SPONGES 49 



plication of appropriate stimuli to it, has no effect on 

 the neighboring oscula. It must, therefore, be admitted 

 that the physiological findings agree with the histologi- 

 cal results in supporting the conclusion that sponges are 

 devoid of true nervous elements (Minchin, 1900). 



Sponges, then, represent that stage of evolution in 

 which a primitive type of muscle tissue has made its ap- 

 pearance unaccompanied with nervous elements. To 

 state this conclusion in the terms used in the earlier part 

 of this discussion, sponges may be said to have among 

 their cell combinations effectors, but no receptors or ad- 

 justers. They mark the beginnings of the neuromuscular 

 mechanism in that they possess the original and most 

 ancient of its constituents, muscle, around which the re- 

 mainder of the system is supposed subsequently to have 

 been evolved (Parker, 1909, 1910 b, 1911, 1914 b). 



