STRUCTUKE OF THE MEDUSA. 19 



the almost immeasurably slender fibres of the 

 upper nerve-ring. A second point of difference 

 consists in the surprising Avcalth of ganglion-cells 

 in the one ring as compared witli the other. Thus, 

 on the whole, there is no doubt that the lower 

 nerve-ring pi-esents a higher grade of structure than 

 does the upper, as shown not only by the greater 

 multiplicity of nerve-cells and fibres, but also by 

 the relation in which these elements stand to the 

 epithelium. For in the case of the lower nerve- 

 ring, the presumably primitive connections of the 

 nervous elements with the epithelium is w^ell-nigh 

 dissolved — this nerve-ring having thus separated 

 itself from its parent structure, and formed for 

 itself an independent layer beneath the epithelium. 

 The two nerve-rings are separated from one another 

 by a very thin membrane, which, in some species 

 at all events, is bored through by strands of nerve- 

 fibres which serve to connect the two nerve-rings 

 with one another. 



The pei'ipheral nervous system is also situated 

 in the ectoderm, and springs from the central 

 nervous system, not by any observable nerve-trunks, 

 but directly as a nervous plexus composed both of 

 cells and fibres. Such a nei'vous plexus admits 

 of being detected in the sub-umbrella of all Medusse, 

 and in some species may be traced also into the 

 tentacles. It invariably lies between the layer 

 of muscle-fibre and that of the epithelium. The 

 processes of neighbouring ganglion-cells in the 

 plexus either coalesce or dwindle in their course 

 to small fibres : at the marmn of the umbrella these 



