HELIOTROPISM 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. 
T have already called attention to Engelmann’s observa- 
tion that Euglena viridis reacts clearly only when the light 
falls upon the oral pole. Huglena 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.’ 
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? (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 
1DARWIN, Die Bildung der Ackererde durch die Thitigkeit der Wiirmer, trans. 
by CARUS, 1882, pp. 11 ff. 
2 observed different facts in an American fresh-water Planarian. [1903] 
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