ACCLIMATION IN PLANARIA DOROTOCEPHALA. 



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metabolic rate is sufficiently high in relation to that of x. All the 

 experimental data agree in pointing to the conclusion that head- 

 frequency increases with increase in rate x in relation to rate y, 

 and decreases with the increase of rate y in relation to rate x. 

 In other words, we may express head-frequency in the general 



rate x 

 formula head-frequency = 



rate y 



In order to show the direct effect of temperature on head- 

 frequency, we may compare the head-frequency of pieces which 

 have been living at a given temperature and undergo regulation 

 at another temperature higher or lower than the first, with the 

 head-frequency of pieces which have been living and undergo 

 regulation at the first temperature. 1 Table VI. shows the effect 

 on head-frequency of a rise from low to medium temperature, 

 Table VII. the effect of a fall in temperature from medium to 

 low, and Table VIII. the effect of a rise from medium to high. 

 In each lot, A, B and C consist of 50 pieces, and the head- 

 frequencies are given in percentages. 



TABLE VI. 



WORMS WHICH HAVE BEEN LIVING FOR Six WEEKS AT Low TEMPERATURE, 



REGULATING AT MEDIUM. 



In Table VI., for example, where the first series of pieces 

 have been living at low and undergo regulation at medium tem- 

 perature, while the second series have been living and undergo 

 regulation at low, the rise in temperature produces a very great 

 increase in head-frequency. In the A pieces, with 24 per cent. 



1 These tables are made up from unpublished records of Professor Child which 

 he has had the great kindness to put at my disposal. Practically all my own data 

 along this line were rendered worthless by the chlorination of the city water. 



