THE NEEVE-NET 119 



ous organization, both of which may exist side by side 

 in the same animal. Judging from the fact that the 

 nerve-net is apparently the exclusive type of nervous 

 organization in the ccelenterates and that it becomes pro- 

 gressively less and less evident the higher the animal 

 series is ascended, it seems fair to conclude that the nerve- 

 net is the more primitive type and that in the course of 

 evolution it has given way more and more to the synaptic 

 system which has finally come to be the dominating plan 

 of nervous organization in the more complex animals. 

 From this standpoint one of these animals might per- 

 fectly well possess both types of nervous structure; 

 nerve-nets having been retained in its more conservative 

 portions and synaptic structures having been developed 

 in its more progressive parts. Thus the nerve-net may be 

 regarded as phylogenetically older than the synaptic sys- 

 tem. If the cell unit of the synaptic system is called the 

 neurone, it would not be inappropriate to designate the 

 more primitive units of the nerve-net as protoneurones 

 (Parker, 1918 a). 



Since the nervous system of the coelenterates consists 

 very largely of nerve-nets, the activities of these ani- 

 mals must reflect in a general way the peculiarities of this 

 type of nervous organization. As the examples that have 

 been given show, the nerve-net is almost always inti- 

 mately associated with the muscles which it excites to 

 action. Commonly the nerve-net is directly superimposed 

 on the muscle layer or the two tissues may be more or 

 less commingled. In either case the activity of the muscle 

 may have a considerable influence on the state of the 

 nerve-net, stretching it or squeezing it or in one way or 

 another changing its physical condition. This relation 

 has been made the basis of some important speculations 



