468 Mr. G. J. Romanes on the [Jan. 11^ 



per minute in water at 45° was placed in water at 19° : the rhythm 

 almost immediately began to slow and the strength of the contractions 

 to diminish, till the rate fell to 10 per minute (quite regular) and the 

 contractions ceased to penetrate the muscular tissue further than an inch 

 or two from the marginal ganglia ; pauses now became frequent, but 

 stimulation always originated a fresh bout of swimming. Next only 

 single contractions were given at long and irregular intervals, and these 

 contractions were so feeble that they were restricted to the immediate 

 vicinity of the lithocyst in which they originated. Soon after this stage 

 irritability disappeared. This process from first to last occupied rather 

 less than five minutes. On now leaving the animal for 10 minutes more, 

 and then transferring it to water at the original temperature of 45°, all 

 the above-mentioned stages were passed through in reverse order. 



Some specimens of Aurelia were frozen into a solid block of sea-water 

 ice. On being released all their gelatinous tissues were seen to be 

 pierced through in every direction by an innumerable multitude of ice- 

 crystals, which had been formed by the freezing in situ of the sea-water 

 which enters so largely into the composition of these tissues. Tet, on 

 being thawed out, the animals recovered, although their original rate of 

 rhythm did not fully return. Their tissues then presented a ragged 

 appearance, which was due to the disintegrating effect produced by the 

 formation of the ice-crystals. 



§ 7. Effects of certain Gases on the Rhythm. — Oxygen forced under 

 pressure into sea-water containing Sarsice has the effect of accelerat- 

 ing the rate of their rhythm. The following observation on a single 

 specimen will serve to render this apparent, the numbers of pulsations 

 being recorded in five-minute intervals. The progressive recovery from 

 exhaustion during the last of the three observations deserves notice. 



In ordinary water 472, 527, 470. 



In oxygenated sea-water 800. 



In ordinary sea-water 268, 350, 430. 



Carbonic acid has the opposite effects to those of oxygen, and, if ad- 

 ministered in too large doses, destroys both spontaneity and irritabihty. 



Nitrous oxide at first accelerates the motions of Sarsia, but eventually 

 retards them. I omitted, however, to push the experiment to the stage 

 of complete anaesthesia. 



Medusae are very sensitive 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 con- 

 tractions feeble, wdiile the pauses between the swimming-bouts become 

 more and more frequent and prolonged. If the water is not changed, all 

 these symptoms become more marked, and, in addition, the rhythm be- 

 comes more irregular. Eventually the swimming-motions entirely cease ; 



