HOW FISHES BREATHE. 19 



wide distribution ; these might be regarded as 

 having provided the staples of survival in every 

 branch of descent. ... A generalised form is 

 like potter's clay, plastic in the hands of nature, 

 readily to be converted into a needed cup or 

 vase ; but, when thus specialised, may never 

 resume unaltered its ancestral condition. The 

 clay survives ; the cup perishes. 7 ' 



CHAPTER II. 



HOW FISHES BREATHE. 



FRESH air is as necessary to a fish as to ourselves, 

 and this air is needed, just as with us, for the 

 sake of its oxygen. The taking in of this life- 

 sustaining gas is known as Kespiration. The 

 process of respiration, or breathing, in the fish is 

 performed by means of gills, not, as with us, by 

 means of lungs; and although the difference 

 between these two forms of respiration may seem 

 to be very considerable, we shall see that the 

 former is the more ancient practice, and gradually 

 gave place to the latter. 



Yertebrated or backboned gill - breathing 

 animals are always aquatic, although the con- 

 verse that luii g-breath ing vertebrates are always 

 terrestrial is by no means true. 



Breathing organs of whatever kind are always 

 intimately associated with the upper end of the 

 alimentary canal or food-pipe. Sometimes this 

 association remains throughout life ; as in the 



