78 STUDIES IN GENERAL PHYSIOLOGY 



side of the vessel, the oral piece moved toward the room 

 side, but not the aboral piece. When the oral piece moved 

 from the room side toward the window, it soon turned about. 

 Under similar conditions the aboral piece continued to creep 

 until it reached the window. When the vessel containing 

 the animals was carefully reversed, the aboral animal was 

 not affected, but the oral animal immediately moved toward 

 the room side. 



It can easily be shown that in leeches the head, which 

 contains the eyes, reacts more energetically toward light 

 than the aboral pole. If some small stones are lying on the 

 bottom of a beaker which contains such animals, and the 

 vessel is suddenly illuminated, the animals push their heads 

 under the stones, while the aboral pole remains at rest even 

 though exposed to the light. It is astonishing to notice 

 how long after the illumination the reaction appears. It is 

 not unusual for thirty to seventy seconds to elapse between 

 the illumination and the beginning of the movement. Hoff- 

 meister observed a still longer latent period in the case of 

 the earthworm. It would be unnecessary to show that in 

 animals which possess eyes the oral pole is more sensitive 

 toward light than the aboral. We may therefore accept it 

 as certain that the oral pole of an animal is more sensitive 

 toward light than the aboral, whether the animal does or 

 does not possess eyes. 



In consequence of this fact, it is difficult for an animal to 

 move perpendicularly or obliquely to rays of light emanating 

 from a sufficiently intense source, for, as the oral pole is 

 more sensitive than the aboral, the former must turn more 

 energetically toward or away from the source of light 

 (depending upon whether the animal is positively or nega- 

 tively heliotropic) than the aboral. 



2. The heliotropic irritability is also different on the 

 ventral and dorsal surfaces of a dorsiventral animal. To 



