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THE POPULAR SCIENCE MONTHLY. 



a single remaining ganglion at one end. (The cuts interposed in the 

 parallelogram may for the present be neglected.) Now, if the end- 

 mark a of the nervo-muscular sheet most remote from the ganglion be 

 gently brushed with a camel's-hair brush — i. e., too gently to start a 

 responsive contractile wave — the ganglion at the other end will shortly 

 afterward discharge, as shown by its starting a contractile wave at its 

 own end of the parallelogram b, thus proving that the stimulus caused 



I If - 1 ' 



Fig. 5. 



by brushing the tissue at the other end, a, must have been conducted 

 all the way along the parallelogram to the terminal ganglion 5, so caus- 

 ing the terminal ganglion to discharge by reflex action. Indeed, in 

 many cases, the passage of this nervous wave of stimulation admits of 

 being actually see^i. For the numberless tentacles which fringe the 

 margin of Aurelia are more highly excitable than is the general con- 

 tractile tissue of the bell ; so that, on brushing the end a of the paral- 

 lelogram 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 contraction being thus set up in the ten- 

 tacular fringe, the passage of which is determined b}^ the passage of 

 the nervous wave of stimulation in the superjacent nervous network. 

 This tentacular wave is here represented as having traversed half the 

 whole distance to the terminal ganglion, and when it reaches that gan- 

 glion it will cause it to discharge by reflex action, so giving rise to a 

 visible wave of muscular contraction passing in the direction b a, 

 opposite to that which the nervous or tentacular wave had previously 

 pursued. Now, this tentacular wave, being an optical expression of 

 the passage of a wave of stimulation, is a sight as beautiful as it is 

 unique ; and it affords a first-rate opportunity of settling this all-impor- 

 tant question, namely — will this conductile or nervous function prove 

 itself as tolerant toward a section of the tissue as the contractile or 

 muscular function has already proved itself to be ? For, if so, we shall 



