The Mechanism of Ciliary Movement. 



115 



In a subsequent paper, however, evidence will be presented which indicates 

 that magnesium plays an important role in the economy of the cell. 



The Effects of the Calcium Ion. — If calcium be omitted from the external 

 medium, and the other conditions be the same as in normal Van't Hoff 

 solution, prolonged ciliary movement takes place. The effect of the absence 

 of calcium is, however, seen in the increased sensitivity of the cell to 

 hydrogen ions. This is seen in the following experiment : — 





Duration of movement in 



Ph. 









Van't Hoff solution, 



Yan't Hoff solution, 





containing calcium. 



"without calcium. 



7-8 



More than 48 hours 



More than 48 hours. 



7-0 



More than 48 hours 



15-45 mins. 



If, after the cessation of movement in the absence of calcium, the alkalinity 

 ■of the solution be raised, rapid recovery takes place. If, on the other hand, 

 calcium is added, the amount of recovery, at least for a time, depends upon 

 the time which has elapsed between the cessation of movement, and the 

 addition of the calcium. If the time is short, rapid and complete recovery 

 takes place within 1 minute. If the time be prolonged, the recovery, on 

 addition of calcium, is slow : the amplitude of the beat is regained almost 

 ■at once, but the rate of both the effective and recovery strokes is slow, 

 and there are often marked pauses at the beginning and end of each stroke. 

 The whole phenomenon recalls the effect of acid, and one might conclude 

 that the effect of the absence of calcium is possibly due to a change in the 

 cell produced by a change in the hydrogen ion concentration. 



The cessation of movement in the absence of calcium, and the recovery 

 of movement on the addition of calcium or hydroxyl ions, is paralleled by 

 the reaction of such solutions on the heart. 



(c) The Effect of varying the Osmotic Pressure of the Surrounding Medium. 



Although the cilia are not sensitive to slight changes in osmotic pressure, 

 yet, if this exceeds a certain value, the cilia are rapidly brought to a com- 

 plete standstill. On reducing the osmotic pressure, instant and complete 

 recovery takes place. These facts are extremely easy to demonstrate, and 

 can be repeated a great many times with the same piece of gill. 



It does not matter whether the increase in osmotic pressure is brought 

 about by the addition to sea-water of non-electrolytes or by balanced 

 electrolytes {e.g., 2h M. Van't Hoff solution). In a solution which is not 



