MR, G. J. ROMANES ON THE LOCOMOTOR SYSTEM OF MEDUSA. 
181 
belief is that hitherto the theory of rhythm as cine to ganglia has attributed far too 
much importance to the ganglionic as distinguished from the contractile tissues, and I 
have founded this belief principally on the facts which have been already stated and 
which certainly prove, at least, this much : that after the removal of their centres of 
spontaneity the contractile tissues of the Medusae display a marked and persistent 
tendency to break into rhythmic action whenever they are supplied with a constant 
stimulus of feeble intensity. Without waiting again to indicate how this fact tends 
to suggest that the natural rhythm of the unmutilated organisms is probably in large 
part due to that alternate process of exhaustion and restoration of excitability on the 
part of the contractile tissues, whereby alone the phenomena of artificial rhythm can be 
explained,* I shall go on to describe some further experiments which were designed 
to test the question whether the influences which affect the character of the natural 
rhythm likewise, and in the same manner, affect the character of the artificial rhythm. 
I took the trouble to perform these experiments because I felt that if they should 
result in answering this question in the affirmative, they would tend still further to sub¬ 
stantiate the view I am endeavouring to uphold, viz.: that the natural rhythm may be 
a function of the contractile as distinguished from the ganglionic tissue. Of the 
modifying causes in question, the first that I tried was temperature. 
(n.) Having already, in my former paper, treated of the effects of temperature on 
the natural rhythm, it will now be sufficient to say that we have seen these effects to 
be similar to those which temperature exerts on the rhythm of ganglionic tissues in 
general. Now I find that temperature exerts precisely the same influence on the arti¬ 
ficial rhythm of deganglionated tissue as it does on the natural rhythm of the 
unmutilated animal. To economise space I shall only quote one of my observations in 
a table which explains itself. I also append tracings of another observation to render 
the difference in the rate of the rhythm more apparent to the eye.t 
Temperature of water 
(Fahr.). 
25° 
45° 
75° 
Number of contractions 
per minute. 
24 
40 
GO 
* It is of importance to point out the fact that some of my previously published experiments appear 
conclusively to prove that the natural stimulation which is supplied by the marginal ganglia of the 
Medusae resembles all the modes of artificial stimulation which are competent to produce artificial rhythm 
in one important particular; the intensity of the stimulation which the marginal ganglia supply is shown 
by these experiments to be about the same as that which is required to produce artificial rhythm in the 
case of artificial stimulation. In proof of this point I may allude particularly to the observations which are 
detailed in V., § 1 (a), ( b ), and (c) of my second paper. 
t As the effects of temperature in modifying the rate of artificial rhythm no doubt arise from the effects 
of temperature on modifying the excitability of the contractile tissues, I think it is desirable here again to 
mention a fact which was briefly stated in my second paper (p. 691), viz.: that in the excitable tissues 
of the Medusae, temperature exerts an immense influence, both on the latent period and on the activity 
