134 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 



action of litliocjsts. When I first observed this 

 action, it appeared to me a mysterious thing why 

 its result was alwa^^s to propagate the contraction- 

 wave in only one direction — the direction, namely, 

 in which the wave happened to be ]mssing before it 

 reached the lithocyst. For instance, suppose we 

 have a strip A D, Avith a lithocyst at each of the 

 equidistant points A, B, C, D ; and suppose that the 

 lithocyst B originates a stimulus : the resulting 

 contraction- wave passes, of course, with equal 

 rapidity in the two opposite directions, B A, B C 

 (arrov/s h a, h c) : the contraction- wave h a therefore 



F'-. 24. 



reaches the lithocyst A at the snme time as the 

 contraction-wave b c reaches the lithocyst C, and 

 so both A and C discharge simultaneously. What, 

 then, should we expect to be the result ? I think 

 we should expect the wave b c to continue on its 

 course to D, alter having been strengthened at C, 

 and a reflex wave a' V to start from A (owing to 

 the discharge at A), which would reach B at the 

 same time as a similar reflex wave c' V starting 

 from C (owing to the discharge at C) ; so that by 

 the time the original wave b c d had reached D, 

 the point B would be the seat of a collision between 

 the two reflex waves a' b' and c' <V . And, not to 



