96 STUDIES IN GENERAL PHYSIOLOGY 



that the animals could not erect themselves. They were so 

 placed in the aquarium that their longitudinal axes lay per- 

 pendicular to the side ab (Fig. 10) of the aquarium, and their 

 pedal extremities M touched the glass wall ab. The side a 

 faced and was parallel to the plane of the window. The 

 animals fastened 



a ^L 3 themselves to the wall 



ab, and then began 

 to react, in their char- 

 acteristic way, to the 

 light, by which the 

 head was turned and 

 the tube became con- 

 cave toward the 

 FIG. 10 source of light. The 



tube MX assumed the position MX^ As soon, however, 

 as the tentacles touched the glass wall ab, the tip N turned 

 away from the glass wall. The heliotropic bending gradually 

 affected all the elements of the tube MX, so that the 

 Spirographis finally reached the position MX 2 , in which it 

 remained throughout the period of observation four 

 months. 



I repeated this experiment a number of times, always 

 with the same result. 



7. The heliotropic phenomena of Spirographis took place 

 both in direct sunlight and in diffuse daylight. The minimum 

 light intensity just sufficient to bring about these phenomena 

 is very small. I have not yet studied the effect of rays of 

 different refrangibility in producing these phenomena. Since 

 thus far the more refrangible rays have proved to be the 

 most effective heliotropically both in plants and animals, it 

 is to be suspected that Spirographis also will prove no excep- 

 tion. 



8. As is well known, Sachs has formulated the law that 



