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ME. a. J. ROMANES ON THE LOCOMOTOE SYSTEM OE MEDUSAS. 
once round the latter. It is comparatively rare to find cases in which contractile waves 
continue to pass after the spiral incision has been carried twice round this organ ; and 
it is still more rare to find cases of waves passing in specimens such as that represented in 
Plate 33, where the spiral section has made two and a half turns round the gonocalyx. 
When such cases as the latter do occur, if the specimen happens to he an ordinary-sized 
one, the contractile strip will be about a yard in length. On the other hand, cases may 
occur in which blocking of the contractile wave supervenes when the contractile strip 
is only an inch long ; and in one case complete blocking of this wave was caused by the 
radial incision half an inch long, made on one side of a freely discharging lithocyst, 
i. e. before the circumferential incision was begun at all. But such extreme variations 
upon the side of intolerance of spiral section are as uncommon as are the extreme 
variations upon the side of tolerance. Now the fact that between these two extremes 
there are to be found all possible grades of tolerance, appears to warrant us in concluding 
that the contractile tissue of Aurelia aurita is not of a functionally homogeneous 
nature. 
And this conclusion is very much strengthened by the additional fact that at 
whatever point in a contractile strip which is being progressively elongated by section 
the contractile wave becomes blocked, the blocking is sure to take place completely and 
exclusively at that point. This fact, it appears to me, can only be properly explained 
by supposing that more or less differentiated lines of discharge pervade the contractile 
tissue of the gonocalyx, and that the sudden and complete blocking of the contractile 
waves which invariably takes place at some determinate point during the progress of 
the section is due to the latter having at that point severed some important line of 
discharge, which had previously served to convey the influence of the lithocyst to the 
undivided parts of the gonocalyx. Nevertheless we must bear in mind that this, 
deduction is supported by no histological evidence, and that it is, moreover, very difficult 
to reconcile with the fact that some specimens of Aurelia aurita endure so enormous 
an amount of the most severe forms of section without suffering loss of physiological 
continuity between any of their parts. 
The deduction also appears difficult to reconcile with another fact, viz. that in some 
cases (which, however, are greatly in the minority) the blocking of contractile waves 
in spiral strips admits, after a time, of being overcome, the contractile waves again passing 
from the strip into the gonocalyx as freely as they did before the section reached the 
point at which the blocking occurred, and this occasionally two or three times in suc- 
cession. I think, however, that there is a theory by which all these paradoxical facts 
may be reconciled ; and so at the present stage of my inquiries I provisionally accept 
the hypothesis of there being present in the locomotor system of Aurelia aurita more 
or less definitely integrated lines of discharge. But, in making this statement, it is 
almost needless to add that I wish a marked distinction to be drawn between the certainty 
of the hypothesis and that of the facts from which it is deduced. 
Blocking of contractile waves in strips may also be caused by making a system of 
