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Summary. 1. Paramecium and echinoid larvae under certain environmental conditions swim downwards in light, upwards in darkness. Thus the phenomenon already described by various authors for arthropods exists not only in animals swimming by muscular movements but in organisms moving by ciliary action. 2. The phenomenon is reversible. 3. The most effective rays are in the ultra‐violet.
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Howard Fox (1925) studied this question.
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