218 THE ORIGIN OF THE NERVOUS SYSTEM 



Pleurobrachia. In a normal individual of Pleuro- 

 brachia which has been undisturbed for a time and shows 

 a slow rhythm, reversal is readily produced by direct 

 mechanical stimulation of any level of the row. Such 

 a stimulation usually gives rise to rhythmic impulses 

 which are transmitted in both directions, and the rhythm 

 of the impulse transmitted orally is almost always 

 more rapid than that of the impulse transmitted api- 

 cally. Evidently the region of stimulation is able to 

 initiate impulses with greater frequency down the 

 gradient than in the opposite direction. Moreover, 

 the first impulses transmitted apically usually disappear 

 within a short distance, often only two or three plates 

 from the level of stimulation, but if the stimulation is 

 continued they gradually extend their range and after 

 a few seconds may be transmitted all the way to the 

 apical end of the row. When impulses passing in 

 opposite directions meet, the result differs in different 

 cases. They may obliterate each other completely, so 

 far as further motor effect is concerned, or one of them 

 may obliterate the other and pass on, sometimes with 

 a visibly lower rate of transmission than before, appar- 

 ently because of its decreased intensity. In general the 

 rate of transmission of the impulse passing toward the 

 apical end is lower than that in the opposite direction, 

 or becomes lower as the impulse travels. 



In normal individuals of Pleurobrachia reversal of 

 direction of transmission over the whole length of the 

 row sometimes occurs apparently spontaneously for a 

 short time, and it may usually be brought about by 

 direct stimulation of the oral end of the row. In this 

 case also the first impulses are transmitted only a short 



