STRUCTURE OF THE MEDUSZ. 19 
the almost immeasurably slender fibres of the 
upper nerve-ring. A second point of difference 
consists in the surprising wealth of ganglion-cells 
in the one ring as compared with the other. Thus, 
on the whole, there is no doubt that the lower 
nerve-ring presents 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 well-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 peripheral 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 nervous plexus admits 
of being detected in the sub-umbrella of all Medusz, 
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 margin of the umbrella these 
