The Movements of the Sivimming-Plates in Ctenophores. 411 



be a synchronism in the action of the corresponding plates in 

 these two rows, and such proves to be the case. This condition is 

 very noticeable when at the beginning of a series of swimming- 

 plate movements, the waves run at varying rates, for when a wave 

 passes rapidly or slowly over one row, it passes at the same rate 

 over the other row of the same quadrant. Similar conditions 

 were observed in Pleurobrachia when its swimming-plates were 

 acting in unison. 



The reversed action of the swimming-plates in ctenophores 

 has been stated to occur by numerous observers, but the expression 

 reversed action in this connection is undoubtedly somewhat 

 ambiguous. In the so-called reversal of cilia and other like organs 

 at least two kinds of reversal are possible: a reversal of the 

 propagation wave, "Reizwelle" of Engelmann, and a reversal of 

 the effective stroke of the cilia (Parker, '05, p. 9). In the first 

 instance the question turns on the sequence in which the cilia 

 beat; thus in the normal action of a series of cilia, element a may 

 beat first and z last, the propagation wave passing from a to z; 

 while in reversed action z would beat first and a last, the wave 

 passing in the reverse direction. In the second instance only the 

 eff^ective stroke of the cilium is concerned; this may be normally 

 toward % or reversed toward a irrespective of the sequence in which 

 the cilia of the series act. 



In ctenophores a reversal of the direction of the propagation 

 wave has often been observed. This was early noticed on frag- 

 ments of Beroe by Eimer ('80, p. 226), an observation confirmed 

 on this and other species by Chun ('80, p. 182) and by Verworn 

 ('90, p. 167; '91, p. 459), though the latter is misquoted in this 

 respect by Piitter ('03, p. 35). Reversal of the propagation wave 

 occurs occasionally in Pleurobrachia. When a rapid wave from 

 the aboral end of the animal reaches the oral limit of a row of 

 plates, it may be reflected aborally over the row again, but it 

 seldom retraces its course for more than one-third the whole 

 length of the row. As Verworn ('90, p. 167; '91, p. 440) observed, 

 these reversed waves can often be induced by stimulating mechani- 

 cally the oral end of a row of swimming-plates. 



In Mnemiopsis I have never observed unquestionably reversed 

 waves, nor have I been able to induce them by special stimulation. 

 Some shght evidence of reversal has been seen when a relatively 

 slowly moving wave near the oral end of its course is overtaken 



