112 



GENERAL PRINCIPLES OF ZOOLOGY 



Diffuse Nervous System. The diffuse type is certainly the most 

 primitive; it shows the two elements, nerve fibres and ganglion cells, 

 distributed through the whole body, or, at least, through certain layers 

 of it. The skin of the body, the ectoderm, is one of the fundamental 

 elements in the nervous system, since it is related to the external world, 

 and hence receives the sensory impressions, so important for the develop- 

 ment of nervous tissue. The corals and hydroid polyps are examples, 



C 



FIG. 77. Third abdominal ganglion of a crayfish (after Retzius). C, connective 

 or longitudinal commissure; G, ganglion cell layer; g', ganglion cells whose neurites 

 enter the connective; g 2 , ganglion cell whose neurites enter the peripheral nerve; 

 L, granules, (Leydig's dotted substance) ; N, peripheral nerve. 



since in them the ectoderm is permeated in all directions by a subepi- 

 thelial spider-weblike network of nerve fibres and ganglion cells, which 

 encroach even upon the entoderm (fig. 57). 



Linear Nervous System. From the diffuse type the other chief 

 types can be derived through concentration, which is chiefly conditioned 

 by the fact that there are a few points which are most advantageously 

 located for the reception of sensory stimuli, and hence for the development 



