134 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
action of lithocysts. When I first observed this 
action, it appeared to me a mysterious thing why 
its result was always to propagate the contraction- 
wave in only one direction—the direction, namely, 
in which the wave happened to be passing before it 
reached the lithocyst. For instance, suppose we 
have a strip A D, with 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 
(arrows b a,bc): the contraction-wave b a therefore 
Fig. 24, 
reaches the lithocyst A at the same time as the 
contraction-wave 6 ¢ 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 be to continue on its 
course to D, after having been strengthened at C, 
and a reflec wave a’ U’ to start from A (owing to 
the discharge at A), which would reach B at the 
same time as a similar reflex wave ¢ b’ starting 
from C (owing to the discharge at C); so that by 
the time the original wave bc d had reached D, 
the point B would be the seat of a collision between 
the two reflex waves a’ b’ and ¢’ d’. And, not to 
