the road to a direct physical explanation of electrotaxis. In criticism of the views of GREELEY, so far as hitherto brought out, the following must be said: 1. All thorough work thus far shows that the essential point in the reaction to the electric current is the method in which the current affects the cilia. No attempt has been made to show how the known effects on the cilia could be produced through the factors emphasized by GREELEY, and it would undoubtedly be difficult or impossible to bring the two into relation. 2. The movement toward the anode is not limited to acid solutions, but is known to take place in a still more striking way in various salt solutions, especially in a solution of sodium chloride. I have observed it even in a solution of sodium bicarbonate, having of course an alkaline reaction. 3. The movement to the anode in such solutions is backward. It has been so described by LOEB and BUDGETT (1897, p. 532), by PÜTTER (1900, p. 297), and so far as I am aware, by every one who has described it carefully, and I can myself confirm this fact. The organisms thus become oriented in the same manner, with anterior end to the cathode, as under usual conditions. Further, these same solutions produce backward swimming even without the use of the electric current. We have then all the existing features of the reaction fully accounted for without taking into consideration the factor considered essential by GREELEY. The electric current taken by itself accounts for the orientation in the usual way; the chemical stimulation taken by itself accounts for the swimming backward; the combination of the two accounts for the swimming backward to the anode. 4. The swimming to the anode continues only as long as the chemical stimulation exists. As soon as the organism has had time to become acclimatized to the chemical, it swims as usual to the cathode. This has been shown by PÜTTER (1900), and by STATKEWITSCH (1903 a), and I can confirm it. Often it is but a few moments that the swimming backward to the anode continues. In view of all these facts, it cannot be held on the evi dence thus far brought forth, that the phenomena observed in acid solutions, as described by GREELEY (1903), have any special significance for the theory of electrotaxis, such as that author assumes. The known facts point to the following general statement of the phenomena. Immersion in chemicals, of various characters, causes the organism to swim backward. If at this time the Paramecia are subjected to the electric current, they continue to swim backward, and, becoming oriented, therefore pass to the anode. This movement to the anode ceases as soon as the stimulating action of the chemical ceases. In order to make out a case for the theory advanced by GREELEY, it will be necessary to show clearly that this general statement is incorrect.1 IV. PRESENT POSITION OF INVESTIGATION OF THE BEHAVIOR OF PARAMECIUM. I believe it may be said that we are now able to make a general, qualitative survey of the chief facts and factors in the behavior of this representative of the unicellular animals. There are doubtless still some dark points; the reaction to the electric current, for example, is still hard to place in the general scheme of behavior, though recent researches have gone far toward clearing up this matter. But it is true that we know, in a general way, most of the chief methods of action of this animal, and the way in which these are affected by the chief classes of external conditions. There still remains the investigation of the intimate physiological processes underlying the gross features of the reactions, and especially the quantitative study of the phenomena which the qualitative examination has brought out. Our present knowledge, then, amounts to a preliminary survey, showing us in the gross the phenomena which require investigation in detail. Attempted quantitative study of phenomena of which the qualitative, purely descriptive, features Since the above was written, GREELEY'S final paper has appeared (Biol. Bull., Vol. 7, pp. 3–32). It raises many interesting questions, which I hope to touch upan later. (Note added during correction of proof.) are uncertain, is likely to be misleading and worthless; this has been too often illustrated in the investigations on the reactions of unicellular animals. We cannot measure things till we at least know what we are measuring; if we attempt it our results have only the appearance of accuracy, and are likely to fall to the ground as soon as the qualitative nature of the phenomena is worked out and found to be different from what we had assumed. It is for this reason that the present writer has limited himself thus far almost entirely to qualitative work. Now that the qualitative survey has been made, I believe that if its results are held clearly in mind, quantitative work can be done with some hope of understanding the significance of the data which our measurements bring out. But in view of the pecu· liar and complicated action system of Paramecium, quantitative results will always have to be interpreted with the greatest care, and it must be realized that that method of investigation which examines only the beginning and end of an experiment, without troubling itself as to what the organism does in the meantime, is likely to be most misleading. Further, in view of the peculiar character of the action system of Paramecium, and the large part it plays in determining the behavior under stimulation, the utmost caution is necessary in transferring the conclusions obtained with this animal to other organisms having a different action system. The work on which the present paper is based was done at the Naples Zoological Station while the author was a Research Assistant of the Carnegie Institution of Washington. It is a pleasure to acknowledge my indebtedness to the Carnegie Institution for making the work possible, and for permission to publish the present paper. Pozzuoli, Italy. April 26, 1904. LITERATURE CITED. Birukoff, B. 1899. Bütschli, O. 1889. Dale, H. H. 1901. Untersuchungen über Galvanotaxis. Arch. f. d. ges. Physiol., Bd. Protozoa. III. Abth. Infusoria. Bronn's Klassen und Ordnungen des Thierreichs, Erster Band. Galvanotaxis and Chemotaxis of Ciliate Infusoria. Part I. Journ. of Physiol., Vol. XXVI, pp. 291-361. Davenport, C. B. 1897. Experimental Morphology, Vol. I, 280 pp. New York. Greeley, A. W. 1903. The Reactions of Paramecia and other Protozoa to Chemical and Electrical Stimuli. Science, Vol. XVII, pp. 980-982. Hobhouse, L. T. 1901. Mind in Evolution. 415 PP. London. Jennings, H. S. 1897. Studies on Reactions to Stimuli in Unicellular Organisms. I. 1899a. The Behavior of Unicellular Organisms. Biol. Lectures from 1900. Studies, etc., V. 1902. 1904. Studies, etc., IX. 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