The reactions of Planarians to light . en unequally illumi-nated, cause the directive response, it may be shown that possi-bly the arrangement of the crescentic pigment shields around thesensory cells of the eyes is such that equal stimulation of both eyesis just as certainly received by the worm when it is in a positiondiagonal to the light as when it is pointed directly away from thelight. 8o Herbert Eugene Walter By reference to Fig. 7, in which the relative size of the eyes issomewhat exaggerated and made diagrammatic for sake of clear-ness, it will be seen that no more light reaches the s
The reactions of Planarians to light . en unequally illumi-nated, cause the directive response, it may be shown that possi-bly the arrangement of the crescentic pigment shields around thesensory cells of the eyes is such that equal stimulation of both eyesis just as certainly received by the worm when it is in a positiondiagonal to the light as when it is pointed directly away from thelight. 8o Herbert Eugene Walter By reference to Fig. 7, in which the relative size of the eyes issomewhat exaggerated and made diagrammatic for sake of clear-ness, it will be seen that no more light reaches the sensory cells ofeither eye from position A, the diagonal position, than from posi-tion B, and that it is only when the light comes from some sourcemore lateral than A that the left eye receives more illuminationthan the right. This view may furnish a possible explanation of the diagonalpaths representing imperfect orientation among planarians, but itcan in nowise apply to the case of earthworms since in them direc-tion eyes are Fig. 7 A, diagonal direction of light; B, posterior direction of light; C, location of sensory cells;D, pigment shield. Degree of Wandering. The degree of wandering decreases withan increase of intensity. It may be found approximately throughthe degree of error in orientation in a unit space under differentintensities of light, for perfect orientation signifies the minimumof random wandering and, conversely, the greater the error oforientation the greater the probable wandering. The error of orientation expressed in percentages was computedas follows. With a negative worm emergence from the circle at The Reactions of Planarians to Light 81 a point directly opposite the light was reckoned as o per cent oferror, whereas emergence at a point directly toward the light wasreckoned as ioo per cent, or a maximum of error in orientation value at these two extremes having been estab-lished, the percentage of error which occurs when the wor
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