200 THE ELEMENTARY NERVOUS SYSTEM 



heart muscle of the vertebrate embryo, but in the evolu- 

 tion of the neuromuscular mechanism they represent an 

 initial stage and as such they are characteristic of only 

 the simplest forms. 



The second step in this series is seen in the receptor- 

 effector system of such animals as the coelenterates. 

 This step has been accomplished by adding to the inde- 

 pendent effector of the lowest forms a receptive element 

 in the nature of a sensor} 1 - surface. Such receptors arose 

 by the modification of those epithelial cells that were in 

 closest proximity to the already differentiated muscle. 

 These cells, in the form of sensory patches, became espe- 



Fio. 45. Diagram of an independent effector, a muscle cell such as occurs in the lowest 



multicellular animals. 



cially receptive to categories of external changes, such as 

 pressure changes, chemical changes, and changes in heat 

 and light. The disturbances that these changes initiate in 

 the receptive protoplasm are the means by which the sub- 

 jacent muscles are brought into action. Thus the novel 

 element in this combination, the receptor, serves as a 

 trigger, so to speak, to discharge the underlying muscle, 

 and, judging from the quickness with which this discharge 

 is accomplished as compared with that with which the in- 

 dependent effectors discharge, the efficiency of the new 

 system is beyond doubt. 



The receptor-effector system in its simplest conceiv- 

 able form consists of a patch of sensory cells attached 

 to a group of muscle cells, but practically every instance 

 illustrative of this particular combination is complicated 

 by the fact that the central branches of the receptive cells 



