12 PUPATION OF 



rates, but the fastest controls all the others and forces them to beat 

 in unison with it. To test this, I took a Cassiopea having 19 margi- 

 nal sense-organs and a normal unexcited rate of 12 to 1 6 pulsations 

 per minute. I then made 19 radial cuts 

 midway between the 19 sense-organs, 

 so as to divide the disk into 19 practi- 

 cally equal sectors, each enervated by a 

 single sense-organ. These radial cuts 

 through the sub-umbrella completely 

 separated the sectors one from another 

 in so far as the transmission of nervous 

 impulses were concerned (fig. 3). Un- 

 der these conditions one of the sectors 

 pulsated 18 times per minute ; 2 pul- 

 sated 17 times; 2 pulsated 16 times; i Fig. 3. 

 pulsated 15 times; 3 pulsated 9 times ; i pulsated 8 times; 4 pulsated 

 7 times; 2 pulsated 6 times ; 2 pulsated 5 times, and i failed to pulsate. 

 The sense-organs gradually change their rates, so that at the end of 

 an hour or two the fastest may sink to second or third place, etc. 

 Quite often one or more of the sense-organs either failed to send out 

 pulsations or did so at very infrequent intervals. These sense- 

 organs appeared normal, however, and if stimulated by being thrown 

 into sea -water containing i per cent excess of K 2 So 4 they initiated 

 pulsations at a rapid rate. 



As Romanes and Eimer showed, if we cut off all but one of the mar- 

 ginal sense-organs this one will maintain a rhythmical pulsation of 

 the disk, whereas if this last sense-organ be removed the disk at once 

 becomes more or less paralyzed. The disks of Aurelia or of Dactylo- 

 metra, however, begin to pulsate irregularly a few minutes after the 

 loss of the last marginal sense-organ, but Cassiopea remains practi- 

 cally paralyzed for about 24 hours after the operation, rarely executing 

 a pulsation unless stimulated. On the following day, however, it 

 occasionally pulsates without apparent stimulation, and three days after 

 the operation the disk rarely remains for a minute without pulsating. 

 The pulsations are, however, isolated, single, and separated by irregular 

 intervals of time, until the marginal sense-organs begin to regenerate. 

 Romanes showed that in Hydromedusae the least discernible rem- 

 nant of the bell-margin if left intact will maintain the rhythmical 

 movement of the bell, but that in Scyphomedusse the marginal sense- 

 organs are the only parts of the rim which normally control the 

 rhythmical pulsation. I find that if one cuts off the tip of the last 

 remaining sense-organ of Cassiopea^ thus removing the otoliths and 



