220 STUDIES IN GENERAL PHYSIOLOGY 



were kept in the same vessel, and at the same distance 

 below the surface of the water. Under a are given the 

 height of the fall in normal, under h in brainless, animals. 

 The figures are given only to show how constant is the differ- 

 ence in the threshold of stimulation between the two ani- 

 mals; the absolute height of the threshold of stimulation 

 cannot be determined from them. 



a b 



8 mm. 65 mm. 



1 75 



10 80 



80 



In two other animals I obtained the following values. 



a b 



6 mm. 22 mm. 



8 20 



The temperature was 13 C. 



This difference in irritability may be due to the fact that 

 the wave produced by the drop of water stimulates the motor 

 nerves or muscles directly, and that the threshold of stimula- 

 tion is higher for these than for the sensory nerve endings. 



3. It can be shown by a different experiment that another 

 path for the conduction of stimuli must exist in the brain- 

 less animal than in the uninjured, where stimuli no doubt 

 travel over the nerves for the most part. If an incision is 

 carefully made in the tube of a Cioiia whose ganglion has 

 been removed, not only the injured portion, but the entire 

 animal, contracts, just as when the same stimulus is applied 

 to an uninjured animal. This also may occur when one 

 point of the oral edge is carefully touched with a needle. I 

 imagine that in a brainless Ciona the motor nerves or muscles 

 lying nearest the point of incision are stimulated mechani- 

 cally, that these contract, and that the jarring or pulling 



