216 THE ORIGIN OF THE NERVOUS SYSTEM 



row may be affected by direct stimulation of the mouth 

 region or other parts of the body surface, even after the 

 apical nervous tissue and the sense organ have been 

 removed. Evidently also the plate row is not a pure 

 effector, for it is able to initiate and transmit impulses. 

 After section of the plate row at any level, transmission 

 of the impulses from the apical region ceases at the level 

 of section, but the portion of the row oral to the level of 

 section begins, usually almost at once, or within a few 

 moments, to beat with a rhythm of its own and the 

 impulses are transmitted orally from the level of section. 

 Moreover, even a single plate isolated by section from 

 its neighbors may show rhythmic beats. 



An investigation of the plate row in the genera, 

 Mnemiopsis (Child, 191 yc), Bolina, Beroe, and Pleuro- 

 brachia (unpublished) by means of the susceptibility 

 method shows that the row is a susceptibility gradient, 

 its apical end being the most susceptible region. When 

 individuals are exposed to the action of inhibiting agents, 

 e.g., cyanide and various anesthetics, the apical end of 

 the row is affected first, and at a certain stage the 

 impulses from the apical organ appear at the apical end 

 of the row but are transmitted only a short distance 

 instead of the whole length of the row, as in normal 

 animals. Under these conditions the lower levels of 

 the row, being less susceptible, develop rhythmic 

 impulses of their own. As I have pointed out (191 yc), 

 they are physiologically isolated. At first there may be 

 only one such rhythm, originating in the region where 

 the apical impulse disappears, but later, as the action 

 of the agent progresses, several different rhythms may 

 appear at different levels of the row, those of the more 



