IONS 



99 



valent cations K^, Na+, Li+ and NH4+ generally induced reversal, 

 while the divalent and trivalent cations did not. The former 

 authors, who used a large number of chemicals, found that the 

 polyvalent cations were able to neutralize monovalent ones, and 

 they concluded that cation unbalance upset the electrical equili- 

 brium of the membrane. 



The concentration of ions present in the medium may modify 

 the reversal response of these protozoan cilia. Okajima (1954b) 



24 r 



0-1 0-2 3 4 0-5 



Concencration of MgClj (docs) and AlClj (crosses) in rriM 



Fig. 25. The effect of magnesium chloride and aluminium 



chloride at various concentrations on the frequency of beat of 



Stentor membranelles (from Sleigh, 1956a). 



found that if Opalina is placed in a calcium -free Ringer solution, 

 it quickly loses the ability to reverse the direction of beat in 

 response to a stimulus; the same happens in potassium chloride 

 isotonic with the normal medium, where the absence of Ca++ is 

 again likely to be the cause. At other potassium concentrations 

 there is little change in the threshold of the reversal response, 

 although the duration of reversal is longest at medium concentra- 

 tion. The higher the concentration of calcium in the medium, 



H 



