CHAPTER VII 



EXPERIMENTS ON ARTHROPODS 



i. Experiments on the lowest animal forms have 

 taught us that the peculiar reactions of animals are 

 determined, first, by the different forms of irrita- 

 bility of the elements composing the tissues, and, 

 second, by the arrangement of the muscle-fibres. 

 The central nervous system does not control response 

 to stimulation : it merely serves as a conductor from 

 the point of stimulation to the muscle through which 

 weaker stimuli may pass, and pass more rapidly than 

 would be possible if the muscle were stimulated 

 directly. 



In the Annelids each ganglion is the relay station 

 for the sensory and motor nerves of the correspond- 

 ing segment. If the head exercises a stronger in- 

 fluence upon the behaviour of the animal than any 

 other segment, as in Nereis, for instance, I believe 

 it is due to the fact that in the oral end more kinds 

 of irritability are present and more peripheral organs 

 are differentiated (sense-organs, mouth, etc.) than in 

 the other segments. The fact that in this case the 

 sympathetic nervous system takes its origin from the 



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