364 STUDIES IN GENERAL PHYSIOLOGY 
as Ciona intestinalis. After the loss of the ganglion, how- 
ever, the animal reacts in the same way when touched as 
before. Only one condition is different. The intensity of 
the stimulus which is necessary to bring about a reaction in 
the brainless Ciona is much stronger than in the intact 
animal. This indicates that the mechanism is different in 
the two cases. In the normal animals the sensory nerves 
are stimulated and the stimulus passes through the ganglion 
to the muscles. In the brainless animal the muscles at the 
irritated point are possibly stimulated directly. The con- 
traction of these muscles is then the cause of the contraction 
of the neighboring muscles and soon.’ Under these circum- 
stances it is more than a mere possibility that in the normal 
Ciona also the characteristic reaction when touched is deter- 
mined not by the brain, but that the brain serves the purpose 
in this case of a better and more rapid conductor of the stimu- 
lus. The nature of the reaction might rather be chiefly 
determined by the arrangement of the muscles. 
The heliotropic phenomena of animals are identical 
in all respects with those of plants. The latter have no 
central nervous system and the remarkable nature of these 
reactions is therefore not necessarily determined in animals 
also by the specific characteristics of their reflex centers. It 
is much more probable that the nervous system plays in this 
only the réle of a conductor of stimuli, while the actual char- 
acter of the process is determined by the following condi- 
tions: (1) The shape of the body and the topographical 
distribution of irritability corresponding with it. (2) The 
changes brought about by the light in the illuminated tissues: 
(3) The conduction of the stimulus to the contractile tissues. 
(4) The arrangement and structure of the latter. 
Examples of the same sort are the observations described 
1It is however possible that the stimulus is conducted to the individual muscle 
fibers through nerves. [1903] 
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