sharks and rays, 

 the air-bladder is wanting altogether. 



But what has all this to do with lungs and 

 lung-like structures 1 



A great deal. In the first place the mode of 

 origin of both air-bladder and lung is precisely 

 -similar as an outgrowth of the gullet. In the 

 second, we can follow by a series of gradations 

 the gradual evolution of the former from a simple 

 air-bag, as in the perch, for instance, to a true 

 lung such as is found in certain remarkable 

 fishes known as the lung-fishes. These fishes 

 are found in muddy rivers, whose waters are 

 often charged with foul gases. At such times 

 the lung fishes come from time to time to the 

 surface to breathe atmospheric air. When the 

 water is less impure they breathe by gills as 

 other fish. But the details of this matter belong 

 rather to text-books of comparative anatomy 

 than to a little work like the present. 



One point more about gill-breathing before we 

 leave this subject. So far, the gills which we 

 have examined have been what are called in- 

 ternal gills. That is to say, they have been 

 concealed within, and protected by, either a 

 series of pouches or chambers communicating 

 with the outer world by slits, or by a single 

 large plate. In the young of many fishes, e.g. : 

 the young dog-fish, the gills are at first external, 

 and take the form of long delicate filaments pro- 

 truding through the outer gill-slit. In the young 

 bichir, or Polypterus, of the Nile, these gills are 

 retained for a somewhat longer period, and are 

 quite large (fig. 3) ; but in the adult, as in all 



