ACCLIMATION IN PLANARIA DOROTOCEPHALA. 313 



140,, i^c, '16). A physiologically young animal shows a higher 

 susceptibility to change in conditions as indicated by direct 

 effect, and also is able to become more rapidly and completely 

 acclimated to changes which are not too extreme. The cells of 

 x undergo extreme dedifferentiation after section and become 

 essentially embryonic cells before giving rise to a head. In an 

 isolated piece then they very soon become physiologically younger 

 than the cells of y, and the higher susceptibility of x to tempera- 

 ture changes as well as the effects of acclimation are unques- 

 tionably associated with this change in physiological condition 

 and increase in metabolic activity. 



In short, the data on head-frequency not only indicate direct 

 and acclimatory effects of temperature of the same sort as those 

 indicated by susceptibility to cyanide and CO production, but 

 they also indicate that these temperature effects are not the same 

 for cells or tissues in different physiological condition. Here 

 again the difficulty of drawing definite conclusions as to existence 

 of a temperature coefficient of physiological processes is evident, 

 from the fact that the rate of increase in the velocity of reaction 

 may vary with the physiological condition of the cells or tissues 

 concerned. 



VIII. SUMMARY. 



1. The susceptibility of Planaria dorotocephala to toxic con- 

 centrations of KNC increases with rise and decreases with fall 

 in temperature. 



2. When animals which have been living at one temperature 

 are subjected to another a gradual change in susceptibility 

 to KNC in the new temperature occurs. In animals brought 

 from a lower to a higher temperature the susceptibility under- 

 goes gradual decrease at the higher temperature, and in animals 

 brought from a higher to a lower temperature the susceptibility 

 undergoes a gradual increase at the lower temperature. These 

 secondary changes in susceptibility are distinguishable after 

 twelve hours' subjection to a new temperature, but extend over 

 days or weeks, apparently gradually approaching a limit. These 

 changes occurring in a temperature higher or lower than that at 

 which the animals have previously been living represent a process 

 of acclimation. 



