REACTIONS OF PARAMOECIUM CAUDATUM TO LIGHT 337 



heating. The experiments were carried on in a darkened room 

 so that the only possible light rays were those from the electric 

 light used to illuminate the animal. 



The size of the hanging drop was such that the path of the 

 animal was very free from the sudden swervings due to hitting 

 the boundaries of the drop. At least one minute was always 

 allowed, for the purpose of overcoming the mechanical stimula- 

 tion due to the transfer of the animal to the hanging drop, 

 before any measured observations were taken. Some series were 

 made in which the maximum stimulus was the first used and 

 then the intensities were gradually decreased until the minimum 

 intensity was reached.' Other series were run in the opposite 

 direction, while a third set consisted in sudden jumps from one 

 extreme intensity to the other. All records were made in a 

 room in which the temperature was always between 2 1° and 22° C. 



The experiments were first made with the object of measuring 

 the possible effect of the various light intensities upon the width 

 of the spiral pathway. Animals were placed either in water or 

 in weak gelatin solutions, which slightly retarded their speed, 

 and careful measurements of the widths of the spirals, under 

 various intensities of light were made. There was no measur- 

 able difference in the width of the spirals, no matter what 

 intensity of stimulus was used, but a measurable change in 

 speed was found, which must have been due to a change of 

 ciliary activity in response to a change in the stimulus. The 

 greater the intensity of the light used, the faster the animal 

 moved, hence the greater the ciliary activity. 



The distances of the light from the hanging drop and the 

 corresponding intensities of the light were as follows: — 



Distance in cm 15 25 50 75 100 125 150 175 200 125 250 



Intensities in C. M.. 1422 512 128 58 32 20.5 14.2 10.5 8 6.5 5.1 



The average rate of speed of the animals in response to each 

 intensity, irrespective of conditions preceding the trials as 

 regards the light environment, and including animals from both 

 conjugating and non-conjugating lines, was as follows: — 



Intensities in C. M . . 1422 512 128 58 32 20.5 14.2 10.5 8 6.5 5.1 

 Average velocity in 

 cm. permin. '. 8 7.95 6.95 6.35 6 5 4.75 4.50 3.75 3.5 3 



