74 STUDIES IN GENERAL PHYSIOLOGY 



convex side of the watch-glass was turned outward. When 

 direct sunlight fell upon the glass, the rays were focused a 

 few centimeters behind the. glass wall. Notwithstanding 

 this fact, the positively heliotropic animals moved in the 

 direction of the rays from the room side of the glass box 

 through the focal point to the front of the box, although the 

 intensity of the light was the greatest in the focus. This 

 could be shown very beautifully in some tiny, positively 

 heliotropic worms I found in the brackish water at Kiel, but 

 whose identity unfortunately I failed to determine. 



Positive heliotropism is encountered more often in the 

 plant kingdom than negative heliotropism. It is worth while 

 to mention the fact that positive heliotropism appears to 

 exist in more species in the animal kingdom also than does 

 negative heliotropism. 



All caterpillars and Lepidoptera, whether they fly by 

 day or night, can, according to my observations, be con- 

 sidered positively heliotropic. Thus far I have tried in vain 

 to find negatively heliotropic Lepidoptera or caterpillars. 

 The great majority of the other winged insects are also 

 positively heliotropic. 



We also encounter positive heliotropism in animals which 

 live in water, and even in mud, and which therefore can 

 never profit by light. I was much interested in some obser- 

 vations I made in this direction on a small Crustacean (Cuma 

 Rathkii) which lives on the bottom of the bay of Kiel. The 

 animal can be fished out of the mud in which it buries itself 

 only with a dragnet. Notwithstanding this fact, the animal 

 is strongly positively heliotropic. When I kept these small 

 crabs in a glass vessel and allowed light to fall upon them 

 from one side only, the moving animals collected at the side 

 of the vessel nearest the light. The resting animals were 

 oriented, and turned their oral poles toward the source of 

 light and their median planes in the direction of the rays. 



