The Movements of the Sivimming-Plates in Ctenophores. 419 



plates are in this respect. The slightest touch will often cause 

 them to vibrate and will even give rise to a wave which, beginning 

 with the plate stimulated, runs orally over the row. This condition 

 is undoubtedly suggestive of such a view as that advanced by 

 Verworn; his ('90, p. 168) ingenious experiment of attaching 

 plate to plate by cotton filaments and thus obtaining a form of 

 mechanical transmission shows how this idea may find applica- 

 tion. When, however, it is remembered that in rest the plates 

 point orally, that the propagation wave ordinarily proceeds from 

 the aboral end of the row, and that the effective stroke of the plate 

 is made in the aboral direction, it is clear that each plate as it goes 

 into action does not strike toward the next plate to act but away 

 from it and hence in a direction unfavorable for mechanical stimu- 

 lation. When the propagation wave is reversed, as happens in 

 Pleurobrachia and probably in many other ctenophores, the action 

 of the plates is such that an oral one may well stimulate mechani- 

 cally the next in turn, and, while I believe that the normal wave 

 depends for its propagation upon a neuroid transmission, I am 

 inclined to the opinion that the reversed waves may depend largely 

 on mechanical transmission. As is well known, these reversed 

 waves seldom extend far and are always insignificant as compared 

 with the normal ones. Hence I do not believe that mechanical 

 stimulation plays any really important part in transmitting the 

 normal wave. 



Direct stimulation seems to be a possible means of transmission 

 over a cut in a row of plates. Since both Eimer ('80, p. 229) and 

 Verworn ('90, p. 167) have recorded the occurrence of this form 

 of transmission in European species, it might be looked for in other 

 forms, though I have beenunableto find any evidence of it in Mnem- 

 iopsis or Pleurobrachia. However, I see no reason why the vibra- 

 tion of a plate on the aboral side of a cut may not stimulate to 

 action a plate on the oral side of the same cut provided the two 

 plates are brought close enough together. The subject is worthy 

 of further investigation. 



Most of the observations that have been brought forward 

 against the mechanical theory might now be urged in favor of the 

 neuroid theory, for transmission without ciliary or plate motion 

 is what is implied by this view. The idea that the movement of 

 the swimming-plates is controlled by nerves was held by some of 

 the older investigators such as Eimer ('73, p. 45) and Krukenberg 



