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A gradient or choice apparatus, based on the 'fluvarium' principle and suitable for testing the responses of the freshwater snail Bulinus (Physopsis) globosus to different partial oxygen tensions was used. In a gradual oxygen gradient established with this apparatus, B. (P.) globosus shows a significant preference for oxygenated water. In experiments with a choice between low and high partial oxygen tensions, B. (P.) globosus shows a greater preference for oxygenated water. Oxygen consumption rates vary considerably among individuals and decreases when lowering the Po 2 in the water.

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