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. 
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