242 



HYDROID POLYPES 



of the velum with the edge of the bell there lies, imme- 

 diately beneath the ectoderm, a layer of peculiar branched 

 cells (Fig. 56, B, n. c\ containing large nuclei and produced 

 into long fibre-like processes. These nerve-cells (see p. 227) 

 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 



B 



771. C 



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

 medusa of a hydroid polype (Eiicopella campamilaria), 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.) 



the velum. An irregular network of similar cells and fibres 

 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 



