76 JELLY-FISH, STAR-FISH, AND SEA-URCHIXS. 



of the neuro-muscular sheet most remote from the 

 ganglion be gently brushed with a camel's hair 

 brush — i.e. too gently to start a responsive contrac- 

 tion-wave — the ganglion at the other end will 

 shortly afterwards discharge, as shown by its start- 

 incr a contraction-wave at its own end of the 

 parallelogram, h; thus proving that the stimulus 

 caused by brushing the tissue at the other end, a, 

 must have been conducted all the way along the 

 parallelogram to the terminal ganglion, 6, so causing 

 the terminal crano^lion to discharcje bv reflex action. 

 Indeed, in many cases, the passage of this nervous 

 wave of stimulation admits of being seen. For the 

 numberless tentxicles which frin^re the maro-in of 

 Aurelia are more higlily excitable than is the 

 general contractile tissue of the bell; so that on 

 brushing the end a of the parallelogram remote 

 from the ganglion, the tentacles at this end respond 

 to the stimulus by a contraction, then those next in 

 the series do the same, and so on — a wave of con- 

 traction being thus set up in the tentacular fringe, 

 the passage of which is determined by the passage 

 of the nervous wave of stimulation in the super- 

 jacent nervous netwoik. This tentacular wave is 

 in the illustration represented as having travelled 

 nearlv half the whole distance to the terminal o-ancr- 

 lion, and when it reaches that ganglion it will cause 

 it to discharge by reflex action, so giving rise to a 

 visible wave of muscular contraction passing in the 

 direction h a, opposite to that which the nervous or 

 tentacular wave had previously pursued. Now this 

 tentacular wave, being an optical expression of a pas- 



