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 condactile elements which 

 are present. Whenever a stimulus wave readies 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 



