4o8 



G. H. Parke 



of this paper to discuss the cause of metachronism in ciliary action 

 as exempHfied in the swimming-plates of these animals. 



The material upon which I worked consisted almost entirely 

 of the common summer ctenophore of the New England coast, 

 Mnemiopsis leidyi A. Agassiz, though I also made some observa- 

 tions on the winter species Pleurobrachia rhododactyla L. Agassiz. 

 The work was done for the most part during the last few summers 

 at the Wood's Hole Laboratory of the United States Bureau of 

 Fisheries, to the officers of which I am under obligations for many 

 kindnesses shown me. 



II. OBSERVATIONS. 



Anatomical. 



Mnemiopsis leidyi is a lobate ctenophore measuring often as 



much as seven or eight centi- 

 metres in leno;th. Its external 

 form is shown in Fig. i, which is 

 a view of the animal so placed 

 that its sagittal plane corresponds 

 to the plane of the paper. The 

 mouth is directed downward and 

 the two large lobes that charac- 

 terize this group of ctenophores 

 are seen at the right and left of 

 it. The aboral pole is pointed 

 upward and four of the eight 

 rows of swimming-plates are 

 shown converging toward it. 

 Their relation to the sense body 

 at the aboral pole can be seen 

 clearly in Fig. 2, where it will 



Side view of Mnemiopsis leidyi. The sagit- bg obsetVed that from the mOSt 

 tal plane corresponds to that of the paper and i i i r i 



the aboral pole is uppermost. Two short sub- aboral plate oi each row a nar- 



transverse rows of swimming plates and two roW band eXtends tO the SenSe 



long subsagittal ones are seen converging to- , , ^j^, , j i r I 



ward the aboral pole. The subsagittal rows DOdy. 1 hcSC bandS, beiOte they 



extend as vibratile lines far over the surface of rcach the Sense bodv, UnitC in 

 the lobes. . . , -' 



pairs and enter that organ as 

 four bands. As will be seen by comparing Figs, i and 2, the 

 rows of swimming-plates are either long or short and the pairs 



Fig. I. 



