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 



