MR. G. J. ROMANES ON THE LOCOMOTOR SYSTEM OF MEDTJSiE. 
729 
lithocyst D, and in the other case at the lithocyst A. Now, although this absence of 
reflex waves appears at first sight mysterious, it admits of an exceedingly simple explana- 
tion. It will be remembered, from my experiments on stimulation, that the contractile 
tissues of the covered-eyed Medusae cannot be made to respond to two successive stimuli 
of minimal, or but slightly more than minimal, intensity, unless such stimuli are sepa- 
rated from one another by a certain considerable interval of time. Now, when in the 
above illustration the contractile wave starts from A, by the time it reaches B the por- 
tion of tissue included between A and B has just been in contraction in response to the 
stimulus from A, while the portion of tissue included between B and C has not been in 
contraction. Consequently the stimulus resulting from a ganglionic discharge being 
presumably of minimal, or but slightly more than minimal, intensity, the tissue included 
between A and B will not respond to the discharge of B ; while the tissue included 
between B and C, not having been just previously in contraction, will respond ; and 
conversely, of course, if the contractile wave had been travelling in the opposite 
direction. 
( c ) Seeing that this explanation is the only one possible, and that it, moreover, follows 
as a deductive necessity from my experiments on stimulation, I think there is no need 
to detail any of the further experiments which I made with the view of confirming it. 
But the following experiment, devised to confirm this explanation, is of interest in 
itself, and on this account I shall state it. Having prepared a contractile strip with 
a single remaining lithocyst at one end, I noted the rhythm exhibited by this lithocyst, 
and then imitated that rhythm by means of single induced shocks thrown in with a key 
at the other end of the strip. The effect of these shocks was, of course, to cause the 
contractile waves to pass in the direction opposite to that in which they passed when 
originated by the lithocyst. Now I found, as I had expected, that so long as I con 
tinued exactly to imitate the rate of the ganglionic rhythm, so long did the waves 
always pass in the direction BA — A being the lithocyst, and B the other end of the 
strip. I also found that if I allowed the rate of the artificially caused rhythm to sink 
slightly below that of the natural rhythm, after every one to six waves (the number 
depending on the degree in which the rate of succession of my induction-shocks ap- 
proximated to the rate of the natural rhythm) which passed from B to A, -one would 
pass from A to B*. 
Of course the only interpretation to be put on these facts is, that every time an arti- 
ficially started wave reached the terminal ganglion it caused the latter to discharge ; but 
that the occurrence of a discharge could not in this case be rendered apparent, because 
of the inadequacy of that discharge to start a reflex wave. But that such discharges 
always took place was manifest, both a 'priori because from analogy we may be sure that 
if there had happened to be any contractile tissue of appropriate width on the other side 
of the ganglion, the discharge of the latter would have been rendered apparent, and a 
* When two such waves met, they neutralized each other at their line of collision — or perhaps, more cor- 
rectly, the tissue on each side of that line, having just been in contraction, was not able again to convey a 
contractile wave passing in the opposite direction to the wave which it had conveyed immediately before. 
