1. The action of certain ions on a marine amoeba has been studied. Movement takes place between the limits pH 6 and pH 10: it is reversibly inhibited at the acid limit but alkaline inhibition is reversible only after a brief immersion.

2. Of the four chief cations of sea-water Ca alone is essential for movement. Movement takes place in balanced solutions of Ca with any alkali metal, with no other cation present. NH4 is toxic. The optimum alkali-metal/Ca ratio for movement, and the velocity attained, increase with the atomic weight of the alkali metal. Viability in these solutions follows a different law : Na > K > Cs > Li and Rb.

3. Ca can be replaced by Sr but not by Mg or Ba. For the optimum movement, Na/Sr < Na/Ca.

4. Although movement occurs in either NaCl or KC1 alone Ca is present, yet it is maintained much longer when K is added to an (NaCl + CaCl2) solution. The effect is most marked where Na/K≑ 50, as in sea-water. This specific action of K on viability is shared by Rb.

5. In a solution of (NaCl + CaCl2) with excess Ca, movement is immediately inhibited, apparently owing to an increased ectoplasmic viscosity. The inhibition is reversible and resembles that caused by acid.

6. Inhibition from lack of Ca differs from that caused by excess. Movement is at first normal, but sooner or later falls to zero: the time required for this is less the lower the Ca concentration. This inhibition is not readily reversible.

7. In the effect of Ca deficiency amoeba resembles contractile tissues. In both cases the effect is counteracted by increased alkalinity. In amoeba the action is complicated because above pH 8 alkali tends to inhibit all movement if Mg is absent.

8. There is a general similarity between the action of ions on amoeba and on contractile tissues. The differences observed seem to be related to the long time required for effects to occurin amoeba. This is probably connected with adaptation to the variable external environment, in-shore sea-water.

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