TRANSMISSION AND CONDUCTION 



213 



structure made up of elements resembling large cilia 



placed side by side and adhering closely to each other. 



In movement each plate behaves like a single cilium. 



Under normal conditions an orally directed metachronism 



appears in the movements of the plates of each row, that 



is, the wave of beats begins 



at the apical end of the row 



with the beat of the first 



plate, the beat of the second 



plate follows slightly later 



and so on down the row. 



The movement of the plates 



and its progression down the 



row presents the appearance 



of a wave traveling in the 



oral direction. Such waves FIG. 67. Side view of the 



r n ,1 j i body of the ctenophore, Pleu- 



follow each other at widely , , . ,, .' 



roorachia (from Hertwig, 1905, 



different intervals, these be- after Chun), 

 ing long, in some species 



several seconds, when the animals are completely 

 undisturbed and quiescent, and much shorter, often 

 only a small fraction of a second, when the animals are 

 excited. Since the progress of the wave along the row 

 is obviously the visible motor effect of a transmitted 

 impulse of some sort, these plate rows constitute most 

 interesting material for the experimental investigation 

 of a relatively primitive type of transmission, and their 

 behavior under various conditions has been repeatedly 

 studied. Extended discussion of the literature of this 

 subject is impossible at this time, but some of the earlier 

 work has been considered in a recent paper (Child, 

 and in an earlier study by Parker (1905). 



