XXII 



NERVOUS SYSTEM 



243 



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 occurs 

 on the inner or concave face of the umbrella, between the 

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



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

 medusa of a hydroid polype (£ucope//a campanularia)^ showing nucleus 

 and transverse striation. 



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

 cells («. c) and muscle-fibres {m. c) on either side. (After von Len- 

 denfeld. ) 



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

 is the central, the network the peripheraj. 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. 



We thus see that while the manubrium of a medusa has 

 the same simple structure as a hydranth, or what comes to 



R 2 



