BRAIN PHYSIOLOGY OF WORMS 355 



are put into a vessel having a cylindrical form, they do not 

 collect as do purely heliotropic animals on the window or 

 room side of the vessel but on the right and left sides of the 

 same. The Planarians from which the head together with 

 the brain and the eyes have been removed behave in exactly 

 the same way. All these experiments are successful as early 

 as the day after the operation. Only perfectly fresh material 

 must be used for these experiments. In warm weather the 

 posterior pieces regenerate a new head with eyes and probably 

 a brain after a week. 



3. It was formerly the custom to conclude that an animal 

 possessed eyes when it reacted to light. And since no one 

 doubted that the reaction to light was a reflex movement it 

 was assumed that such animals possessed a central nervous 

 system also. When I found that the orientation of animals 

 toward light is determined by the same conditions as the 

 orientation of plant organs toward the same stimulus I drew 

 the self-evident conclusion that the orientation of the animals 

 toward the light could not possibly rest upon characteristics 

 which are possessed only by the eyes or only by the brain, 

 as plants do not possess such organs. The sensitiveness of 

 the eye to light must rather rest upon the fact that the eyes 

 have a condition in common with heliotropic plants, namely, 

 elements which suffer some change under the influence of 

 the light. For the rest, however, these elements need not 

 be either morphologically or chemically identical in the dif- 

 ferent organisms. An analogous conclusion may be drawn 

 for the brain. When an animal with a brain shows the same 

 reaction as a plant which possesses no brain it follows that 

 the reaction toward light is not the result of a "specific 

 energy" of the brain, but that the brain in that case only 

 performs a function which is possible in plants without 

 nerves. This function need be nothing else than the con- 

 duction of the light stimulus, which, as is well known, occurs 



