NEUROID TRANSMISSION 



67 



animals the swimming plates form eight well-defined 

 rows that extend from the aboral pole toward, if not ac- 

 tually to, the oral pole. The effective stroke of each plate 

 is in the aboral direction, thus carrying the animal through 

 the water with its mouth forward. The wave of ciliary 

 action, however, sweeps over each row from its aboral to 

 its oral end and thus takes a 

 course the reverse of that indi- 

 cated by the effective stroke. 

 Hence it may be concluded that 

 the effective stroke and the 

 wave of ciliary action are inde- 

 pendent factors; for though 

 they usually agree in direction 

 they may be directly opposed 

 as in the example just given. 



The regularity with which 

 one cilium beats after another, 

 the coordinated metachronism 

 of the series, is the feature 

 of the ciliated epithelia that 

 most recalls nervous control and that requires explana- 

 tion. It might be supposed that this regularity was due 

 to the mechanical influence of a given cilium on the one 

 next following and so forth. Thus if cilium A begins to 

 beat, it will strike toward cilium B, which on being struck 

 will thus be called into action and by a similar process 

 excite C and so on. This operation, at least so far as the 

 effective stroke is concerned, is not unlike that seen in the 

 successive toppling over of a row of bricks each on end 

 where the fall of the first brick knocks over the second and 

 so on. Although this explanation finds an easy application 

 to the usual form of ciliary beat in which the effective 



Fio. 15. Side view of the cteno- 

 phore Mnemiopsis leidyi. Of the eight 

 rows of swimming plates four are 

 shown, two long ones and two short. 

 All start from the aboral pole o and 

 converge on the oral pole o. 



