OF FISHES. 39 



heart of the gills, which is equivalent to the heart 

 of the lungs, in mammalia, are cold-blooded ani- 

 mals. On the contrary, those having two hearts, 

 are warm-blooded animals. A whale has no gills, 

 but lungs, and consequently breathes air, — and 

 therefore, necessarily has a double heart. Since 

 it has lungs, and a double heart, it also necessari- 

 ly, has warm blood ; and therefore, a whale, as 

 Dr Mitchell rightly declared, is not a fish. And 

 why ? because the fish is without lungs, has but a 

 single heart, and cannot breathe air alone, or wa- 

 ter alone, but a mixture of both. 



GILLS. 



In the economy of fishes, the gills fulfill the of- 

 fice of lungs ; — they are so constructed, that 

 there is a free exposure, in their fringes, of the 

 impure venous blood, to the direct action of the 

 water. 



Deprive the water of its air, by an air-pump, 

 and it will no longer sustain aquatic life. The 

 simple act of soaking the fimbriae of the gills, in 

 this fluid, is not sufficient ; it is necessary to have 

 the water forcibly driven through them by an ac- 

 tion of the jaws. 



If the operculum, or gill cover, be confined and 

 closed with a ligature, suffocation takes place im- 



