These experiments were made in order to ascertain whether the Medusæ, after having been thus completely frozen, would survive on being again thawed out, and, if so, whether the freezing process would exert any permanent influence on the rate of their rhythm. Now in all the cases the Medusæ, after having been thawed out, presented a ragged appearance, which was due to the disintegrating effect exerted by the ice-crystals while forming in the tissues; yet notwithstanding this mechanical injury superimposed on the physiological effects of such extreme cold, all the Medusæ recovered on being restored to sea-water of the normal temperature. The time occupied by the process of recovery varied in different individuals from a few minutes to half an hour or more, and it was observable 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. _Effects of Certain Gases on the Rhythm._ _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-minute 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. _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 Medusæ will fully recover on being again restored to normal sea-water.