The Movements of the Sivimming-Plates in Ctenophores. 409 



formed by the unions into the bands consist always of a long rov/ 

 combined with a short one. Since the long rows lie near the 

 sagittal plane and the short ones near the transverse, they have 

 been called, respectively, subsagittal and subtransverse. The 

 combination of a long subsagittal row with a short subtransverse 

 one to form a pair has long been known to be one of the structural 

 characteristics of the lobate ctenophores, and, as will be shown 

 later, this feature is not without its physiological significance. 

 Each such pair, as can be seen in Fig. 2, is restricted to a quad- 

 rant of the animal's body. 



The number of swimming-plates in the subsagittal and the sub- 

 transverse rows varies more or less with the size of the animals. 

 Thus in a small specimen eight 

 millimeters long the subsagittal rows 

 contained each about 19 plates, the 

 subtransverse ones about 13; while in 

 a large individual sixty millimeters 

 long, there were about 73 plates in 

 each subsagittal row and about 39 in 

 the subtransverse ones. In the speci- 

 men from which Figs, i and 2 were 

 drawn, there were about 29 plates in 

 each subsagittal row and about 17 in 

 each subtransverse one. 



In Mnemiopsis the bands that lead 

 from the sense body to the swimming- 



„l^ _ "I'^J • ^L ^ Aboral view of Mnemiopsis leidyi. 



plates are ciliated, as in other cteno- The four subsagittal rows of sw.mming- 



phoreS, and, as SamaSSa ('92, p. 229) plates, two from each lobe, and the 



hi r ^1 1 u ^ r _ four subtransverse ones converge on the 



as shown for other lobate forms, a sense body at the ahoral pole. 



band of cilia connects plate with plate. 



In this species, however, the spaces between the plates seem to 

 be much more sparsely provided with cilia than in other lobate 

 ctenophores, if, in fact, cilia are not sometimes entirely absent 

 from these regions. 



The second species upon which I worked, Pleurobrachia 

 rhododactyla, was of simpler structure than Mnemiopsis. It 

 belongs to the Cydippidae and has the typical form of an oblong 

 spheroid. Its eight rows of swimming-plates are of about equal 

 length and can be readily distinguished as subsagittal or sub- 

 transverse only by their relations to other parts in the animal's 



Fig. 2. 



