78 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
disc, without once interrupting the continuity of 
the tracing; for on coming to the end of a divided 
thread, one could always double back on it and 
choose another thread which might be running in 
the required direction. And this is what we are 
now compelled to believe takes place in the fibres 
of this nervous network, if we assume that these 
visible fibres are the only conductile elements which 
are present. Whenever a stimulus wave reaches a 
cut, we must conclude that it doubles back and 
passes into the neighbouring fibres, and so on, time 
after time, till it succeeds in passing round and 
round any number of overlapping cuts. 
This is, no doubt, as I have already observed, a 
very remarkable fact ; but it becomes still more so 
when we have regard to the histological researches 
of Professor Schafer on the structural character of 
this nerve-plexus. For these researches have shown 
that the nerve-fibres which so thickly overspread 
the muscular sheet of Aurelia do not constitute a 
true plexus, but that each fibre is comparatively 
short and nowhere joins with any of the other 
‘fibres; that is to say, although the constituent 
fibres of the network cross and recross one another 
in all directions—sometimes, indeed, twisting round 
one another like the strands of a rope—they can 
never be actually seen to join, but remain anatomi- 
cally insulated throughout their length. So that the 
simile by which I have represented this nervous 
network—the simile, namely, of a sheet of muslin 
overspreading the whole of the muscular sheet—is, 
as a simile, even more accurate than has hitherto 
