NER rO us S YSTEM. 



333 



cephalic ganglia, and two well-developed lateral ganglionic cords placed 

 close beneath the epidermis. These cords, instead of giving off definite 

 nerves, as in animals with a fully differentiated nervous system, are con- 

 nected with a continuous subdermal nervous plexus. 



The features of the embryology and the anatomy of the nervous 

 system, to which attention has just been called, point to the following 

 general conclusions as to the evolution of the nervous system. 



(1) The nervous system of the higher Metazoa appears to have 

 been evolved in the course of a long series of generations from a 



Fig. 237. Isolated cells belonging to the upper nerve-eing of Carmakina 

 HASTATA. (After 0. and R. Hertwig.) 



A. Nenro-epithelial sense-cell. c. sense-hair. 



B. Transitional cell between a neuro-epithelial cell and a ganglion-cell. 



differentiation of some of the superficial epithelial cells of the body, 

 though it is possible that some parts of the system may have been 

 formed by a differentiation of the alimentary epithelium. 



(2) An early feature in the differentiation consisted in the growth 

 of a series of delicate processes of the inner ends of certain epithelial 

 cells, which became at the same time specially differentiated as sense- 

 cells (figs. 236 and 237). 



(3) These processes gave rise to a subepithelial nervous plexus, 

 in which ganglion-cells, formed from sense-cells which travelled in- 

 wards and lost their epithelial character (fig. 237 B), soon formed an 

 important part. 



(4) Local differentiations of the nervous network, which was no 

 doubt distributed over the whole body, took place partly in the 

 formation of organs of special sense, and partly in other ways, and 

 such differentiations gave rise to a central nervous system. The 

 central nervous system was at first continuous with the epidermis, 

 but became separated from it and travelled inwards. 



(5) Nerves, such as we find them in the higher types, originated 

 from special differentiations of the nervous network, radiating from 

 the parts of the central nervous system. 



The following points amongst others are still very obscure : — 



(I) The steps by which the protoplasmic processes from the primitive 



