22 PULSATION OF JBLLYFISHES. 



with the electrodes the contraction travels all the distance around A , 

 but the sector B does not contract. The path of least electrical resist- 

 ance is evidently through the long strip of sub-umbrella tissue, while 

 the short path across the cuts interposes a greater resistance. 



PULSATION WITHOUT MARGINAL SENSE-ORGANS. 



Romanes, Eimer, von Uexkiill, and others, have shown that in Scy- 

 phomedusae the marginal sense-organs are centers which discharge the 

 stimuli producing the rhythmical movements of the disk; and that 

 if we remove these sense-organs, a more or less complete paralysis of 

 the disk occurs. In some forms, such as Aurelia and Dactylometra, 

 this paralysis lasts but a few minutes, and then more or less irregular 

 contractions commence. In Rhizosioma pulmo, according to Hargitt, 

 the paralysis is much more pronounced than in Aurelia. In Cassiopea 

 xamachana the paralysis is practically complete for at least 24 hours, 

 the disk responding only to definite stimuli, and very rarely giving 

 a contraction without evident cause. On the second day after the 

 operation the disk is much more sensitive to stimuli of all sorts 

 and gives occasional isolated contractions without apparent stimu- 

 lation, and at the end of a week the disk can rarely be observed for a 

 minute without one's seeing it give a number of quick, isolated contrac- 

 tions. Regular rhythmical pulsation never sets in, however, unless 

 the marginal sense-organs be regenerated. 



Hitherto, disks without sense-organs have always been maintained 

 in sustained pulsation by constant artificial stimulation, or by being 

 placed in more or less injurious stimulating solutions. It will be 

 recalled that Romanes obtained regular pulsation in the disks of 

 Aurelia by passing through them a constant, or faradaic, current of 

 electricity of minimal strength. He thus demonstrated that rhythmi- 

 cal movements might result from a constant stimulus, and he showed 

 that one contraction could not follow another until the sub-umbrella 

 tissue had recovered from the exhaustion caused by the previous 

 contraction ; then, and then only, can the tissue respond to the ever- 

 present stimulus. Romanes concluded, therefore, that the ganglia of 

 the marginal sense-organs may exert a constant stimulus, and yet give 

 rise to periodic contractions. Romanes also found that the paralyzed 

 bell of Sarsia could be set into a "flurried shivering" pulsation for 

 one hour by a solution of 10 to 20 drops of acetic acid in 1000 cc. of 

 sea-water, and that it would also respond by rhythmic contractions 

 to a solution of 5 per cent glycerin in sea- water. 



In 1900 lyoeb found that the paralyzed disk of Gonionemus will 

 pulsate rhythmically for an hour in a solution of ^n NaCl or ^n 



