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MR, G. J. ROMANES ON THE LOCOMOTOR SYSTEM OF MEDUSiE. 
tissues of the higher animals, and this whether the current he passed in the direction 
of the length or in that of the thickness of the muscular fibres. 
Waves of contraction in the excitable tissues of Aurelia , when started by a very weak 
stimulus, may be so feeble as to be barely perceptible. This fact in itself raises the pre¬ 
sumption that the so-called waves of stimulation, which were described in my former 
paper, may really be but waves of contraction which are too feeble to be perceptible 
at all; and hence that the phenomena of reflex action, &c., as they occur in the 
Medusas may be due, not to nerve, but to muscle. This presumption, however, is 
excluded by the additional fact that very often severe section of the excitable tissues 
has the effect of completely blocking even the strongest waves of contraction while 
still allowing the waves of stimulation to pass freely, as shown by tentacular waves, 
reflex action of the lithocyst, and writhing response of the polypite continuing to assert 
themselves beyond the region in which the contractile waves have been blocked by 
section. 
Exhaustion of a narrow isthmus of junction tissue sometimes has the effect of 
blocking the passage of contractile waves in that isthmus, their passage, however, 
being resumed when the junction tissue is allowed time to recover its full excitability. 
It has been several times observed that the ganglionic influence of a lithocyst admits 
of asserting itself at a distance from the seat of the lithocyst itself, and this even 
when severe forms of section are interposed between the lithocyst and the distant point 
in the tissue at which the contractile waves are originating. The interpretation of 
tins fact need not again be rendered. 
It has been uniformly observed that the contractile tissues of Aurelia, after having- 
been completely severed with a sharp scalpel, in from four to eight hours regenerate 
themselves sufficiently to admit of the re-establishment of physiological continuity 
across the line of previous severance. 
IY. LITERATURE. 
I shall terminate this concluding paper on the locomotor system of Medusae with 
as brief a reference as possible to the existing literature of the subject. In my first 
paper I have already had occasion to render an epitome of the memoirs of Professors 
Agassiz and Haeckel, so that it becomes needless in this place again to state the 
results at which these observers arrived. Neither is it desirable to discuss in this 
place the results which were arrived at by the numerous observers prior to the date of 
Professor Haeckel’s work. It is not desirable to do so, because these results, in so far 
as they professed to be positive, were for the most part erroneous, and in so far as they 
were combative of error, now possess only an historical interest. Those who may care 
to consult the literature of this subject on account of any such interest which it may 
have for them, will require more particularly to consult the following works :— 
Ehrenberg, c Die Acalephen des rothen Meeres und der Organismus der Medusen der 
