. Carnegie Institution of Washington publication. 54 THE BKHAVIOR OF LOWER ORGANISMS- cone, or the frustum of a cone, as indicated in Fig. 21. The result, as seen from above, is that all the specimens seem to vibrate from side to side ; in other words, they are taken with a sudden oscillation or trem- bling. This oscillation when the intensity of the light is suddenly changed was observed by Strasburger (1878, pp. 25 and 50) in flagellate swarm-spores; he speaks of it as " Erschtitterung " or "Zit- ; During this oscillation the anterior end becomes pointed successively
. Carnegie Institution of Washington publication. 54 THE BKHAVIOR OF LOWER ORGANISMS- cone, or the frustum of a cone, as indicated in Fig. 21. The result, as seen from above, is that all the specimens seem to vibrate from side to side ; in other words, they are taken with a sudden oscillation or trem- bling. This oscillation when the intensity of the light is suddenly changed was observed by Strasburger (1878, pp. 25 and 50) in flagellate swarm-spores; he speaks of it as " Erschtitterung " or "Zit- ; During this oscillation the anterior end becomes pointed successively in many different directions, as Fig. 21 shows. When, now, the usual forward course is resumed (with only the usual amount of swerving toward the dorsal side), the animal follows one of these directions. Thus its path is changed (Fig. 22). Strasburger (1878, p. 25) noticed that the path followed after the oscillation was oblique to the former path. As a study of Figs. 21 and 22 will show, this is a necessary consequence of the increased swerving toward the dorsal side, to which the oscillation itself is due. All these relations become much clearer if a model of an actual spiral is studied ; it is difficult to represent them upon a plane surface. If the stimulus is stronger, as when there is a greater decrease in illumination, the swerving toward the dorsal side is much greater; the or- ganism wheels far to that side, so that the spiral course seems entirely interrupted. But there is really nothing in this reaction differing in prin- ciple from what is happening in the normal forward swimming. If the swerving toward the dorsal side is long continued the specimen may be seen to swing first far to the (observer's) right, then, after it has revolved on the long axis, far to the (observer's) left; in reality it swings an equal amount upward and downward and in intermediate directions. It may, however, swing at once so far to the dorsal side that the new. FIG. 22.* * FIG. 22.—Show
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