EXPERIMENTS ON ASCIDIANS 37 



For some time after the operation, in most cases for 

 about twenty-four hours, the animals remained con- 

 tracted. At the end of this period they began to re- 

 lax again. To my great surprise, I found that the 

 typical reflex continued. If we let a drop of water 

 fall on such an animal, the typical reflex act is pro- 

 duced just as in the normal animal. Hence the reflex 

 cannot be determined by specific structures of the 

 ganglion. But what does determine the reflexes, and 

 what is the function of the ganglion ? 



The answer to the first question must be that the 

 reflex is determined by the structure and arrange- 

 ment of the peripheral parts, especially the muscles. 

 The mechanical stimulus throws the muscles directly 

 into activity, and the stimulation is transmitted from 

 muscle-element to muscle-element directly, as in the 

 heart or the ureter. But is the central nervous system 

 superfluous in this animal ? We get the answer to 

 this question if we determine the threshold of stimul- 

 ation. The threshold of stimulation for producing 

 this reflex is higher in animals which have been 

 operated upon than in normal animals. As the 

 source of the stimulus, I used the kinetic energy of 

 drops of water, which fell from a pipette upon the 

 animal. Since the weight of the falling drop in the 

 pipette is always the same, the minimum of the height 

 from which a falling drop can produce a contraction is 

 a convenient measure of the irritability ; the latter is 

 of course equal to the reciprocal value of the thresh- 

 old of stimulation. In one case there were in an 



