HELIOTROPISM OF ANIMALS 15 
When the source of light or the vessel was carefully turned 
about, the animals changed their orientation until their 
median planes were again in the direction of the rays of 
light. The directing force of the light exhibited itself here 
in the same manner as in the Euglenz in the experiments of 
Stahl. 
I next placed a small glass box filled with mud in the 
same vessel with the crabs. The animals did not scent the 
mud; at least not one of them moved into the box containing 
the mud. When I disturbed the animals (by touching them 
with a pencil), they first swam upward and then, if I did not 
disturb them further, slowly fell to the bottom. If an 
animal happened to fall upon the mud, it immediately became 
lively as soon as it touched the mud. It burrowed into it 
eagerly, after which it was impossible to get the animals to 
react to light. The other animals which fell to the glass 
bottom of the vessel remained inactive. 
Thus we see that contact with the mud had a greater 
effect than light; contact-irritability is more intense than 
heliotropism. It is in this way that it happens that the 
animal, besides being a poor swimmer, lives away from the 
light in spite of its positive heliotropism. 
XII. THE DEPENDENCE OF THE ORIENTATION UPON THE 
FORM OF THE BODY 
In the introduction I have called attention to the fact that 
the orientation of an animal toward light, like every phe- 
nomenon of irritability, is determined by two factors: first, 
external causes—in this case the light—and, second, inter- 
nal causes, namely, the structure of the animal. 
So far as the structure of the animals is concerned, we 
are dealing in this paper exclusively with animals whose 
bodies consist of two morphologically symmetrical halves, 
and which have morphologically different ventral and dorsal 
Digitized by Microsoft® 
