172 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
minutes to half an hour or more, and it was observ- 
able 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 Sarsiz has the effect of greatly 
accelerating the rate of their rhythm. The follow- 
ing observation on a single specimen will serve to 
render this apparent. 
Number of pulsations given by Sarsia in succes- 
sive five-minute intervals. 
In ordinary sea-water sOee - 472, 527, 470 
In oxygenated sea-water . : - 800 
In ordinary sea-water. ° - 268, 350, 450 
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 swim- 
ming bouts was greatly curtailed, the swimming 
motions, in fact, being almost quite continuous 
throughout the five minutes that the Medusa was 
