268 STUDIES IN GENERAL PHYSIOLOGY 



Nor do the animals collect at D on the room side of the dish, 

 but rather at E. The movements, therefore, occur in the 

 direction of the rays of light. If the experiment is to be 

 demonstrated to others, a shadow may be thrown into the 

 vessel by a rod, in which case one can see directly that the 



animals move parallel to 

 the shadow. 



Attention need scarcely 

 be called to the fact that 

 if rays of light strike the 

 animal simultaneously 

 from various directions, 

 and the animal is able to 

 move freely in all direc- 

 tions, the more intense rays 

 will determine the direc- 

 tion of the progressive movements. 



That it makes no difference to the negatively heliotropic 

 Limulus larvae whether they go from regions of less intense 

 light to regions of greater intensity that is to say, from 

 the "dark" into the "light"- but that only the direction 

 of rays of light determines the direction of the progressive 

 movements, is shown by the following experiment. Let AB 

 in Fig. 64 again be the plane of the window ; SS l the hori- 

 zontal projection of the sun's rays falling into the room 

 obliquely from without and above. The horizontal part of 

 the window frame casts the shadow CD upon the table. The 

 strip CD will, of course, be illuminated by reflected daylight. 

 I placed the vessel ef containing the Limulus larvse upon 

 the table so that the window side e of the dish lay in the 

 shadow, while the room side / of the dish was in the sunlight. 

 At the beginning of the experiment the larv were collected 

 in the shadow on the side of the dish nearest the window. 

 They at once began to move to the room side in the path of 



