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The ability of Hydra to carry out the feeding reflex in response to reduced glutathione was inhibited by either (i) standing in distilled water, (ii) the presence of ethylenediamine tetraacetic acid, or (iii) the presence of magnesium ions. These three types of inhibition were reversed instantaneously by the addition of calcium ions.
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Lenhoff et al. (1959) studied this question.
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