26 THE STORY OF FISH LIFE. 



other fishes, they disappear, being exchanged for 

 the more easily protected internal gills. These 

 external gills become still more interesting when 

 we remember that the larval frogs and newts 

 also breathe by external gills, whilst in certain 

 aquatic salamanders {Nedurus and Proteus) these 

 external gills are retained throughout life. 



The breathing of fishes is attended by some 

 very characteristic movements of the mouth 



rarely properly under- 

 stood by the lay 

 mind. " He drinks 

 like a fish" is a charge, 

 and a very serious 



Fig. 3,— HeadofayoungPo/j/i)<«rMS one, ofteu launched 

 (bichir), showing the external ^ • , 



gill. by one man against 



another. Often it is 

 as false and unfounded as the comparison. To 

 begin with, fishes when they drink — if they 

 drink — drink water. But it is not the nature 

 of the draught but the frequency of its re- 

 petition to which allusion is made in this 

 quotation. It is apparently supposed that the 

 constant and rhythmical opening and closing of 

 the mouth is a proof of the act of drinking. 

 Nothing could be further from the truth. 

 This is the outward sign of the act of breath- 

 ing, and corresponds to the heaving sides and 

 the steaming nostrils of the galloping horse. 

 In opening the mouth water is drawn in ; in 

 closing it, it is forced out through the gill-slits 

 in the gullet, over the gills, that the oxygen may 

 be extracted by the blood, and out by the gill- 

 slits or slit, as the case may be. No water is 



