THE NERVOUS SYSTEM 



169 



discussed later, lack myelin but are provided with sheath cells. 

 Finally, nerve terminations may consist of axis cylinders (naked 

 axons) only. 



The entire nervous system, including the sensory portions of 

 the sense organs, develops from the ectoderm, with the possible 

 exception of taste cells, which in some cases of vertebrates at 

 least are said to be endodermal in origin. An ectodermal origin 

 of the nervous system is what one would expect since the ecto- 

 derm is the primitive outer covering of the body and therefore the 



A B 



Fig. 111. — Nervous mechanism of Hydra. A, epitheliomuscular cell from ecto- 

 derm; B, ectodermal nerve plexus. The long fibrils in the background are the 

 contractile parts of epitheliomuscular cells. They lie in the mesoglea. {From 

 Schneider.) 



portion of the body through which the environment is sensed. 

 Neurons apparently are ectodermal cells that have gradually 

 taken on a nervous function, accompanied by structural differ- 

 entiation from other ectodermal cells. 



Diffuse Nervous System. — A primitive form of nervous system 

 is found in Hydra and related forms, consisting of nerve cells 

 with branching processes, located in the ectoderm. The cells 

 are arranged in a loose network, showing no marked tendency to 

 be concentrated at any points to form nerve centers or ganglia. 

 The nearest approach to centralization is about the mouth 

 (hypostome) where the nerve cells are more numerous than 

 elsewhere. The arrangement of the nervous system of Hydra 

 suggests a crude mechanism for receiving stimuli and correlating 



