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 so on. 1 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 role 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 



1 It is however possible that the stimulus is conducted to the individual muscle 

 fibers through nerves. [1903J 



