MR, G. J. ROMANES ON THE LOCOMOTOR SYSTEM OF MEDUSAE. 
191 
of this point entailed a number of experiments, I think it is desirable in this con¬ 
cluding paper to state their general result, lest other observers should suppose that the 
evidence as to the occurrence of waves of stimulation is vitiated by the occurrence of 
these almost imperceptible waves of contraction. 
That there is here a veiy probable source of error opened up is, of course, manifest. 
As stated in my previous paper, the proof as to the occurrence of waves of stimulation 
is two-fold: First, by stimulating a part of the irritable tissue at a distance from a 
lithocyst, too gently to start a visible wave of contraction from that part, a wave of 
stimulation may nevertheless be proved to have been started; for, shortly after apply¬ 
ing the stimulus, the distant lithocyst will discharge its ganglionic influence, so giving 
rise to a visible wave of contraction, which starts, not from the seat of stimulation, but 
from that of the lithocyst. Secondly, the invisible wave of stimulation through the 
excitable tissue of the umbrella may often be seen to have its passage visibly recorded 
by the numerous tentacles which in Aurelia fringe the margin of the umbrella—one 
tentacle after another successively retracting until the “tentacular wave” of retraction 
which started immediately below the seat of stimulation in the umbrella has passed 
throughout the whole series of tentacles. In umbrellas with their lithocysts removed 
these tentacular waves course all the way round the margin without exciting any 
visible contraction in the umbrella-tissue which was the immediate seat of stimulation; 
but if any of the lithocysts are left in situ, they invariably discharge their ganglionic 
influence shortly after a tentacular wave reaches them. 
These facts, of course, tend to show that in the excitable tissues of Aurelia there 
is some element which performs the essential function of nerve—the function, namely, 
of conveying stimuli irrespective of the passage of contractile waves. But in view 
of the fact that in these same tissues contractile waves may occur of so feeble an 
intensity as to be almost invisible, it becomes necessary to be quite sure that such 
waves may not sometimes be so feeble as to be quite invisible, and therefore that what 
I have called nervous waves of stimulation are not in reality only invisible muscular 
waves of contraction. The evidence which I have to adduce in order to show that 
such is not the case is happily unequivocal. I cannot, indeed, prove that muscular 
waves of contraction may not occur in the excitable tissues of Aurelia of too feeble an 
intensity to admit of becoming visible ; but I am able to prove that, whether or not 
such is the case, other waves of a distinctively nervous kind undoubtedly pass through 
these tissues. The proof of this consists in the fact that sometimes waves of stimu¬ 
lation will continue to pass from one tissue area to another after the passage of 
contractile waves between these two areas has been blocked by section ; or, reverting 
to the terms employed in my previous paper, excitational continuity may in some- 
cases remain intact after contractional continuity has been destroyed. And forasmuch 
as it is impossible to suppose that an invisible, or very feeble, wave of contraction can 
force a passage where a visible, or very strong, wave of the same kind fails to do so, 
the following observations must be regarded as eliminating the possible source of error 
