. Carnegie Institution of Washington publication. REACTIONS TO LIGHT IN CILIATKS AND FLAGELLATES. 37 orientation theory. This observation was made repeatedly in a very large number of cases ; not a single exception to it was observed. The swinging of the anterior end is continued past the point where the light falls directly upon it until the animal is directed again into the shadow, as illustrated in the diagram (Fig. 12, £, 1,2, 3, 4). 2. The Stentor may reach the line with the aboral side directed away from the source of light. In this case it turns (as usual) toward the aboral side, thus s
. Carnegie Institution of Washington publication. REACTIONS TO LIGHT IN CILIATKS AND FLAGELLATES. 37 orientation theory. This observation was made repeatedly in a very large number of cases ; not a single exception to it was observed. The swinging of the anterior end is continued past the point where the light falls directly upon it until the animal is directed again into the shadow, as illustrated in the diagram (Fig. 12, £, 1,2, 3, 4). 2. The Stentor may reach the line with the aboral side directed away from the source of light. In this case it turns (as usual) toward the aboral side, thus swinging its anterior end aivay from the source of light. 3. The Stentor may reach the line with the aboral side directed upward or downward or in some intermediate position. In every case it turns toward the right aboral side, in whichever way this is directed. x a. s A B FIG. 13 * The writer wishes it to be understood that the foregoing statements as to the direction in which the animal turns are presented, not merely as interpretations in accordance with a certain theory, but as direct, unequivocal observations, many times repeated. Thus, on passing from a darker to a lighter area, even when the light comes from one side. the Stentors react merely to the difference in illumination, without regard to the direction from which the light comes. The direction of turning is determined throughout by an internal factor, not by the side of the animal on which the light falls, nor by the direction of the rays of light. We have put the theory of orientation by direct differential *FiG. 13.—Another method of testing the manner in which Stentor reacts to light. For a side view of this apparatus, see Fig. 14. Light comes from the left side, in the direction indicated by the arrows. A screen (s) is interposed between the source of light and the vessel containing the Stentors. This screen is of such a height (as illustrated in Fig. 14) that it cuts off the light from the half (A) of the v
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