210 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
occur in Sarsia, tend more in favour of the exhaus- 
tion than of the resistance theory.* 
I will now sum up this rather lengthy discussion. 
The two theories of ganglionic action may be 
stated antithetically thus: in both theories the 
accumulation of energy by ganglia is supposed to 
be a continuous process; but while the resistance 
theory supposes the rhythm to be exclusively due 
to an intermittent and periodic discharge of this 
accumulated energy on the part of the ganglionic 
tissues, the exhaustion theory supposes that the 
rhythm is largely due to a periodic process of 
exhaustion and recovery on the part of the respond- 
* The evidence, however, is not altogether exclusive of the 
resistance theory, for it is quite possible that in addition to the 
high irritability of the manubrium there may be conductile lines 
of low resistance connecting this organ with the marginal ganglia. 
I entertain this supposition because, as explained in my Royal 
Society papers, I see reason to believe that the natural swimming 
movements of Sarsia are probably in part due to an intermittent 
discharge of the ganglia. I think, therefore, that in this par- 
ticular case the ganglia supply a tolerably constant stimulation 
to the manubrium, while it is only at intervals that their energy 
overflows into the bell, and that the higher degree of irritability 
on the part of the manubrium ensures the tonic response of this 
organ at a small cost of nervous energy. How far the rhythm of 
the nectocalyx is to be attributed to the resistance mechanism of 
the ganglia, and how far to the alternate exhaustion and recovery 
of the contractile tissues, I think it is impossible te determine, 
seeing that it is impossible exactly to imitate the natural gan- 
glionic stimulation by artificial means. But it is, I think, of 
importance to have ascertained at least this much, that in Sarsia 
the tonus of one organ and the rhythm of another, which appa- 
rently both received their stimulation from the same ganglia, 
must at any rate in part be attributed to a differential irrita- 
bility of these organs, as distinguished from their differential 
stimulation. 
