174 
MR. G. J. ROMANES ON THE LOCOMOTOR SYSTEM OF MEDUSiE. 
the mutilated nectocalyx under these circumstances are most unmistakably of a 
rhythmic nature. Viewed from a little distance, indeed, these motions are not dis¬ 
tinguishable from the natural swimming motions of the unmutilated animal, except 
that, not being of quite such a powerful character, they are not so effective for 
locomotion. Viewed more closely, however, it may frequently be seen that the whole 
bell does not contract simultaneously, but that, as it were, clouds of contraction pass 
now over one part and now over another. Still, whether the contractions are partial 
or universal, they are always more or less rhythmical. As this is the only case that 
has ever been observed of rhythm as due to a constant chemical stimulus, I have 
studied it with much care, and shall now give a full description of what appears to 
me an important body of physiological facts. 
Ten to twenty drops of acetic acid having been added to 1000 c.c. of sea-water, and 
the paralyzed bell of Sarsia having been placed in the mixture, an interval of about 
half a minute will elapse before any movement begins. Sooner or later, however, the 
artificial rhythm is sure to be induced, and it will then continue for a variable time— 
occasionally as long as an hour, and generally for a considerable number of minutes. 
After it ceases it may often be made to recommence, either by adding a few more 
drops of acid to the sea-water, or by supplying an additional stimulus to the bell by 
nipping it with the forceps. Eventually, however, all movement ceases, owing to the 
destruction of irritability by the action of the acid. By this time the whole inner 
surface of the bell has become strongly opalescent, owing to the destructive influence 
of the acid on the epithelial cells which overspread the irritable tissues. The latter 
fact is worth mentioning, because in no case does the artificial rhythm set in until this 
opalescence has begun to show itself; and as this opalescence is but an optical expres¬ 
sion of the damage which the epithelial coat is undergoing, the explanation of the time 
which elapses after the first immersion of the bell in the acidulated water and the 
commencement of the artificial rhythm, no doubt is that during this time the acid has 
not obtained sufficient access to the excitable tissues to serve as an adequate 
stimulus. 
During the soaking stage of the experiment— i.e., before the artificial rhythm begins 
—the excitability of the tissues may be observed progressively and abnormally to 
increase; for soon after the soaking stage begins, in response to a single nip with the 
forceps the bell may give two or three locomotor contractions, instead of a single one, 
as is invariably the case with a paralyzed bell of Sarsia in normal water. Later on 
during the soaking stage four or five successive contractions may be yielded in response 
to a single mechanical stimulus, and shortly after this a whole bout of rhythmic con¬ 
tractions may be started by the same means. Indeed, in some cases the artificial 
rhythm in acidulated water requires such a single additional stimulus for its inaugura¬ 
tion—the shivering movement failing to begin spontaneously, but beginning imme¬ 
diately upon the application of the additional stimulus. Similarly, after the shivering 
movements have ceased, a fresh bout may very often be started by again giving the 
