210 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 



occur in Sarsia, tend more in favour of the exhaus- 

 tion than of the resistance theor}^* 



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 aUoorefher exclusive of the 

 resistance theory, for it is quite possible that in addition to the 

 hi^h 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 Eoyal 

 Society papers, I see reason to believe that the natural swimming 

 movements of Sai'sia 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 to 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 dilierent ial 

 stimulation. 



