XXIII 



NERVOUS SYSTEM 



245 



cells (Fig. 56, B, ;/. c], containing large nuclei and produced 

 into long fibre-like processes. These nerve-cells (see p. 230) 

 are so disposed as to form a double ring round the margin 

 of the bell, one ring (Fig. 55, D, nv) being immediately 

 above, the other (nv'} immediately below the insertion of 



the velum. 



An irregular network of similar cells and fibres 



B 



m.c 



n.c 



m.c 



FIG. 56. A, Muscle fibres from the inner face of the bell of the 

 medusa of a hydroid polype (Eucopella campamdaria}, showing nucleus 

 and transverse striation. 



B, portion of the nerve-ring of the same, showing two large nerve- 

 cells (;/. c] and muscle-fibres (in. c] on either side. (After von Len- 

 denfeld. ) 



occurs on the inner or concave face of the bell, between the 

 ectoderm and the layer of muscle-fibres. The whole consti- 

 tutes the nervous system of the medusa ; the double nerve-ring 

 is the central, the network the peripheral nervous system. 



Some of the processes of the nerve-cells are connected 

 with ordinary ectoderm-cells, which thus as it were connect 

 the nervous system with the external world : others, in some 

 instances at least, are probably directly connected with 

 muscle-fibres. 



