The Movements of the Sivtmming-Plates in Ctenophores. 42 1 



opinion, this factor would never of itself result in giving rise even 

 to a single full w^ave, though it might, if vigorously started, carry 

 a w^ave over a small number of plates. Its influence at most would 

 be only of a subordinate character. Chun's view that both neu- 

 roid and mechanical factors take part in transmission has been 

 adopted by Piitter ('03, p. 98). 



While mechanical transmission may be of only subordinate 

 importance, mechanical stimulation must be regarded as of no 

 small significance. It has already been pointed out that a plate, 

 if mechanically stimulated, may become the point of origin of a 

 wave which will be transmitted over the row of plates in all 

 respects normally. Hence mechanical stimulation will not only 

 bring a plate into action but will induce the formation of a normal 

 neuroid propagation wave. 



If the ciliated epithelia of the higher animals and such special- 

 ized structures as the swimming-plates of the ctenophores are con- 

 trolled by impulses that are passed in a definite direction and 

 within circumscribed limits from cell to cell, it seems highly 

 probable that many of the coordinated responses of the lower 

 metazoans and of the early larval stages of the higher forms may 

 depend upon this form of mechanism rather than on any kind 

 of true nervous organization. Thus it may well be that the slow 

 but uniform responses of sponges to local stimulation may be due 

 to neuroid transmission through their epithelial layers and not 

 through true nervous tissue, which, as is well known, has been 

 sought for in vain in these animals. The exact orientation to light 

 of larval sea urchins at the blastula or gastrula stage involves a 

 certain coordinated beat of the cilia which, in the absence of ner- 

 vous elements, may well be due to neuroid transmission. Thus 

 animals in such early stagess of growth may carry out by means of 

 their epithelia reactions which in later stages would be performed 

 by a true nervous mechanism. Conditions of this kind lead me 

 to believe that before primitive metazoans possessed any nervous 

 organs whatever, they probably had in their epithelia organs 

 which exhibited the most fundamental property of nervous tissue, 

 namely, a capacity to transmit in a prescribed direction impulses 

 to motion. From epithelia of this kind sense organs and central 

 nervous organs were probably evolved, and yet this evolution did 

 not bring about the entire suppression of these primitive pre- 

 nervous mechanisms; for the ciliated epitheila of the highest 



