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 

 freezino-. In no case, however, that I observed did 

 tlie rate of the rhythm after the freezing return 

 fully to that which had been manifested before the 

 freezing. 



Effects of Certain Gases on the Rhythn. 



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 Sarsire 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 . . . 472, 527, 470 



In oxygenated sea-water . . . 800 



]u 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 swim- 

 ming bouts was greatly curtailed, the swimming 

 motions, in fact, being almost quite continuous 

 throuo-hout the five minutes tliat the Medusa was 



