CONTRIBUTIONS FROM THE ZOOLOGICAL LABORATORY OF THE MUSEUM 

 OF COMPARATIVE ZOOLOGY AT HARVARD COLLEGE. E. L. MARK, 

 Director.— No. 169. 



THE MOVEMENTS OF THE SWIMMING-PLATES IN 



CTENOPHORES, WITH REFERENCE TO THE 



THEORIES OF CILIARY METACHRONISM. 



BY 



G. H. PARKER. 



With 2 Figures. 



( I. INTRODUCTION. 



Since the publication in 1880 of Chun's elaborate monograph 

 on the ctenophores, it has been generally admitted, contrary to 

 the opinion of many of the older investigators, that the swimming- 

 plates of these animals are their principal organs of locomotion. 

 Moreover, the ciliary nature of these organs may now be regarded 

 as well established, and their relatively enormous size has already 

 made them favored objects with investigators of ciliary phenom- 

 ena. As is well known, these swimming-plates are arranged in 

 rows and the members of each row, like ordinary cilia, beat 

 metachronally, not svnchronally. The explanation of this pecu- 

 liarity has called forth two somewhat opposing views. According 

 to the first of these, which has been developed chiefly by Engel- 

 mann ('68, p. 475; '79, p. 388), it is maintained that one element 

 beats immediately after its next neighbor in a given order because 

 of a nerve-like impulse that is supposed to pass from cell to cell 

 and thus to bring into action in regular sequence the overlying 

 elements. This may be called the neuroid theory of ciliary action. 

 According to the second view, advanced in the main by Verworn 

 ('90, p. 175), the cause of metachronal action is not to be sought 

 for in the cell-body proper, but rather in the mechanical effect of 

 one cilium on another, in that the action of one cilium mechanically 

 stimulates the next one to action. This may be called the me- 

 chanical theory of ciliary action. Because of the minute size of 

 ordinary cilia, experimental tests of these two theories are not 

 easily carried out; hence the anatomical conditions presented in 

 ctenophores are of unusual importance. It is the principal object 



