NATURAL RHYTHM. 161 
Another circular incision carried round midway between the 
former one and the margin caused rhythm toriseto ... 24 
Rate again gradually declined, andin a quarter ofanhourwas 12 
Another circular incision was carried round as close to the 
margin as was compatible with leaving the physiological 
continuity of all the lithocysts intact. Rhythmroseto 14 
Within a few minutes it fell to ... eee ose eee se 7:6 
Excepting the cases where the effects of shock 
are apparent, some such series of phenomena as 
those just recorded are always sure to ensue when 
a covered-eyed Medusa is mutilated in the way 
described, and this kind of mutilation, besides pro- 
ducing such marked effects on the rate of the 
rhythm, also produces an effect in impairing the 
regularity of the rhythm. In some specimens 
the latter effect is more marked than it is in others. 
The following series of observations will serve to 
give a good idea of this effect :— 
An Aurelia manifested a regular and sustained 
rhythm of 36. Immediately after the removal of 
the manubrium, the rate of rhythm in successive 
minutes was as follows: 40, 39, 37, 35, 32, 30, 29, 
26, 24, 18, 14 (40 seconds’ pause), 16, 15, 14, 15, 16 
(40 seconds’ pause), 22, 20, 19, 15, 16, 17, 14, 13, 13, 
fs 16,.16, 17, 18,14, 12, 13,11, 12, 9,15, 16, 14, 12, 
9, etc., the rhythm now continuing very irregular. 
An hour after the operation, the following were the 
number of contractions given in one-rinute inter- 
vals, the observations being taken at intervals of 
ten minutes: 15, 15, 12, 22, 14, ete. 
In this experiment, therefore, as soon as the ac- 
celeration and slowing-stages had been passed, viz. 
about a quarter of an hour after the operation, a 
