454 



GENERAL PHYSIOLOGY 



tactic, i.e., swim in crowds away from the warmer side, while with 

 temperatures below this limit they show positive thermotaxis, since 

 they leave the cooler side. There is here a phenomenon com- 

 pletely analogous to chemotaxis and phototaxis, in which the 

 organisms likewise turn from both sides toward a certain degree of 

 intensity of the stimulus. A simple calculation shows how small 

 the difference in temperature can be at the two poles of the body 

 of the Paramcecium, and still produce a thermotactic effect. The 

 length of the surface of the liquid, the smallest just effective 



FIG. 228. Apparatus for the investigation of thermotaxis. A flat trough of black ebonite (Fig. 

 227), in which is liquid containing Paramcecia, rests in a depression upon a metallic plate. 

 The plate has three tubes, through which from ti beaker, by means of a tube, water of a desired 

 temperature can be passed. Above the trough thermometers are attached to a stand, which 

 dip into the liquid containing the Paramcecia and at any moment allow the temperature 

 prevailing in different places to be read off. (After Mendelssohn.) 



differences in temperature at its two ends, and the length of the 

 body of the Paramcecium must be known. In such a calculation, 

 which, of course, can give only approximate values, Jensen found 

 that Paramcecia are still thermotactic when at the two ends of 

 their body, 0*2 mm. in length, a difference of temperature of 0'01 C. 

 prevails. There is here expressed a delicacy of distinction in the 

 intensity of stimuli, which finds analogies both in the data obtained 

 by Pfeffer for chemotaxis and in the slight differences of stimulus 

 effective in phototaxis, but which leave the differential capacity of 

 the human consciousness far behind. 



