148 THE ELEMENTARY NERVOUS SYSTEM 



found to be very noticeably insensitive to food. It, there- 

 fore, seemed clear that it was the food in the digestive cav- 

 ity rather than any accidental overflow that had influ- 

 enced the tentacles. 



Reasons have already been pointed out for believing 

 that the change in the responses of the tentacles after 

 continuous feeding is due to sensory fatigue and not to a 

 general metabolic change; this seems also to be true in 

 the particular instance under consideration. Though the 

 meat juice injected into the digestive cavity unquestiona- 

 bly serves as material for metabolism and eventually must 

 have its influence on the animal's general state, its first 

 condition is that of a component of the fluid mixture that 

 bathes the inner surfaces of the actinian. These surfaces 

 include the cavities of the tentacles. As shown elsewhere 

 (Parker, 1917 b), substances in solution in the digestive 

 space of such organs as the large tentacles of Condylac- 

 tis penetrate in a very short time the thin walls of these 

 parts and thus make their way to the exterior. In doing 

 so they must come in contact with the sensory ectoderm. 

 Since the changes in the reactions of the tentacles pro- 

 duced by food juices injected into the digestive cavity 

 are in the direction of diminished response, and since 

 these changes come over the tentacles with consider- 

 able rapidity and before a modified metabolism depen- 

 dent upon new food could have got much headway, it is 

 believed that the loss of responsiveness in this instance, 

 like that in the former case, is due to sensory fatigue and 

 not to changed metabolism. In the first instance the 

 fatigue was produced by the direct application of stim- 

 ulating substances to the exterior of the tentacles ; in the 

 second to the transfusion of those substances from the 

 cavities of the tentacles to their sensory mechanism. If 



