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PARKER— ORIGIN AND SIGNIFICANCE OF [April 21, 



more sluggish coelenterates, as to suggest that the muscles in question 

 act not through the intervention of nerves, but under direct stimula- 

 tion, and since sponges have yielded no evidence anatomical or 

 physiological of possessing nervous elements of any kind, I have 

 concluded that their muscles normally act under direct stimulation. 

 In other words, sponges are metazoans with effectors but without 

 receptors ; and in so far as their neuromuscular mechanism is con- 

 cerned, they are metazoans one degree simpler than the majority of 

 coelenterates. 



If this conclusion concerning the neuromuscular mechanism in 

 sponges is correct, it follows that, of the three elements concerned, 

 the effector or muscle is the most primitive and has developed as an 

 organ quite independent of nerve, as seen in the sponges (Fig. 3, 



T 



nnmnim 



A 





B D 



Fig. 3. Diagram to illustrate the early stages in the differentiation of 

 the neuromuscular mechanism. A, epithelial stage ; B, differentiated muscle- 

 cell at stage of sponge ; C, partially differentiated nerve-cell in proximity to 

 fully differentiated muscle-cell; D, nerve- and muscle-cell of coelenterate stage. 



A, B). Next in sequence would appear the receptor or sense organ 

 which, derived from the cells in the neighborhood of a developed 

 effector (C), would serve as a more efficient means {D) of calling 

 this organ into action than direct stimulation. This stage is repre- 

 sented by many coelenterates ; and their quick responses, as compared 

 with those of sponges, are dependent, I believe, upon this advance 

 in organization. Finally, in forms somewhat more advanced than 

 the coelenterates, central nervous organs or adjustors would begin 

 to differentiate in the region between the receptors and effectors ; 

 and these would develop in the higher animals, lirst, as organs of 

 transmission wherebv the whole musculature of a given form could 



