HELIOTROPISM OF ANIMALS 75 



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 Euglenae 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, 

 arid which have morphologically different ventral and dorsal 



