202 THE ELEMENTAEY NERVOUS SYSTEM 



have other functions than the control of muscles, it ap- 

 pears that neither of these names is especially appro- 

 priate. They are the cells in the simpler types of nerv- 

 ous system from which the neurones of the more complex 

 types have been derived. Hence the name protoneurone, 

 as already suggested, is not inappropriate (Parker, 

 1918 a). A receptor-effector system whose nerve-net in- 

 cludes protoneurones is, therefore, a step nearer a cen- 

 tralized nervous system of the 

 higher animals than one in which 

 the protoneurone is not present. 

 Although a receptor-effector 

 system may consist of highly 

 differentiated sense organs 

 connected by a protoneurone 

 nerve-net with distantly located 

 muscles, it nevertheless is very 

 far from representing even a 

 simple central nervous system. 

 First of all, such a receptor- 

 effector system exhibits in a 

 marked degree a state of diffuse- 

 ness. The animals possessing such systems have no single 

 nervous organ to which their nervous experience 

 may be said to be referred and from which their im- 

 pulses to response emanate. Their nervous activities are 

 uncentralized. Each important organ, like the tentacle 

 or the foot of an actinian or the column of Corymorplia, 

 has within itself its own neuromuscular organization, and 

 the autonomy thus conferred on this part is one of the 

 characteristics of the effector-receptor system. 



This system is further characterized by great diffuse- 

 ness of transmission. Any nervous impulse started up 



m 



Fio. 47. Diagram of a com- 

 plex type of receptor-effector sys- 

 tem such as is seen in many parts 

 of sea-anemones. It consists not 

 only of receptors r, with their 

 nerve-nets, and of muscle cells m, 

 but also of the so-called ganglion 

 cells g in the nerve-net. 



