SECTION III. CENTRAL NERVOUS 

 I ORGANS 



CHAPTER XIV 



CONCLUSION 



IT is intended that the present volume shall include 

 an extended discussion of only the simplest examples of 

 the elementary nervous system. In this concluding chap- 

 ter, however, a brief outline will be given of the relations 

 of this system to the central nervous system of the more 

 complex animals. An outline of this kind must perforce 

 block out only main contours, for, even were they known, 

 it would be impossible in so limited a space as a single 

 chapter to follow the intricacies to be met with in the 

 evolution of that most complicated system of organs, the 

 central nervous system. 



It was pointed out in the earlier part of this volume 

 that the neuromuscular system probably did not origi- 

 nate primarily as a nervous structure. The first trace of 

 this system is to be seen in independent effectors, the 

 smooth muscle of the lowest multicellular animals (Fig. 

 45). This tissue, as seen in the oscular and pore sphinc- 

 ters of sponges, represents muscle unassociated with 

 nerve and acting under direct stimulation from the en- 

 vironment. Such independent effectors are apparently 

 open to only a limited range of stimuli, particularly to 

 those of a physical type, and are relatively slow and slug- 

 gish in response. They reappear in the higher animals, 

 as in the acontial muscles of the sea-anemones and in the 



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