410 G. H. Parker. 



body. In a specimen of average length, about sixteen millimetres, 

 there were approximately 40 plates in each row. 



Physiological. 



The resting position of the swimming-plates in both Mnemiop- 

 sis and Pleurobrachia is one in which the individual plate is turned 

 close to the body of the animal and with its tip directed 

 orally. In action the plate makes a vigorous stroke aborally and 

 then returns to its resting position. In consequence of such 

 movements carried out more or less simultaneously by certain 

 plates in each row, the animal's body is moved through the w^ater 

 with the mouth forw^ard. The plates in any one row strike one 

 after another beginning at the aboral end, /. e., to use the term 

 proposed by Verworn ('90, p. 152), they beat metachronally. 

 Ordinarily the first plate to strike is the most aboral one and the 

 others follow in sequence giving rise, by the order of their beat, to 

 a wave-like appearance which progresses, of course, in an oral 

 direction. 



Chun ('80, p. 172) has shown that when in a normal animal a 

 wave starts over one row, a like wave also starts over the other row of 

 the same quadrant, i. e., the two rows of any quadrant act in 

 uni on. This relation was observed by Verworn ('91, p. 456) in 

 all the ctenophores that he studied, and it is certainly an invariable 

 occurrence in Mnemiopsis, but not in Pleurobrachia. In Pleuro- 

 brachia, though the rows of plates on the same quadrant are 

 often seen to beat in unison, they also frequently beat independ- 

 ently. That their beating in unison is not a mere matter of 

 accident is seen from the fact that, whereas rows on the same 

 quadrant often beat in unison, adjacent ones belonging to different 

 quadrants do not beat in this manner. There can be no question, 

 I believe, that the rule laid down by Chun, to the effect that rows 

 on the same quadrant always beat in unison, has its exceptions, 

 for in Pleurobrachia the two rows of any quadrant may beat 

 independently. As might be expected from the researches of 

 Chun ('80, p. 172), the removal of the sense body from Mnemiop- 

 sis or from Pleurobrachia is invariably followed by a complete loss 

 of the partial or perfect unison of action between rows of plates. 



Since there is an agreement in the metachronism of the 

 two rows of plates on any quadrant in Mnemiopsis, there should 



