REACTIONS OF FISHES TO SALTS ,277 



result fatally either to the individual or to the species. The 

 mechanism, therefore, which is working to preserve the life of 

 the organism is so delicate that it produces beneficial reactions 

 jLipon the part of the animal far in advance of life and death com- 

 plications. The working of this mechanism is undoubtedly 

 closely correlated with quantitative and perhaps qualitative 

 changes in metabolism. These changes in metabolism will 

 have a direct relation to the amount of CO2 given off by the 

 organism. 



It has been shown that a slight increase ii'i the carbon dioxide 

 content of an animal's blood results in a marked increase in the 

 general irritability, and this increase in irritability would alone 

 result in an increase in the range and vigor of the movements 

 made by the organism. Thus no factor other than increased 

 metabolism need be hypothecated to account for the stimulus 

 which starts the breeding migration of so many animals. The 

 directive factors which result in the animal's coming into special 

 conditions for the breeding activities are another matter. These 

 can be none other than the factors, physical and chemical, which 

 are present in the environment. In the general metabolism of 

 fishes, the stage of development of the sex organs plays an im- 

 portant role, and it is very probable that the state of metabolism 

 in these organs furnishes the initial stimulus which causes the 

 animals to start upon the breeding migrations at a given period 

 of the life cycle. Treadwell ('15) points out that the eggs of 

 the Atlantic palola give off an increasing amount of CO2 as the 

 swarming season approaches, and concludes that this indicates 

 that there is probably an internal stimulus which is important 

 in producing the swarm. There can be little doubt but that 

 such internal stimulus is acting; the important fact, however, is 

 that it has been shown that such internal changes in the physio- 

 logical state of the animal may result in very marked changes 

 in the animal's reactions to environmental factors. AUee ('12) 

 has shown that, in isopods, a high rate of metabolism is cor- 

 related with a high percent of positive responses to current and 

 that a lowering of the metabolic rate in the animals will diminish 

 and even reverse the rheotactic reaction. 



