NERVOUS SYSTEM 



245 



cells (Fig. 56, B, n. 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 



n.c 



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

 medusa of a hydroid polype (Eucopella campannlaria], showing nucleus 

 and transverse striation. 



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

 cells (n. c) and muscle-fibres (m. 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. 



