192 
MR, G. J. ROMANES ON THE LOCOMOTOR SYSTEM OF MEDUSHL 
with which I am now dealing ; they establish the fact that whether or not wholly 
invisible waves of contraction occur in Aurelia, waves of stimulation undoubtedly 
occur. For the sake of brevity I shall quote these observations from my notes 
verbatim :— 
“ An Aurelia of medium size. One half of the margin removed in the form of a 
strip two inches broad, the remainder of the umbrella being left attached. After 
some overlapping sections had completely and permanently destroyed contractional 
continuity in the strip, excitational continuity clearly remained intact, as shown by 
the occurrence of a decided tentacular wave at the opposite side of the umbrella 
whenever the free end of the strip was irritated. That contractional continuity had 
in this case been effectively destroyed was proved by tetanising the free end of the 
strip four hours after the section with strong faradaic currents, and also with spirit. 
Under the influence of such strong stimulation of the free end of the strip, the lobes 
of the polypite violently writhed, although the umbrella remained quiescent.” 
This response on the part of the polypite is of conclusive value as proving the 
occasional persistence of excitational continuity after contractional continuity has been 
destroyed. My notes contain the record of several other observations confirmatory of 
this fact, but it is needless to quote them at length. The writhing movements of 
the polypite under these circumstances are often very decided ; but they are usually 
wonderfully late in beginning. This is probably owing to the prolonged and uncertain 
period of latency which, as stated in my previous paper, is manifested by the polypite 
of Aurelia; for even in experiments where the contractional continuity of the umbrella 
tissue has not been destroyed by section, the response of the polypite to strong stimu¬ 
lation of these tissues does not begin till after the violent contractions of the umbrella 
have been in progress for a considerable time. 
§ 2. Effects of Exhaustion in impeding the passage of contractile loaves .—In my 
various modes of section of Aurelia I have several times observed a fact that is worth 
recording. It sometimes happens that when the connecting isthmus between two 
almost severed areas of excitable tissue is very narrow, the passage of contractile 
waves across the isthmus depends upon the freshness, or freedom from exhaustion, of 
the tissue which constitutes the isthmus. That is to say, on faradising one of the two 
tissue areas which the isthmus serves to connect, the resultino- contractile waves will 
at first pass freely across the isthmus to spread all over the other tissue area; but 
after several such waves have passed in rapid succession across the isthmus, it may 
happen in some preparations that every now and then a contractile wave fails to pass 
across the isthmus. When this is the case, if the stimulation is still continued, a 
greater and greater proportion of waves fail to pass across the isthmus, until perhaps 
only one in every five or six waves become propagated from the one area to the other. 
If now single induction shocks be substituted for the faradaic stimulation, it may be 
found that by leaving an interval of four or five seconds between the successive 
shocks, every wave that is started in the one area will be propagated across the 
