680 
ME. Gr. J. EOMANES ON THE LOCOMOTOE SYSTEM OF MEDUSAE. 
that those specimens which recovered soonest had the rate of their rhythm least 
affected by the freezing. In no case, however, that I observed did the rate of the 
rhythm after the freezing return fully to that which had been manifested before the 
freezing. 
§ 7. Effects of certain gases on the Bhythm. ( a ) Oxygen. — I will now conclude my 
remarks on rhythm by very briefly describing the effects of certain gases. Oxygen forced 
under pressure into sea-water containing Sarsia has the effect of greatly accelerating the 
rate of their rhythm. The following observation on a single specimen will serve to 
render this apparent. 
Number of pulsations given by Sarsia in successive five minutes’ intervals. 
In ordinary sea-water 472, 527, 470 
In oxygenated sea-water . . . . 800 
In ordinary sea-water 268, 350, 430 
It will be seen from this observation that the acceleration of the rhythm due to the 
oxygenation was most marked. Indeed the pulsations followed one another so rapidly 
that it was no easy matter to count them. It must also be stated that while the animal 
was under the influence of oxygen, the duration of the natural pauses between the 
swimming-bouts was greatly curtailed — the swimming-motions, in fact, being almost 
quite continuous throughout the five minutes that the Medusa was exposed to such 
influence. Lastly, it will be observed from the above Table that the unnatural amount 
of activity displayed by the organism while in the oxygenated water entailed on it a 
considerable 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. 
(b) 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 admi- 
nistered 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. 
(c) 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 doubtless have supervened had the pressure of the gas been 
sufficiently great. 
(d) Deficient aeration. — It may now be stated that the Medusae are exceedingly sensi- 
tive to such slight carbonization of the water in which they are contained 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 symptoms become 
more marked, and in addition the rhythm becomes very irregular. Eventually the 
swimming-motions entirely cease ; but almost immediately after the animals are restored 
