1. The reaction of the catfish, Schilbeodes gyrinus Mitchill, to hydrochloric acid over a wide range of concentrations (from pH 1.82 to pH 6.83) has been studied under experimental conditions which reduced to a minimum all other stimuli.

2. As the [H+J increases within the limits mentioned, the reaction time of the fish decreases. In other words, the rate of the stimulation processes is an increasing function of the hydrogen ion concentration.

3. The effective stimulus is the hydrogen ion, since NaCl solutions of equivalent concentration were not stimulating.

4. Stimulation by hydrochloric acid is therefore correlated with the potential of the cation resulting from dissociation of the acid molecule.

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