41 8 G. H. Parker. 



rule the slight stretching of a region would bring the plates of that 

 part to a standstill, though it did not interfere seriously with trans- 

 mission. But it must be noted that in such an operation much 

 care must be used not to overstrain the tissue, for otherwise a 

 permanent cessation of action will follow. Avoiding this difficulty, 

 however, mechanical strain, like low temperature, may be made 

 to check motion without interfering with transmission. 



III. THEORETIC CONSIDERATIONS. 



The results of the experiments just described, in which the 

 swimming-plates of ctenophores were removed or restrained, or 

 the row chilled or stretched locally, afford good grounds for deny- 

 ing to the mechanical theory any essential part in the explanation 

 of ciliary metachronism. It ordinary transmission is really depen- 

 dent upon the mechanical action of one element on the next, it is 

 inconceivable how such a process can be accomplished when these 

 elements for any reason cease to move. That transmission does 

 take place after the swimming-plates have been brought to a stand- 

 still by physical restraint, cold, etc., is unquestionable. Verworn 

 ('90, p. 172) admitted surprise when he found that in Cestus 

 transmission occurred even after the removal of a plate and he 

 was led to assume a continuous band of cilia to account for this 

 condition. In Mnemiopsis no such band is present and yet 

 transmission takes place even after the removal of a plate. 



The failure of a wave to pass when the plates in Beroe are 

 restrained from moving is not, as Verworn believed, a satisfactory 

 test of the nature of transmission, for, notwithstanding the care 

 used in restraining the plate, the operation may influence the 

 deeper parts of the tissue and thus check transmission as well 

 as plate movement. The fact that transmission does occur 

 in Mnemiopsis when the plates are restrained, shows how treacher- 

 ous such negative evidence is. These facts, together with the 

 evidence from chilled and stretched rows, show, I believe, that the 

 mechanical theory is not a necessary part of the explanation of 

 ciliary metachronism. 



Although the mechanical theory may not be the correct explana- 

 tion of transmission, its rejection does not imply a rejection of the 

 idea that the plates are open to mechanical stimulation. Every- 

 one who has worked with ctenophores knows how sensitive the 



