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THE POPULAR SCIENCE MONTHLY 



exist in the body of a protozoan and in fact there is experimental 

 evidence to show that in certain infusorians the superficial protoplasm 

 is somewhat differentiated as a receptive surface and that this proto- 

 plasm also serves as a conducting organ whereby, for instance, the 

 activity of certain groups of specialized cilia in these animals is coor- 

 dinated. These conditions, however, are found within the substance 

 of a single cell and are so remote from those of a true nervous mech- 

 anism that, interesting and significant as they are, they had better be 

 termed neuroid than nervous. They show at best that the protoplasm 

 of the protozoan harbors operations that may develop in the multi- 

 cellular animals into reflex proc- 

 esses rather than that the proto- 

 zoans possess these processes, and 

 that we must look among the 

 simplest metazoans for the begin- 

 nings of a true neuromuscular 

 mechanism. 



In making a quest for the 

 first stages in the development of 

 the nervous system, it is impor- 

 tant to keep in mind the relative 

 significance of the three physio- 

 logical elements already pointed 

 out: the receptors, the adjustors 

 and the effectors. A little reflec- 

 tion will show that these three 

 are not likely to prove all of 

 primary significance. 



A receptor or sense organ 

 alone would be of no service 

 whatever to an animal; it would 

 resemble a telephone receiver dis- 

 connected from the rest of the 

 system. In a similar way the 

 adjuster or central organ is use- 

 less without at least some other 

 element in the reflex apparatus. 

 The only mechanism sufficient in itself is the effector, which, if it can 

 be brought into action by direct stimulation, may accomplish something 

 serviceable to the animal. It is therefore improbable that we shall find 

 multicellular animals that possess either receptors or adjustors without 

 effectors, but it is conceivable that primitive metazoans may have ef- 

 fectors without other parts of the typical neuromuscular mechanism. 

 In a search for the earliest traces of the neuromuscular mechanism, 



Fig. 2. Diagram of the Canal System 

 of a Calcareous Sponge (modified from 

 Haeckel). The lateral Inlet pores receive 

 water from the exterior, as shown by the 

 arrows on the sides ; the osculum at the 

 apex discharges water to the exterior. 



