204 JELLY-FISH, STAR-FISH, AND SEA-URCHINS, 
fluence that any given degree of temperature exerts 
on the rate of the natural and of the artificial 
rhythm respectively. Further, it will be remembered 
that in warm water the natural rhythm, besides 
being quicker, is not so regular as it is in cold 
water; thus also it is with the artificial rhythm. 
Again, water below 20° or above 85° suspends the 
natural rhythm, 7.e. stops the contractions; and 
the artificial rhythm is suspended at about the 
same degrees. Lastly, just as there are considerable 
individual variations in the extent to which the 
natural rhythm is affected by temperature, so the 
artificial rhythm is in some cases more influenced by 
this cause than in others, though in all cases it 
further resembles the natural rhythm in showing 
some considerable degree of modification under 
such influence. 
On the whole, then, it would be impossible to 
imagine two cases more completely parallel than 
are these of the effects of temperature on natural 
and on artificial rhythm respectively; and as it 
must be considered in the last degree improbable 
that all these coincidences are accidental, I conclude 
that the effects of temperature on the natural 
rhythm of Medusz (and so, in all probability, on 
the natural rhythm of other ganglio-muscular 
tissues) are for the most part exerted, not on the 
ganglionic, but on the contractile element. 
In order to test the effects of gases on the 
artificial rhythm, I took a severed quadrant of 
Aurelia, and floated it in sea-water, with its 
muscular surface just above the level of the water. 
