GEOTROPISM OF PARAMECIUM AND SPIROSTOMUM. 7 



loaded animals which move upward in this stream after dis- 

 persing above into the weaker part of the magnetic field tend to 

 sink again and cause a return circulation to the bottom. The 

 magnet is effective in producing this circulation by diminishing 

 the effect of gravity on animals containing iron. It also exerts a 

 passive pull upon them and they gradually swing into their 

 finally oriented position in a vertical path under the combined 

 influence of the magnet and gravity" (4, p. 189). Harper could 

 not get any definite results showing a direct effect of the magnet. 

 "The movement toward the magnet," he says, "is the most 

 diffused feature of the circulation, so that superficially it might 

 be set down as the result of the random movements of the 

 animals" (4, p. 185). 



i. Experiments on Paramecium caudatum. 



Experiment a. Judging from the results of both his experi- 

 ments, Harper contributes little tp the solution of our problem, 

 except his criticism on Lyon's experiments already mentioned. 

 Hence the present writer looked for a method that would obviate 

 Harper's objection to Lyon's conclusions as much as possible. 

 He took advantage of the fact that the paramecia are positively 

 rheotropic, that is, they show the positive reaction to water 

 currents as already stated. For this purpose, he repeated Lyon's 

 method of centrifugalization with a capillary tube like that 

 described by Lyon. A glass tube about 4 mm. in diameter and 

 7 cm. in length was drawn at the middle into a capillary tube so 

 fine that paramecia (and spirostoma) could not turn around in it. 



The parts of abc and cde were both about 3.5 cm. long. The 

 end of e was "sealed " in Lyon's case, but it was left open in those 

 used by the writer for the following reasons. The heavier ends 

 of the animals, when centrifuged in the tubes so prepared, are 

 not only passively thrown to the end of the tube away from the 

 axis of the centrifuge so that they close the open end of the tube 

 themselves, but at the same time the current of water is also 

 thrown in the same direction with the animals. The animals 

 being positively rheotropic, as Jennings found (9, pp. 468-473), x 



1 The present writer also found that their positive rheotropism is often stronger 

 than their negative geotropism. 



