MR. Gr. J. ROMANES ON THE LOCOMOTOR SYSTEM OF MEDUSAE. 
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isthmus to the other area. But if the interval between the successive shocks be 
reduced to two or three seconds, every now and then a wave will fail to pass across 
the isthmus. And if the interval be still further reduced to one second, or half a 
second, comparatively few of the waves will pass across. Now, however, if the tissue 
be allowed five minutes’ rest from stimulation, and the single shocks be thrown in 
at one second’s filtervals, all the first six or ten waves will pass across the isthmus, 
after which they begin to become blocked as before. It may be observed also that 
when the waves are thus blocked, owing to exhaustion of the connecting isthmus, 
they may again be made to force a passage by increasing the intensity of the stimu¬ 
lation, and so giving rise to stronger waves having a greater power of penetration. 
Thus, on re-enforcing the electrical stimulus with the simultaneous application of a 
drop of spirit, the resulting waves of contraction are almost sure to pass across the 
isthmus, even though this has been exhausted in the manner just described. 
§ 3.—Another fact, which I have several times observed during my sections of Aurelia , 
also deserves to be recorded. I have observed it under several modes of section, but 
it will be only necessary to describe one observation. 
Fig. 11. 
In the Aurelia, of a portion of which the accompanying woodcut is a representation, 
seven of the lithocysts were removed, while the remaining one was almost entirely 
isolated from the general contractile tissue by the incisions a a, b b, c c. The lithocyst 
continued to animate the tissue area xxxx, and through the connecting passage y 
the contractile waves spread over the remainder of the subumbrella tissue zzzz. 
So far, of course, the facts were normal; but very frequently it was observed that 
the contractile waves did not start from the lithocyst, or from the area xxxx , but 
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