v MEDUSAE 319 



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 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 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 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 meso- 

 glcea : 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 move- 

 ments 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 permanently paralysed. 



It is not, however, rendered incapable of movement, for 

 a sharp pinch, i.e., an external stimulus, causes a single con- 

 traction, showing that the muscles still retain their irritability. 

 But no movement takes place without such external stimu- 



