AN EXPERIMENTAL STUDY OF ACCLIMATION TO TEMPERATURE IN PLANARIA DOROTOCEPHALA

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Abstract
1. The susceptibility of Planaria dorotocephala to toxic concentrations 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 undergoes 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. 3. Estimations of CO2 production by a colorimetric method and by the Tashiro biometer give essentially the same results as the susceptibility method. The increase in CO2 production which occurs when animals are brought from a lower to a higher temperature is followed by a gradual decrease at the higher temperature; and the decrease in CO2 production which occurs when animals are brought from a higher temperature is followed by a gradual increase in CO2 production at the lower temperature. 4. Experiments on the effect of temperature on head-frequency in the regulation of pieces give result similar to those on susceptibility and on CO2 production. Rise in temperature increases, fall in temperature decreases, head-frequency, but acclimation determines changes in head-frequency in opposite directions from those directly determined by change of temperature. Animals which have become acclimated to a high temperature show a lower head-frequency in regulation than those brought into that temperature at the beginning of regulation; and animals acclimated to low temperature show a higher head-frequency than those brought into that temperature at the beginning of regulation. 5. All the experimental data agree in indicating that, within the temperature range of the experiments, acclimation to a change in temperature upward consists in changes which manifest themselves physiologically as a gradual decrease in rate of metabolism or oxidation; and acclimation to a change in temperature downward consists in changes which manifest themselves physiologically as a gradual increase in rate of metabolism or oxidation. 6. The working of the regulating mechanism concerned in acclimation to temperature resembles as regards its action on metabolism the temperature-regulating mechanism of warm-blooded animals, but is very much slower and less effective as a compensatory mechanism than the latter. It may perhaps be regarded as representing the general basis from which a temperature-regulating mechanism has developed.

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