The Mechanism of Ciliary Movement. 



109 



It is interesting to mention a few experiments which were performed on 

 the ciliated plates of the Ctenophore PleurobracKia (see fig. 5). 



Like the lateral cilia of Mytilus there is here definite metachronial 

 movement, hut the movement of each cell is dependent upon a stimulus 

 passing to it from the cell next to it. If the ciliated comb is cut, then all 

 those cilia situated on the oral side of the cut cease to beat, and come to rest 

 at the beginning of the effective beat. Any cell of this oral portion can be 

 thrown into motion by stimulating the cell immediately above it. As far as 

 one can see, these cilia resemble skeletal muscle rather than cardiac muscle, 

 since some form of external stimulus is necessary to produce a mechanical 

 response from the individual cells. 



II. The Influence of the Environment on Ciliary Activity. 



It has already been shown (Gray, 12) that a satisfactory medium for 

 ■ciliary activity is provided by a Van't Hoff's solution containing NaCl, KOI, 

 MgCl2 and CaClg, in the same proportions as in sea-water, and whose 

 hydrogen ion concentration is about Pn 7'8. We can therefore regard the 

 other constituents of normal sea- water as unessential. 



(a) The Effect of Varying the Hydrogen-ion Concentration. 



The fact that the cilia on the gill of Mytilus cease to move when the 

 hydrogen ion concentration of the surrounding medium reaches a limiting 

 value, and that the cilia will resume their movements when the acidity of 

 the medium is reduced (Gray, 12), led to the following investigation of 

 Bernstein's well known hypothesis. 



If the hypothesis be sound, then the inhibiting powers of an acid medium 

 should be inversely proportional to the rate at which the acid can enter the 

 •cell. It has been shown that mineral acids enter living cells very slowly, if 

 at all, whereas the fatty acids enter readily. Pieces of living gill, stained 

 with neutral red, undergo no change in colour when placed in Van't Hoff 

 solution, to which sufficient hydrochloric acid has been added to produce a 

 hydrogen ion concentration of Pn 3"4 : when placed in a similar solution, 

 made acid with acetic or butyric acid, the gill instantly changes from a dull 

 brick red colour to one of brilliant red — indicating that the acid has 

 entered the cells. Corresponding facts apply to the alkalies : ammonia 

 rapidly enters a living cell, whereas sodium hydroxide does not. 



It is obvious, therefore, that a method is available for deciding whether the 

 cessation of movement of cilia in an acid medium is due to an interference 

 with the electrical properties of the cell surface, or whether the affected 

 elements lie within the cell itself. 



