512 



THE EYE IN EVOLUTION 



Figs. 666 to 670. — The Evolution of the Nervous System. 



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Fig. 666. — Two individuals of a 

 protozoan colony joined by inter- 

 cellular bridges. 



Fig. 667. — The impossibility of a 

 direct cell-to-cell link in a complex 

 multicellular organism. Five cells 

 require 10 two-way channels. 



Fig. 668.- 



-The nerve-net — a common network linking up all the cells with 

 a trunk pathway between two important cells. 



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Fig. 669. — The ganglionic nervous 

 system of Arthropods. 



Fig. 670. — The central nervous system 

 of Primates. 



organism becomes coordinated in its response to a single stimulus. Thus a 

 nervous system was born. A subepithelial nerve-net of this type made 

 its first appearance in Coelenterates, but it is obvious that a diffuse network 

 without short-circuiting and centralization is both wasteful and inefficient. 

 Fig. 6G7 shows that to link up a large number of cells in this way becomes 

 a practical impossibility : to link up 5 requires 10 two-way interconnections ; 

 the most that can reasonably be done is to send out a call of general aware- 

 ness. It has been a commonplace to compare the nervous system with a 

 telepl; exchange ; if any single subscriber is to be put m contact with 

 any ot conomically, cables rather than a multiplicity of individual wires 



