ME. Gr. J. EOMANES ON THE LOCOMOTOE SYSTEM OE MEDUSAE. 
717 
facts to be detailed in this subsection. For if it can be proved that the contractile 
tissues of Aurelia are pervaded by tissue-tracts which display the essentially nervous 
function of establishing what I have termed excitational continuity between different 
parts, then I think we may be more prepared to believe that the passage of contractile 
waves depends on the presence of tissue-tracts presenting a nervous character. For 
the evidence being already in favour of the hypothesis that the passage of contrac- 
tile waves depends on the presence of certain differentiated elements whose function is 
presumably nervous, suctpevidence would be further strengthened if it could be shown 
that in the very same tissue there occur other differentiated elements whose function is 
demonstrably nervous. Now that such elements as the last mentioned do occur in all 
parts of the excitable tissues of Aurelia aurita is a fact concerning which there can be 
no question. 
( b ) If any part of the polypite of Aurelia be stimulated, the natural rhythm becomes 
accelerated and the systoles more powerful. Again, if any part of the irritable surface 
of the swimming-bell be stimulated, not only do the swimming-motions become more 
rapid and powerful, but the lobes of the polypite w 7 rithe about in answer to the stimu- 
lation. These complementary facts clearly prove the occurrence of reflex action between 
the polypite and the swimming-bell reciprocally. Similarly, if seven lithocysts be 
removed, and the remaining one be of a somewhat weakly character, so as not to inter- 
fere with the observation by its excessive spontaneity, it may often be observed that if 
a portion of the irritable surface of the swimming-bell be submitted to a stimulus of 
somewhat less than minimal intensity as regards the starting of a contractile wave 
from the point of stimulation, a contractile wave will nevertheless start from the single 
remaining lithocyst so soon as time has been allowed for the stimulus to travel from 
the point of its application to that of the lithocyst. The greater the distance between 
these two points, the less certain is the experiment to succeed ; but in many specimens 
it may succeed every time, even though the stimulus be applied at the opposite side of 
the swimming-bell from that in which the lithocyst is placed. In such cases, however, 
it is very important to observe that stimulation of certain tracts of tissue is followed 
by this reflex response, while stimulation of other tracts, e\en though these be situated 
much nearer to the lithocyst, is not so followed. Such excitable tracts are by no means 
constant as to their position in different individuals ; but the fact of their occurrence 
tends to show that the reflex action we are contemplating is effected, not by means of 
an equally diffused excitability on the part of the whole nervo-muscular sheet, but by 
means of functionally differentiated tissue-tracts, which, during the experiment, act as 
afferent nerves. 
(c) Next I must draw attention to a fact which will presently be proved of the highest 
importance in relation to the subject we are considering. It may be remembered, from 
Plate 32 in my former paper, that the entire margin of Aurelia aurita is fringed by 
a number of exceedingly delicate tentacles. These tentacles are highly retractile; 
and the point to which I have now to direct attention is this, that when any portion 
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