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.’ 
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. Jt 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 
FIG. 100 
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. 
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