76 STUDIES IN GENERAL PHYSIOLOGY 
surfaces and morphologically different oral and aboral poles. 
We disregard all other detailed morphological peculiarities, 
because those mentioned suffice to explain the orienting 
movements of an animal, as they do for the movements of 
plants. The distribution of irritability on the surface of 
an animal corresponds to the above-mentioned morphologi- 
cal relations. Elements at the surface of the body sym- 
metrically situated with reference to the median plane have 
equal irritabilities. This condition compels the animal to 
orient itself toward a source of light in such a way that the 
rays of light strike the symmetrical points in the body at 
equal angles; this is the case when the animal places its 
median plane in the direction of the rays of light. Points 
on the dorsal or ventral surface equidistant from the 
median plane have unequal irritabilities, the irritability 
being in general the greater the nearer the points are to the 
oral pole. In the same way, the irritability of a dorsal ele- 
ment is different from the irritability of the opposite ventral 
element. If these assumptions regarding the connection 
between irritability and the main structure of an animal are 
correct, it follows, without further discussion, that an animal 
with bilateral symmetry is compelled to place its median 
plane in the direction of the rays of light and to move in 
this direction either toward or away from the source of 
light. We must therefore prove that the described distribu- 
tion of irritability on the surface of an animal is not fiction, 
but reality. 
1. The oral pole of an animal is more irritable heliotropi- 
cally than the aboral pole, no matter whether the animal 
has eyes or not. 
I have already mentioned that the blind adult Musca 
larva immediately places the entire median plane of its body 
in the direction of the rays of light when sunlight strikes only 
its oral pole. When, however, the oral pole remains in the 
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