NATURAL RHYTHM. 173 



exposed to such influence. Lastly, it will be ob- 

 served from the above table that the unnatural 

 amount of activity displayed by the organism while 

 in the oxygenated water entailed on it a consider- 

 able degree of exhaustion, as shown by the fact 

 that even a quarter of an hour after its restoration 

 to normal water its original degree of energy had 

 not quite returned. 



Carbonic acid. — As might be expected, this gas 

 has the opposite effects to those of oxygen. It is 

 therefore needless to say more about this agent, 

 except that if administered in large doses it destroys 

 both spontaneity and irritability. Nevertheless, if 

 its action is not allowed to last too long, the Medusae 

 will fully recover on being again restored to normal 

 sea-water. 



Nitrous oxide. — This gas at first accelerates the 

 motions of Sarsia, but eventually retards them. 

 I omitted, however, to push the experiment to the 

 stage of complete anaesthesia, which would doubt- 

 less have supervened had the pressure of the gas 

 been sufficiently great. 



Deficient aeration. — It may now be stated that 

 the Medusae are exceedingly sensitive to such slight 

 carbonization of the water in which they are con- 

 tained as results from their being confined in a 

 limited body of it for a few hours. The rhythm 

 becomes slowed and the contractions feeble, while 

 the pauses between the swimming bouts become 

 more frequent and prolonged. If the water is not 

 changed, all these sym[)toms become more marked, 

 and, in addition, the rhythm becomes very irregular. 



