1. Experiments with strychnine were performed to test two assumptions important in the development of a theory for the mechanisms involved in the tropisms exhibited by Daphnia.

2. After a brief interval in strychnine solution Daphnia exhibits a reversal of the primary sign (a) of phototropism, from negative to positive; and (b) of galvanotropism, from anodic to cathodic. In both cases the orientation of the body remains the same.

3. The mechanism responsible for the sign of phototropism and galvanotropism in Daphnia is therefore distinct from that underlying orientation.

4. Evidence is obtained indicating that changes in sign of tropism, produced by changes in illumination or by subjection to strychnine, involve the control of antagonistic muscles in the swimming appendages which are reciprocally innervated.

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