70 LIFE IN THE SEA [OH. 



Such sudden temperature changes must however be 

 very exceptional in the sea, and a nektic animal, such 

 as a fish, would usually be able to respond to them 

 by means of a migration into water of more favour- 

 able conditions. Temperature changes must have 

 for their usual effects changes in the intensity and 

 mode of metabolism of the animal ; and there is 

 plenty of experimental evidence that this is the case. 

 We have to regard the life-processes of an animal as 

 the result in some way of a regulated series of 

 chemical reactions, and we know that the velocity of 

 a chemical reaction increases as the temperature rises. 

 The amount of oxygen absorbed in respiration and 

 that of the carbonic acid excreted are greater or less 

 according as the temperature is greater or less ; and 

 the rapidity of respiration varies in the same way. 

 In cold water the respiratory movements of the 

 mouth and gills of a fish slow down greatly and 

 often assume the Cheyne-Stokes type, that is there 

 are relatively long pauses followed by groups of re- 

 spirations. This means that the rate of metabolism, 

 that is the amount of chemical change going on in the 

 tissues of the animal, decreases as the temperature 

 falls. Towards the end of the year the quantity of 

 food taken decreases and many fishes such as the 

 plaice and flounder cease altogether to feed during 

 the months of December to February. The stomach 

 and intestine are usually empty during this period ; 



