HELIOTBOPISM OF ANIMALS 79 



my knowledge, no observations on this subject have as yet 

 been made on animals. 



Planarians and leeches afford an example of the differ- 

 ence between dorsal and ventral irritability. In leeches the 

 ventral surface, which has no eyes, is more sensitive to light 

 than the dorsal surface. It has already been said that this 

 animal leaves the dorsal side of its aboral pole exposed to the 

 light, if only its head is protected from the light. Such 

 animals stick to the side of a beaker, so that their ventral 

 surfaces, which carry the suckers, are directed outward. If 

 diffuse light of a sufficient intensity falls upon the ventral 

 surfaces of the animals, most of them leave the window and 

 move to the room side. The animals then turn their dorsal 

 surfaces to the light. 



In this case, as in all the others, only the more refrangible 

 rays are chiefly active. When the animals are covered with 

 red glass, orientation does not follow, or only after some time. 

 If blue glass is held over them, the orientation takes place 

 just as in diffuse daylight. 



The difference between the irritability of the ventral and 

 the dorsal surfaces of dorsiventral animals is therefore com- 

 parable with that between the oral and the aboral poles. 



3. The dependence of the irritability of a dorsiventral 

 animal on the symmetry of its body must yet be discussed. 

 Those elements of the body of a dorsiventral animal which 

 occupy symmetrical positions with reference to the median 

 plane have equal irritabilities. 



The facts which prove this are to such an extent objects 

 of daily experience that a brief allusion to them will suffice. 

 If a touch on one side of the animal calls forth a movement 

 to the left, then the same stimulus applied to the opposite 

 symmetrical spot on the body will cause the same amount of 

 movement to the right. An object appearing in the right 

 field of vision causes the same amount of movement as one 



