1877.] 



Locomotor System of Medusa, 



477 



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. Lastly, 

 water below 20° or above 85° suspends the natural rhythm ; and the 

 artificial rhythm is suspended at about the same degrees. 



(c) Oxygen accelerates, while carbonic acid retards and eventually sus- 

 pends the artificial rhythm, in just the same way as these gases act on 

 the natural rhythm. 



(d) When the marginal ganglia of Sarsia are removed, the manubrium 

 shortly afterwards relaxes to five or six times its normal length. There 

 can be no doubt that this effect is due to the muscular fibres of 

 the manubrium having been previously kept in a state of tonic con- 

 traction by means of a continuous ganglionic discharge from the margin. 

 Now physiologists are unanimous in regarding muscular tonus as a kind 

 of gentle tetanus due to a persistent ganglionic stimulation, and against 

 this opinion nothing can be said. But, in accordance with the accepted 

 theory of ganglionic action, physiologists further suppose that the only 

 reason why some muscles are thrown into a state of tonus by ganglionic 

 stimulation, while other muscles are thrown into a state of rhythmic 

 action by the same means, is because the resistance to the passage of the 

 stimulation from the ganglion to the muscle is less in the former than 

 in the latter case. On the other hand, the new theory of ganglionic 

 action explains the difference by supposing a different degree of irrita- 

 bility on the part of the muscles in the two cases ; for it will be remem- 

 bered that in my experiments on paralyzed Aurelia, if the continuous 

 stimulation were of somewhat more than minimal intensity, tetanus was 

 the result, while if such stimulation were but of minimal intensity, the 

 result was rhythmic action. JSTow I find in the case of Sarsia that the mus- 

 cular tissue of the manubrium is more excitable than the muscular tissue 

 of the bell ; so that, for this and other reasons, the facts here accord more 

 closely with the exhaustion than with the resistance theory of ganglionic 

 action. But, in conclusion, I should like to say that as yet I regard the 

 former theory as of a merely provisional character, and that I have pub- 

 lished it thus prematurely in order that if, as my experiments strongly 

 suggest, it is the true theory of rhythm, other physiologists may be able 

 to test it on rhythmically contracting tissues in general. 



IT, SECTioir. 



§ 1. (A) Ueflex action.— ^]iQ occurrence of reflex action in theMedusse 

 is of a very marked and unmistakable character. For instance, if the 

 manubrium of Sarsia or of Aurelia be irritated, the swimming-organ re- 

 sponds to the irritation by giving one or more contractions; if the 

 marginal ganglia be now removed, the swimming-organ no longer 

 responds even to the most violent irritation of the manubrium. Again, in 

 Aurelia, if only one lithocyst be left in situ, and if, during a pause in the 



