180 
MR. G. J. ROMANES ON THE LOCOMOTOR SYSTEM OF MEDUSJE. 
suddenly snipping off the opalescent portion of the tissue-mass— i.e., the portion which 
had been previously alone submitted to the influence of the irritant—all movement in 
the remainder of the tissue-mass instantly ceased. This experiment I performed 
repeatedly, sometimes exposing a large and sometimes a small portion of the tissue to 
the influence of the irritant. As I invariably obtained the same result, there can be 
no doubt that in the case of chemical, as in that of the after-effects of electrical 
stimulation, the artificial rhythm depends for its manifestation on the presence of a 
constant stimulus, and is not merely some kind of obscure fluttering motion which, having 
been started by a stimulus, is afterwards kept up independently of any stimulus. 
Such being the case, I naturally expected that if I were to supply a constant 
stimulus of a thermal kind I should also obtain the phenomena of artificial rhythm. 
In this, however, my expectations have not been realised. With no species of Medusa 
have I been able to obtain the slightest indication of artificial rhythm by immersing 
the paralyzed animals in heated water. I can only explain this fact by supposing that 
the stimulus which is supplied by the heated medium is of too uniform a character 
over the whole extent of the excitable tissues; it would seem that in order to produce 
artificial rhythm there must be a differential intensity of stimulation in different parts 
of the responding tissue, for no doubt even the excitatory influence of acidulated 
water is not of nearly so uniform an intensity over the whole of the tissue area as is 
that of heated water. 
In now quitting the subject of artificial rhythm as it is manifested by the paralyzed 
bells of Sarsia, it is desirable again to observe that sustained artificial rhythm cannot 
be produced either by means of chemical irritation or as the after-effect of electrical 
stimulation in the case of any of the species of covered-eyed Medusae that I have met 
with. In order to evoke any response at all, stronger solutions of the irritants require 
to be employed in the case of the covered- than in that of-the naked-eyed Medusae, 
and when the responses do occur they are not of so suggestive a character as those 
which I have thought it worth while so fully to describe. Nevertheless, even in the 
covered-eyed Medusae well marked, though comparatively brief, displays of artificial 
rhythm may often be observed as the result of constant chemical stimulation. Thus, 
for instance, in the case of Aurelia, if the paralyzed umbrella be immersed in a solu¬ 
tion of glycerine (10 to 20 per cent.) a few rhythmic pulsations of normal rate are 
usually given, but shortly after these pulsations occur the tissue begins to go into a 
tetanus, which progressively and rapidly becomes more and more pronounced until it 
ends in violent tonic spasm. So that the history of events really resembles that of 
Sarsia under similar circumstances, except that the stage of artificial rhythm which 
inaugurates the spasm is of a character comparatively less pronounced. 
(in.) Thus far, then, I have detailed all the facts which I have been able to collect 
with reference to the phenomena of artificial rhythm as produced by different kinds of 
constant stimulation. It will not be forgotten that the interest attaching to these 
facts arises from the bearing which they have on the theory of natural rhythm. My 
