ME, Gr. J. EOMANES ON THE LOCOMOTOE SYSTEM OF MEDUSA. 
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“ 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 note will serve to convey 
a just estimation of these facts : — 
“An Aurelia , whose rhythm in water at 40° was very regular at 18 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 Medusse 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 Medusae, so it becomes presumable that considerable elevations act 
very destructively on the nervo-muscular tissues of these 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 instead of quicken it. 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 perma- 
nently slowed. And, in favour of the explanation just offered, it may be further pointed 
out that the first effects 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 immer- 
sion, 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. 
(b) Cold . — 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 20 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 10 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 
