NATURAL RHYTHM. 161 



Another circular incision carried round midway between the 



former one and the margin caused rhythm to rise to ... 21 



Rate again gradually declined, and in a quarter of an honr was 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. RliyLhm rose to 14i 



Within a few miuutes it fell to ... ... ... ... ... 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, 

 15, 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 uinute inter- 

 vals, the observations being taken at intervals of 

 ten minutes: 15, ^D, 12, 22, 14, etc. 



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 



