EVOLUTION OF NERVOUS SYSTEM 89 



ten trials after which only the meat will be taken and the filter 

 paper will be discharged into the seawater without being 

 brought to the mouth. If, after having developed this state 

 of affairs on one side of the mouth, the experiment is now 

 transferred to the opposite side, both the filter paper and the 

 meat will again be taken in till this side has also been brought 

 to a state of discriminating. Thus the experience of one part 

 of the animal has no perceptible influence on another in the 

 sense that there is no common nervous center where the experi- 

 ence of a given part may be put to the service of the rest. It is 

 as though we had to burn each finger in turn before we discover 

 that fire is bad for the hand. These examples bring into 

 strong contrast the types of reaction characteristic of the 

 higher and the lower multicellular animals and serve to illus- 

 trate the difference between the centralized and the diffuse 

 nervous system. 



In a diffuse nervous system, such as that possessed by a sea- 

 anemone, the external surfaces of the animal serve as recep- 

 tors and these communicate directly with the subjacent muscu- 

 lature. The nerve-net serves to spread the impulses throughout 

 the body but without involving any central organ. Thus the 

 sea-anemone may be said to possess receptors and effectors 

 without an adjuster or central nervous organ properly so 

 called. This central organ is, therefore, a feature of the 

 higher animals and in comparison with receptors and effectors 

 it must be looked upon as a more recent evolutionary acquisi- 

 tion. Our sense organs did not develop in consequence of a 

 central nervous system but our central nervous organs devel- 

 oped because our very early ancestors had already acquired 

 receptors and muscles. 



If lowly organized animals, such as sea-anemones, possess 

 nothing worthy of the name of a central nervous organ, it 

 follows that their so-called sense organs must differ consider- 

 ably in function from those of the higher animals. In highly 



