An Aurelia whose rhythm in water at 40° was very regular at eighteen per minute, was suddenly transferred to water at 80°. In the immediately succeeding minutes the rhythm was 22, 20, 14. The latter rate continued for nearly half an hour, when the observation terminated. The effect of very warm water, therefore, is to slow the rhythm, as well, I may add, as to enfeeble the vigour of the contractions. The case of Medusæ thus differs, in the former respect, from that of the heart; and I think the reason of the difference is to be found in the following considerations. Even slight elevations of temperature are quickly fatal to the Medusæ, so it becomes presumable that considerable elevations act very destructively on the neuro-muscular tissues of those animals. This destructive effect of high temperatures may, therefore, very probably counteract the stimulating effect which such temperatures would otherwise exert on the natural rhythm, and hence a point would somewhere be reached at which the destructive effect would so far overcome the stimulating effect as to slow the rhythm. That this is probably the true, as it certainly is the only explanation to be rendered, will, I think, be conceded when I further state that if 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 temperature 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 immersion 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 becomes 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, I must say a few words on the effects of cold. The following may be regarded as typical experiments.