HELIOTKOPISM OF ANIMALS 77 



shade, while the aboral pole is exposed to the sun, the ani- 

 mal does not bring its median plane into the direction of the 

 'rays of light and does not move in this direction, although it 

 may become very restless. Light therefore prescribes the 

 direction of the progressive movements in these animals 

 when it strikes the oral pole, but it does not have this effect 

 when the light falls only upon the aboral pole. 



I have already called attention to Engelmann's observa- 

 tion that Euglena viridis reacts clearly only when the light 

 falls upon the oral pole. Euglena viridis possesses a pig- 

 ment spot at this pole which is called the "eye-spot." 

 Physiologically this expression is a very unhappy one, as we 

 do not know that this spot has anything in common with the 

 functions of our eye. Engelmann, however, expressly states 

 that the most sensitive spot of the Euglena is not the pig- 

 ment spot, but the colorless protoplasm lying just in front of 

 it. 



The earthworm has no eyes. Hoffmeister found that the 

 animals recede from the light when the anterior end of the 

 body is illuminated. If, however, the oral pole is shaded 

 and the rest of the body is illuminated, no effect is produced. 

 Darwin repeated and corroborated this experiment. 1 



Fresh-water Planarians have eyes and are negatively helio- 

 tropic, but their sensitiveness to light is not very great. I 

 cut Planarians in two in the middle. The oral piece reacted 

 to light soon after the operation, just as the whole animal. 

 Not once, however, did I see indications of a heliotropic 

 movement in the aboral piece before regeneration had 

 set in 2 (which is very complete in these animals), even 

 though the aboral piece was by no means inactive, but crept 

 around very energetically in the glass and reacted promptly 

 when touched. When I placed both pieces on the window 



l DARWIN, Die Bildung der Ackererde durch die Thdtigkeit der Warmer, trans, 

 by CAKUS, 1882, pp. 11 ff . 



2 1 observed different facts in an American fresh-water Planarian. [1903] 



