REACTIONS OF PARAMOECIUM CAUDATUM TO LIGHT 339 



vious work had led me to expect. In the beginning no change 

 in the response of the animal to the increase or decrease of the 

 stimulus was observable. The rate at which the animal had 

 been traveling before the change in stimulus was made, was 

 continued, and this lasted sometimes for as long as two minutes 

 after the change had been made. If, however, the new stimulus 

 was maintained for a longer period, a change in the rate of 

 locomotion was noted, and gradually the response became the 

 one found to be normal for the given intensity of stimulus. 

 This slowness of response to sudden changes of the light may 

 perhaps account for the fact that former observers have not 

 noted the increased speed of Paramoecium in response to an 

 increase of the light stimulus. 



Another curious fact was observed in the behavior of animals 

 that were undergoing conjugation. In such cases about 40 per 

 cent of the animals were entirely irresponsive, or only very 

 slightly responsive, to any change in the intensity of the light 

 stimulus. Often the difference in response was noticeable only 

 when the records for the two extremes of intensities were com- 

 pared. In non-conjugating lines only 10 per cent of the indi- 

 viduals were poorly responsive to changes in the intensity of 

 the light. 



In 2 per cent of the animals tested there was found negative, 

 or rather inhibitory, response to increased light. The greater 

 the intensity of the light, the slower the locomotion until at 

 the upper limit (1422 C. M.) practically all movement ceased. 

 The power of motion was slowly regained as the intensity of 

 the light was decreased. 



There were no differences found between the responses of the 

 animals kept in daylight, in electric light, or in darkness for 

 the twelve hours preceding an experiment. 



Both fresh and stagnant water were used in order to ascertain 

 whether the amount of oxygen present has any effect on the 

 activity of Paramoecium, but no such effect could be detected. 



Although the light was also used to illuminate certain por- 

 tions of the field to a greater extent than other portions, there 

 was absolutely no evidence in the motions of Paramoecium of a 

 directive or an orienting effect of this unequal lighting. 



The conclusions that I have been led to in this study of the 

 reactions of Paramoecium in response to increases and decreases 

 of the light are as follows: — 



