24 THE BEHAVIOR OF LOWER ORGANISMS. 



been so slow in appreciating the real nature of the reactions of the in- 

 fusoria. If Stylonychia or Oxytricha or any other of the Hypotricha 

 had been taken as the usual type for study on reactions, many of the 

 theories now maintained could never have been put forth. The body 

 of Paramecium is comparatively little differentiated, so that it is diffi- 

 cult to distinguish oral and aboral sides, and, to multiply this difficulty 

 many times, the animal revolves rapidly on its long axis, so that oral 

 and aboral sides never retain for two successive instants the same 

 position. It is not wonderful, therefore, that the method of reaction 

 by turning toward the aboral side was not observed in the first investi- 

 gations on Paramecium and that many still find it difficult to observe. 

 Nevertheless, it was on Paramecium itself that this reaction method was 

 first observed (Jennings, 1899), an ^ i ts existence was confirmed later 

 on the organisms where its observation presents no difficulties. Aside 

 from the direct observations of the method of reaction, the following 

 facts throw light on the way in which the collections take place. 



As described in the second of my studies 

 (Jennings, 1899, pp. 314, 315), the collect- 

 ing of Paramecia in regions of optimum 

 temperature may be produced in the follow- 

 ing manner : The infusoria are mounted in 

 ^ water which is above the optimum temper- 



ature (say 30) on a slide beneath a cover 



glass supported at its ends by glass rods. Into this slide is introduced 

 with the capillary pipette a little cooler water (say at 24), which 

 covers a small circular area in the center of the slide. Very soon the 

 Paramecia have collected in this region till a dense group is formed. 

 The same result may be obtained by placing a drop of ice water on the 

 top of the cover glass of a slide of Paramecia which has been warmed 

 considerably above the optimum temperature. (Fig. 9.) 



Are these collections due to the orienting of Paramecium by the 

 heat, as maintained by Mendelssohn for thermotaxis in general? Ob- 

 servation shows that they are not ; that on the contrary the Paramecia 

 gather in the optimum region in the same manner as they gather in a 

 drop of weak acid, as described in my studies. The Paramecia on the 

 heated slide are swimming rapidly in all directions. They do not 

 change their course or become oriented in the least when a spot in a 

 certain part of the slide is cooled. But as a consequence of their 



*Fio. 9. Collection of Paramecia due to the reaction to temperature change. 

 The slide rests on a vessel of water at a temperature of 45. An elongated drop 

 of ice water is placed on the upper surface of the cover glass. The Paramecia 

 quickly collect beneath the drop of ice water. 



