In the case of Aurelia aurita, the characteristic effects of temperature on rhythm may be better studied than in that of Sarsia, from the fact that the natural motions are more rhythmical and sustained in the former than in the latter genus. I have, therefore, in this connection made more observations on Aurelia than on Sarsia. The following may be taken as a typical experiment. A small and active specimen of Aurelia contracted with the greatest regularity 33 times per minute in water kept at 34°; but on transference to water kept at 49°, the contractions always became irregular, in respect (_a_) of not having a perfectly constant rhythm, and (_b_) of exhibiting frequent pauses, which was never the case in colder water. The rate of rhythm in the warmer water varied from 37 to 49; and as in these observations no allowance was made for the occurrence of the pauses, the actual rate of rhythm during the swimming motions was about 60 per minute. The following are some sample observations in the case of this specimen:-- ----------------------------+---------------------+---------------- Temperature of water (Fahr.)|Number of pulsations.|Seconds of rest. ----------------------------+---------------------+---------------- 40° | 41 | 5 " | 49 | 4 Transferred to 34° | 33 | 0 " " | 33 | 0 " " | 33 | 0 " " | 33 | 0 Transferred to 49° | 45 | 4 " " | 39 | 10 " " | 37 | 15 Transferred to 34° | 20 | 0 " " | 30 | 0 " " | 33 | 0 " " | 33 | 0 " " | 33 | 0 " " | 33 | 0 ----------------------------+---------------------+---------------- This rate continued quite regularly for a quarter of an hour, when the observation terminated. It might naturally be supposed that when the alterations of temperature between 34° and 49° produce such marked effects on the rhythm, still greater alterations would be attended with still greater effects. Such, however, is not the case. Water at 70° or 80°, for instance, has the effect of permanently _diminishing_ the rate of the rhythm, after having temporarily raised it for a few seconds. The following experiment will serve to convey a just estimation of these facts.