NATURAL RHYTHM. 169 



an Aurclia be loft 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, bat 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 eftect takes some^vhat 

 more time to become fully developed than does the 

 stimulating effect. 



Before leaving the subject of temperature in 

 relation to rhythm, I 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 



