326 W. C. ALLEE 



that their general activity as well as their rheotactic response 

 is affected. 



c. Increase of temperature 



It was shown in a previous section that decreasing the tem- 

 perature caused a lowering of the positive response; theoretically 

 then an increase of temperature should show reversed results. 

 The first experiment to test the effects of increased temperature 

 was carried on by keeping the animals near a hot electric plate. 

 The second, by keeping the animals in an automatically regulated 

 heating tank. The results listed in the third part of table 26 

 were obtained by keeping the isopods in a melting ice pack 

 exposed to a warm room temperature. 



The first part of the table shows that the effects produced by 

 an increase of 6° operating for twelve hours, will persist for at 

 least that much longer although the isopods are returned to the 

 original temperature. The second set of trials indicates that the 

 effect of continued high temperature is only temporary and the 

 last three tests show that an increase of only 4° may cause a 

 most decided increase of positiveness. The last experiments 

 were upon stream Aselli during the breeding season. 



Other experiments show that a sudden increase of 10° in tem- 

 perature may cause either an increase or a decrease in the positive 

 reaction, but if an increased response is still given after a 10° 

 raise in temperature, more heat always results in a diminution of 

 the positive response. Whether a 10° increase in temperature 

 will cause an increase or decrease in the positivity, seems correlated 

 with the general amount of activity of the isopods. 



3. EFFECT OF OXYGEN ON SIZE 



The relation between rate of growth and the ultimate size of 

 the isopods, with the amount of oxygen present in the water has 

 been mentioned already, yet this subject is of enough importance 

 to merit a short general treatment. Colton ('08, pp. 410-447) 

 demonstrated for snails, that the size may be correlated with the 

 general oxygen supply, but the exact amount of oxygen present 



