350 STUDIES IN GENERAL PHYSIOLOGY 



body to come to rest upon its ventral surface. We cannot 

 compel the animal to bring its back in contact with solid 

 bodies, and at the same time expose its ventral surface to the 

 water. 1 



The question now arises whether these phenomena of 

 orientation are a function of the brain 

 as are the spontaneous progressive 

 movements. Strange to say this is 

 not the case. The brainless Thysano- 

 zoon returns to the ventral position 

 when it is laid upon its back, only the 

 reaction occurs more slowly than in 

 the normal animal, or in that portion 

 of the animal containing the brain. 

 Reactions to light could not be demon- 

 strated. 



3. If, instead of making a complete 



transverse section of the animal, only the longitudinal nerves 

 are cut, and the two pieces are left united with each other 

 by a very thin bridge of protoplasm at one side (Fig. 100), 

 the aboral piece is not innervated directly by the nerves from 

 the brain. A conduction of the impulse by way of the 

 lateral nerve plexus is, of course, still possible. 



When, immediately, after the operation, I laid such an 

 animal upon the bottom of the aquarium the oral piece at 

 once began to move, while the aboral piece tried to attach 

 itself to the bottom. It responded, however, to the pull 

 which the oral piece exerted upon it, and took part in a per- 

 fectly co-ordinated manner in the progressive movements, as 

 if no interruption had occurred. After some time the oral 

 piece turned about, crept over the back of the aboral piece 

 whereby the latter was dragged along passively, and laid 



1 The righting of starfish which have been laid upon their backs is also only a 

 case of stereotropism, and has nothing to do with the effects of gravity. 



