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MR. G. J. ROMANES ON THE LOCOMOTOR SYSTEM OF MEDUSAE. 
the adjacent lithocyst, thus causing it to originate another wave which, in turn, is just 
able to pass to the next lithocyst in the series, and so on, each lithocyst in turn acting 
like a reinforcing battery to the passage of the contractile wave”*. Now this fact, I 
think, sufficiently explains the mechanism of ganglionic action in those cases where one 
or more lithocysts are prepotent over the others ; that is to say, the prepotent lithocyst 
first originates a contractile wave, which is then successively reinforced by all the other 
lithocysts during its passage round the swimming-bell. In this way the passage of a con- 
tractile wave is no doubt somewhat accelerated ; for I found, in marginal strips, that the 
rate of transit from a terminal lithocyst to the other end of the strip was somewhat 
lowered by excising the seven intermediate lithocysts. 
(b) I may here state, in passing, a point of some little interest in connexion with this 
reinforcing action of lithocysts. When I first observed this action, it appeared to me a 
mysterious thing why its result was always to propagate the contractile wave in only 
one direction — the direction, namely, in which the wave happened to be passing before 
it reached the lithocyst. 
Fig. 18. 
For instance, suppose we have a strip A D with a lithocyst at each of the equidistant 
points ABCD. Suppose now that the lithocyst B originates a stimulus : the resulting 
contractile wave passes, of course, with equal rapidity in the two opposite directions, 
B A, B C (arrows b «, b c). The contractile wave h a therefore reaches the lithocyst A % 
at the same time as the contractile 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, after having been streng- 
thened at C, and a reflex wave d V to start from A (owing to the discharge at A) which 
would reach B at the same time as a similar reflex wave d V starting from C (owing to the 
discharge at C) ; so that by the time the original wave bed had reached D, the point B 
would be the seat of a collision between the two reflex waves d V and d V. And, not to 
push the supposed case further, it is evident that if such reflex waves were to occur, 
the resulting confusion would very soon require to end in tetanus. As a matter of fact, 
these reflex waves do not occur ; and the question is, why do they not 1 Why is it that 
a wave is only reinforced in the direction in which it happens to be travelling — so that 
if, for instance, it happens to start from A in the above series, it is successively propa- 
gated by B C in the direction ABCD, and in that direction only, whereas, if it 
happens to start from D, it is propagated by the same lithocysts in the opposite direc- 
tion, D C B A, and in that direction only \ — the wave in the one ease terminating at the 
* Croonian Lecture, 1876, Phil. Trans, p. 311. 
