10 THE BEHAVIOR OF I.OWKR ORGANISMS. 



left the heated region (c). If one end is heated, while the other is 

 cooled, the Paramecia gather in the intermediate region. 



How are these movements to be explained? Mendelssohn applies 

 to the phenomena Verworn's schema for the orientation of a ciliate 

 organism to a one-sided stimulus (see Figs, i and 2). As we wish to 

 deal thoroughly with this schema, it will be well to set it forth here, as 

 applied by Mendelssohn to heat and cold, with some fullness. 



The temperature being higher at one end of the trough than at the 

 other, that side or end of the animal directed to the heated end of the 

 trough has a higher temperature than has the opposite side or end 

 (see Fig. 4). This difference in temperature causes a difference in the 

 beat of the cilia. In negative thermotaxis the higher temperature 

 causes the cilia to contract more strongly, as indicated by the heavier 

 shade (on the left side) in the figure; hence the animal is turned 

 toward the opposite side, or away from the source of heat, until it 

 comes into a position where the heat acts equally on the two sides. 

 The Paramecium then of course has its anterior end directed from the 



heated region, and its ordinary 

 swimming carries it away. In 

 positive thermotaxis, on the 

 other hand, the lower tempera- 

 ture causes stronger contrac- 

 Fio. 4.* tions ; hence the cilia on the 



side next the cold region con- 

 tract more strongly, turning the anterior end in the opposite direction. 

 The Paramecium then swims away, as a result of its normal forward 

 movement. 



Mendelssohn studied the subject primarily from a quantitative stand- 

 point, determining the optimum temperature, the rate of reaction, the 

 effects of different temperatures, etc. For this purpose he constructed 

 a very ingenious and delicate apparatus, which permitted accurate 

 quantitative results. Relying then upon his valuable papers for these 

 matters, I have devoted myself entirely to a study of the mechanism of 

 the reactions. For this purpose an apparatus was used that is similar 



* FIG. 4. Diagram of the thermotactic reaction of Paramecium as conceived 

 by Mendelssohn, after Mendelssohn (1902, b). The heavier cilia on the left side 

 show those contracting most strongly and hence those most effective in turning 

 the organism or driving it forward. In negative thermotaxis the left end would 

 have the higher temperature, causing the cilia of the left side of the organism 

 a to beat more strongly. As a result, the organism turns, occupying suc- 

 cessively the positions , 3, c, d. In the latter position there is no further 

 cause for turning, and the animal swims directly away from the heated end. 

 The same diagram illustrates also positive thermotaxis, if the left end is sup- 

 posed to be cooled below the optimum. 



