NERVOUS SYSTEM. 



403 



FIG. 236. NEURO- 

 EPITHELIALSENSE- 

 CELLS OFAURELIA 



AURITA. (From 



Lankester ; after 

 Schafer.) 



moreover, present in the network cells of the same character as the multipolar 

 ganglion-cells in the nervous system of Vertebrates, and some of these cells 

 are characterised by sending a process into the superjacent epithelium. 

 Such cells are obviously intermediate between neuro- 

 epithelial cells and ganglion-cells ; and it is probable 

 that the nerve-cells are, in fact, sense-cells which have 

 travelled inwards and lost their epithelial character. 



In the Craspedote Medusae (Hertwigs, No. 320) 

 the differentiation of the nervous system is carried 

 somewhat further. There is here a definite double 

 ring, placed at the insertion of the velum, and usually 

 connected with sense-organs. The two parts of the 

 ring belong respectively to the epithelial layers on 

 the upper and lower surfaces of the velum, and are not 

 separated from these layers ; they are formed of fine 

 nerve-fibres and ganglion-cells. The epithelium above 

 the nerve rings contains sense-cells (fig. 237) with a 

 stiff hair at their free extremity, and a nervous pro- 

 longation at the opposite end, which joins the nerve- 

 fibres of the ring. Between such cells and true ganglion- 

 cells an intermediate type of cell has been found (fig. 

 237 B) which sends a process upwards amongst the 

 epithelial cells, but does not reach the surface. Such cells, as the Hertwigs 

 have pointed out, are clearly sense-cells partially transformed into ganglion- 

 cells. 



A still higher type of nervous system has been met with amongst some 

 primitive Nemertines (Hubrecht, No. 323), consisting of a pair of large 

 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 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 



26 2 



