146 GENERAL PRINCIPLES OF ZOOLOGY. 



grouped may be termed the diffuse, the linear, the gangli- 

 onic, and the tubular types. 



Diffuse Nervous System. The diffuse type of nervous 

 system is certainly the most ancestral ; it shows the two 

 elements, nerve fibres and ganglion cells, regularly distrib- 

 uted through the whole body, or, at least, through certain 

 layers of the body. The skin of the body, the ectoderm, 

 is to be looked upon as one of the fundamental elements in 

 the nervous system, since it mediates contact with the ex- 

 ternal world, and hence receives the sensory impressions, so 

 important for the development of nervous tissue. The 

 corals and liydroid polyps serve as examples, since in them 

 the ectoderm is permeated in all directions by a delicate, 

 spider-weblike network of nerve fibres and ganglion cells, 

 which encroach even upon the entoderm. 



Linear Nervous System. From the diffuse type the 

 other chief types can be derived through localization, which 

 indeed is chiefly conditioned by the fact that there are a 

 few points which are most advantageously located for the 

 reception of sensory impressions, and hence for the devel- 

 opment of nervous elements. In the medusa such a 

 place is the rim of the bell ; consequently a stronger neri'c- 

 cord remarkably rich in ganglion cells is found here. This, 

 as well as the nerve-ring and the five ambulacral nerves of 

 echinoderms, may be called a central organ, thereby dis- 

 tinguishing the rest of the nervous network as i\\& peripheral 

 nervous system. 



Ganglionic Central Nervous System. Manifold tran- 

 sitional forms lead us to the ganglionic central nervous sys- 

 tem of the worms, molluscs, and arthropods (Fig. 71). The 

 central nervous system here consists of two or more ganglia; 

 each ganglion being a rounded bunch of regularly arranged 

 nerve-fibres and ganglion-cells. The former constitute the 

 nucleus of the mass, and, since they cross in all directions, 

 give the appearance of fine granulation ; this fact has led 

 to the unsuitable, because misleading, name of " Leydig's 

 granular substance." The ganglion-cells, on the other 

 hand, collect in a thick layer surrounding the " Leydig's 



