GENERAL CHARACTERISTICS. 321 



corresponding to the right auricle and ventricle of the 

 warm-blooded vertebrates. The process of respiration 

 now described is as indispensable to Fishes as the 

 respiration of air to other animals ; and they exhibit the 

 same symptoms of distress when it is stopped, and rap- 

 idly perish. Still, it is not by itself, nor by the oxygen 

 which enters into its chemical composition, that the water 

 acts upon the blood. It is only the small quantity of air 

 that is mixed with the water which serves for the respira- 

 tion of these animals, and if they be put in water which 

 has been deprived of air, they immediately die. It is 

 sometimes necessary for them to come to the surface for 

 atmospheric air, when the supply of that substance has 

 become deficient in the water in which they live. 



Most fishes are furnished with a membranous bag 

 filled with air, and called the swimming-bladder. This 

 organ is probably a rudimentary lung, but its true func- 

 tion is not known ; although some believe that by it 

 Fishes have the power of varying' their specific gravity, 

 and thus more easily rising and descending. Others be- 

 lieve it aids in hearing, as there is a connection between 

 it and the chamber of the ear. 



Fishes in general are characterized by great voracity. 

 They feed mainly upon smaller members of their class, 

 and other small animals ; although some are vegetable 

 feeders. Most of them swallow their prey whole. Some, 

 which feed on shell-fish, crush their food by means of the 

 powerful crushing and grinding teeth in the gullet. 



Most fishes are oviparous in their manner of reproduc- 

 tion ; but some species bring forth living young. They 

 produce a far greater number of eggs than any other 

 vertebrates.* Some species prepare a place for their 



* A Salmon sometimes contains as many as 20,000 eggs ; a Perch, 28,320 ; 

 a Herring, 36,960; a Mackerel, 546,000; a Flounder, 1,357,400; a Stur- 

 geon, 7,635,200 ; a Cod, 9,344,000 ; and a species of Upeneus, 13,000,000 ! 

 14* U 



