412 G. H. Porker. 



by a more rapid one. This is seen to sweep over the slower wave 

 and, as it does so, what seems to be a reversed wave starts from 

 the point of colHsion and runs aborally over not more than six or 

 eight plates at most. This short wave is the only evidence of 

 reversal that I have found in Mnemiopsis; it is my belief 

 that this reversal of the swimming-plate action, so common in 

 many ctenophores, is almost entirely absent from this species. 



According to previous mvestigators, ctenophores can reverse 

 the effective stroke of the swmiming-plates as well as change the 

 direction of the propagation wave. Under ordinary conditions, 

 the effective stroke carries the animal with the oral end forward; 

 when this is reversed, the animal moves with the aboral end 

 ahead. Chun .('80, p. 181) mei]tions that this reversed form of 

 locomotion is a regular though rare occurrence with all cteno- 

 phores, especially when by normal locomotion their oral ends 

 collide with some fixed body. Verworn ('91, p. 432) states that 

 he has on rare occasions observed this reversed swimming- in 

 Eucharis and Callianira, but not in other species of ctenophores. 

 I have never seen any evidence of the reversal of the effective 

 stroke in either Pleurobrachia or Mnemiopsis and I am inclined 

 to believe that Chun's statement that the effective stroke can be 

 reversed in all ctenophores, may be a mistake based upon a con- 

 fusion of this form of reversal with the reversal of the propagation 

 wave. In Pleurobrachia it can be easily shown that when the 

 propagation wave reverses from an oral to an aboral direction the 

 swimming-plates continue their effective stroke in an aboral 

 direction as before. 



When a row of swimming-plates in Pleurobrachia or Mnemiop- 

 sis is cut through so as to divide it into oral and aboral portions, 

 the plates in both parts cease to move for a short time and when 

 they resume their activity, the two parts are found to beat differ- 

 ently, /. e., their propagation waves are found to be independent. 

 In this respect the American species agrees with the European 

 forms experimented upon by Eimer ('80, p. 227), Verworn ('90 

 p. 167), and others. If a row in Mnemiopsis is cut with care, the 

 aboral part almost immediately begins to beat metachronally 

 with reference to its fellow of the same quadrant, and the oral part 

 reestablishes independent movements in a few minutes or even 

 seconds. In the quickness of recovery of the oral part Mnemiop- 

 sis is in strong contrast with Beroe and Eucharis, in which, accord- 



