EXPERIMENTS ON MEDUSA 



and more ganglion-cells, and is connected with the 

 upper ring by nerve-fibres. In addition to this ring, 

 which is called the central nervous system, there is 

 also a peripheral nerv- 

 ous system, a plexus, 

 consisting of nerves 

 and scattered ganglion 

 cells, spread out over 

 the whole subumbrella 

 (b, Fig. i), between 

 the epithelium and 

 the muscle-layer. The 

 convex surface of the 

 umbrella consists of a 

 non-contractile, gelat- 

 inous mass, and no 

 nervous elements are 

 to be found in it. 



Acalepha^ (Fig. 2) 

 have no continuous 

 nerve-ring, but a row of separate nerve-centres (S, Fig. 

 2) extends around the margin of the umbrella, lying in 

 the ectoderm, which covers the basis of the marginal 

 bodies (sense organs). The number of these centres 

 corresponds, at least in Aurelia aurita, with the num- 

 ber of sense organs. This nervous system contains 

 no ganglion-cells, but processes called nerve-fibres go 

 out from special epithelial cells. The muscle-layer 

 of the umbrella also is said to contain a peripheral 

 nervous plexus (i). 



FIG. i. HYDROMEDUSA. (Gonionemus 

 vertens.) 



umbrella; , subumbrella with muscles; c, man- 

 ubrium ; d, margin of the swimming-bell with the 

 nerve-ring. 



