ACCLIMATION IN PLANARIA DOROTOCEPHALA. 



309 



TABLE XI. 



periods up to one week is not entirely uniform; while with three 

 weeks at the acclimation temperature, there is uniformity, head- 

 frequency being higher than that of the control when the regu- 

 lation temperature is higher than that of acclimation and vice 

 versa. As might be expected, the head-frequency method of 

 demonstrating the effect of acclimation is not as delicate as the 

 other methods employed, and consequently longer periods of 

 acclimation are required to give consistent and uniform results. 

 Since the physiological methods have shown that at least some 

 degree of acclimation to temperature occurs within a few hours 

 (see pp. 307-8 above), it is evident that when pieces are brought 

 into the new temperature for regulation the effect of any tem- 

 perature to which they have been previously subjected will be 

 more or less compensated by the beginning of acclimation to the 

 new temperature even before head-frequency is determined. 

 Consequently, this method can be expected to show only the 

 larger effects resulting from longer periods of acclimation. 



VII. DISCUSSION. 



The experimental data on susceptibility, on CO 2 production as 

 estimated both colorimetrically and by the biometer, and on 



