NATURAL RHYTHM. 169 
an Aurelia be left for some little time in water at 
80°, and then again transferred to water at 30° or 
40°, its original rate of rhythm at the latter tem- 
perature does not again return, but the rhythm 
remains permanently slowed. And, in favour of 
the explanation just offered, it may be further 
pointed out that the first effect of sudden immer- 
sion in heated water is to quicken the rhythm, it 
not being for a few seconds, or for even a minute 
or two after the immersion, that the rhythm be- 
comes slowed. Lastly, the slowing takes place 
gradually ; and this is what we should expect if, 
as is probable, the destructive effect takes somewhat 
more time to become fully developed than does the 
stimulating effect. 
Before leaving the subject of temperature in 
relation to rhythm, J must say a few words on the 
effects of cold. The following may be regarded as 
typical experiments. 
An Aurelia presenting a regular rhythm of 
twenty per minute in water at 45° was placed in 
water at 19°. Soon after the transference the 
rhythm began to slow, and the strength of the 
contractions to diminish. Both these phenomena 
rapidly became more and more pronounced, till the 
rhythm fell to ten per minute (still quite regular), 
and the contractions ceased to penetrate the muscular 
tissue further than an inch or so from the marginal 
ganglia. Shortly after this stage pauses became 
frequent, but mechanical or other irritation always 
originated a fresh swimming bout. Next, only one 
very feeble contraction was given at long and 
