ACCLIMATION IN PLANARIA DOROTOCEPHALA. 293 



plained on the ground that this high temperature is close to the 

 limit at which these animals can live at all after a sudden change; 

 it has been found that they cannot live in a temperature above 30 

 unless the temperature is raised very gradually. 



The susceptibility method shows then that exposure to a 

 given temperature for even so short a time as 12 hours produces 

 a change in physiological condition (Tables II., III.). When 

 such a 12-hour period of exposure to a given temperature is 

 followed by exposure to a higher temperature, the susceptibility 

 determined at the higher temperature is higher than that of 

 animals which have been living indefinitely at this higher tem- 

 perature. The susceptibility determined at a temperature lower 

 than that of the 12 hours' period is lower than the suscepti- 

 bility of worms which have been living indefinitely at that lower 

 temperature. With increase in the period of exposure to a given 

 temperature this effect shows in general an increase (Table I.). 



V. CARBON DIOXIDE PRODUCTION. 



The second method used for demonstrating the differences 

 in metabolic rate at different temperatures consisted in a com- 

 parison of the COg output of different lots. The measurement 

 of COa has long been used by physiologists as one of the best 

 methods available for estimating the change in metabolic rate 

 under experimental conditions. In these experiments carbon 

 dioxide output was measured in two ways: by the colorimeter 

 method and by the biometer. 



A. The Colorimetric Method. 



The colorimetric method was an adaptation of that used 

 extensively by Haas in the laboratory of Plant Physiology 

 of Harvard University. It consisted essentially in measuring 

 the comparative CO?, output of two lots of worms in terms of the 

 color change in an indicator solution in which the worms were 

 tested. The following indicators were tried: 



Alizarin Congo red Phenolpthalein 



Methyl orange Benzo-purpurin Phenolsulphone-pthalein 



Neutral red Liquid litmus 



Of these the turning point of the first three proved to be neither 

 sufficiently close to the PH of the well water, nor sharply dif- 



