v MEDUSA 319 



ring round the margin of the bell, one ring (Fig. 80, D, 

 nv) being immediately above, the other (nv r ) immediately 

 below the insertion of 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 constitutes the nervous system 

 of the medusa ; the double nerve-ring is the central, the 

 network the peripheral nervous system (p. 155). 



Some of the processes of the nerve-cells are connected 

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

 nect 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 polype, the umbrella 

 has undergone a very remarkable differentiation of its 

 tissues. Its ordinary ectoderm-cells, instead of being 

 large and eminently contractile, form little more than 

 a thin cellular skin or epithelium (p. 109) over the 

 gelatinous mesogloea : they have largely given up the 

 function of contractility to the muscle-processes or fibres, 

 and serve merely as a protective and sensitive layer. 



Similarly the function of automatism, possessed by the 

 whole body of Hydra, is made over to the group of 

 specially modified ectodermal cells which constitute the 

 central nervous system. If a Hydra is cut into any 

 number of pieces each of them is able to perform the 

 ordinary movements of expansion and contraction, but 

 if the nerve-ring of a medusa is removed by cutting away 

 the edge of the umbrella, the rhythmical swimming 

 movements stop dead : the umbrella is in fact perma- 

 nently paralysed. 



It is not, however, rendered incapable of movement, 

 for a sharp pinch, i.e., an external stimulus, causes a 

 single contraction, showing that the muscles still retain 



