668 
ME. Gr. J. ROMANES ON THE LOCOMOTOR SYSTEM OP MEDUSAE. 
previous paper, there is a general tendency for the smaller segments of an Aurelia divided 
in this way to contract less frequently than the larger segments; and more careful 
observation this year has not resulted in establishing any more precise a relationship. 
It would be tedious and unnecessary to quote my observations in this connexion ; but 
as these observations brought out very clearly a fact which I had previously suspected, 
I may detail one experiment to illustrate this point. The fact in question is, that the 
potency of the lithocysts in any given segment of a divided Aurelia has more to do with 
the frequency of its pulsations than has the size of the segment. As mentioned in my 
other paper, one or more lithocysts may often be observed to be permanently prepotent 
over the others ; and I may here observe that the segmentation experiments just described 
have shown the converse to be true, viz. that one or more lithocysts are often perma- 
nently feebler than the others. Well, if a specimen of Aurelia exhibiting decided 
prepotency in one or more of its lithocysts be watched for a considerable length of time, 
so as to be sure that the prepotency is not of a merely temporary character, and if 
the animal be then divided into segments in such a way that the prepotent lithocysts 
shall occupy the smaller segments, it may be observed, provided time be left for 
the tissues to recover shock, that the segments containing the prepotent lithocysts, 
notwithstanding their smaller size, contract more frequently than do the larger segments. 
Conversely, if the larger segments happen to contain feeble lithocysts, their contractions 
will be but few. I have, indeed, seen cases in which the lithocysts appeared to be quite 
functionless, so far as the origination of stimuli was concerned. 
The following observations were made on a healthy specimen of Aurelia having all 
its lithocysts in good condition, but prepotency being well marked in the case of one of 
them, and also, though in a lesser degree, in the case of another. I divided the animal 
so as to leave one of these two prepotent lithocysts in each of the eighth-part segments, 
and the next most powerful lithocyst in the quadrant segment. In the following descrip- 
tion I shall call the two eighth-part segments A and B, the former letter designating 
the segment containing the most powerful lithocyst. The Aurelia before being divided 
manifested for several hours a very regular and sustained rhythm of 32 per minute. 
After its division the various segments contracted at the following rates in one minute 
intervals : — 
Time after operation. 
Segment -J. 
Segment J. 
Segment f A. 
Segment a B. 
J hour. 
20 
25 
27 
15 
1 „ 
20 
25 
27 
15 
2 hours. 
29 
25 
27 
16 
4 ” 
19 
16 
27 
12 
Next morning the water which contained the segments was somewhat foul, and this, 
as is always the case, gave rise to abnormally long pauses. This effect was much more 
marked in the case of some of the segments than in that of others. I therefore observed 
the segments over five minutes intervals, instead of over one minute intervals as on the 
