72 THE ELEMENTARY NEEVOUS SYSTEM 



are attached in the common American ctenophore Mne- 

 miopsis is cut across even in a very superficial way, the 

 ordinary transmission waves can be seen to proceed from 

 the aboral pole over the line of plates orally to the cut, 

 where they cease. Beyond the cut the plates for some 

 time after the operation are in incessant vibration, the 

 waves beginning at the cut end of the series and running 

 to the end near the mouth. The oral course of the trans- 

 mission wave is so universally characteristic of the cten- 

 ophores that the polarity of their rows of swimming 

 plates may be said to be as pronounced as that of the rows 

 of cilia in a ciliated epithelium. 



When a spot about midway the length of a row of 

 plates in Mnemiopsis is touched, the region about it im- 

 mediately becomes depressed and the edges of the de- 

 pression fold over and cover in the plates. This reaction 

 was observed long ago by Verworn (1890) on Beroe, and, 

 though it has been questioned by Bauer (1910), the ob- 

 servation seems to be abundantly confirmed by Lillie 

 (1906) and by Kinoshita (1910), who demonstrated fur- 

 ther that it could occur on a fragment of a Beroe over 

 which the row of swimming plates extended. By this 

 means in Mnemiopsis, and probably in a number of other 

 ctenophores, half a dozen plates or so near the middle of 

 a row may become so much restrained that they will not 

 show the least motion. Nevertheless, transmission- waves 

 that arrive at the aboral entrance to this depression 

 emerge from its oral end with the greatest regularity. 

 This may happen while the covered region is under the 

 closest inspection through a lens and during which not 

 the least sign of movement can be detected in the re- 

 strained plates. Thus the mechanical movement of the 

 swimming plates is no more necessary for the transmis- 



