STIMULI AND THEIR ACTIONS 453 



microscope is so placed that it receives bright sunlight and reflects 

 it through the diaphragm. After the introduction of an opaque 

 plate between the stage and the mirror, an Amceba is so placed by 

 the aid of direct light that, its direction of motion remaining con- 

 stant, it must creep beyond the edge of the black paper. As 

 soon as the anterior end of the Amoeba has passed over the edge of 

 the opening, the opaque plate between the mirror and the stage is 

 suddenly removed, so that the concentrated rays of the sun fall 

 upon that end, while the posterior end is still in the shade of the 

 paper. The result is that the Amoeba immediately changes its 

 direction and flows back into the shade (Fig. 226). That this 

 is a pure heat-effect of the sun's rays and not a light-effect can be 

 decided at once by excluding either the chemically acting light- 

 rays by the introduction of an absorbing solution of iodine in 

 carbon bi-sulphide, or the heat-rays by the introduction of plates of 

 ice or alum. In the former case the thermotactic effect is as 



FIG. 227. Thermotaxis of Paramcecium. In a black ebonite trough, 10 cm. in length, aro 

 numerous Paramceda ; upon unilateral warming of the trough to 24 28" C. they move toward 

 the cooler side. (After Mendelssohn.) 



distinct as in pure sunlight, in the latter it is wanting in spite of 

 the great illumination. Careful tests show that Amoeba is not at 

 all irritable to light. But therm ometric measurement of the 

 temperature in the drop directly over the opening in the black 

 paper shows that at least a temperature of 35 C. must be reached, 

 if the effect is to appear. 



The thermotactic action of different degrees of temperature may 

 be studied best in cilate Infusoria, like Paramcecium, which can 

 be bred in great numbers. If a small ebonite trough be placed 

 upon a metallic plate, and liquid containing Paramceda be placed 

 in it, by warming or cooling differences in the temperature which 

 can be measured by a thermometer can be obtained at the two 

 ends of the liquid. These differences have a pronounced thermo- 

 tactic effect (Fig. 227). The accompanying apparatus, constructed 

 by Mendelssohn ('95), allows heating or cooling with hot or cold 

 water (Fig. 228). With this it is shown that Paramceda at 

 temperatures of more than 24 C. to 28 C. are negatively thermo- 



38' 



