CONCLUSION 



203 



in it at any point is very likely to spread wave-like 

 throughout its whole extent. This form of diffuse trans- 

 mission is in strong contrast with that kind of transmis- 

 sion that is carried out in the nervous systems of the 

 higher animals where impulses follow definite paths in- 

 ward and outward, reaching finally only special groups 

 of muscles or even single muscles. These two features, 



FIG. 48. Transverse section of the ventral nerve-chain of the marine worm Sigalion 

 showing this chain as a thickened portion of the superficial ectoderm in which the sequence 

 of tissues from the exterior inward is superficial epithelium e, ganglion cells g, and nerve 

 fibers/. (After Hatschek, 1888.) 



the absence of a central station and the diffuseness of 

 transmission, both of which are aspects of the same gen- 

 eral condition, are the most striking characteristics of 

 the receptor-effector system and bring this system into 

 strong contrast with that final type of neuromuscular 

 organization that is characteristic of the highest animals 

 and in which a central nervous organ or adjuster is well 

 differentiated. 



This most highly differentiated type of neuromuscular 

 system in which an adjuster or central organ has arisen 



