The reactions of Planarians to light . .Its average emergence from the circle for the first four trials was450, which is a normal negative result, since 900 represents com-plete indifference. In the next four trials, however, the averagewas ioo°, that is, slightly positive, and in the last four, 1240,which is decidedly positive, as shown diagrammatically in Fig. 6. The Reactions of Planarians to Light 79 In the other instance an individual worm, Planaria gonocephala,made the erratic average emergence from a circle of 1450, just350 short of absolute positiveness. This worm was carefullyisolated
The reactions of Planarians to light . .Its average emergence from the circle for the first four trials was450, which is a normal negative result, since 900 represents com-plete indifference. In the next four trials, however, the averagewas ioo°, that is, slightly positive, and in the last four, 1240,which is decidedly positive, as shown diagrammatically in Fig. 6. The Reactions of Planarians to Light 79 In the other instance an individual worm, Planaria gonocephala,made the erratic average emergence from a circle of 1450, just350 short of absolute positiveness. This worm was carefullyisolated and tested again four days later under identical externalconditions when it was found to have returned to a normal nega-tive condition by showing an average record of 560. Accuracy of Orientation. It was found to be frequently the casethat when negative worms were subjected to directive light theirfirst movement instead of being directly away from the source oflight formed a path in a diagonal direction. This tendency to 100° Light. Fig. 6 The arrow at the left represents the constant direction of the light. In each of the three setsof trials each worm was headed successively toward o°, the upper (in the diagram) 900, 1800, and thelower 900. The point of average emergence for the first set of trials—supposing the records of thelower semicircle to have been transferred to the upper semicircle—was at 450, of the second set,at ioo°, and of the third set, at 1240. » travel diagonally away from the light has also been noted in thecase of the earthworm by Smith (02, p. 469). If the negative phototaxis of planarians is to be explained onthe theory of tropisms, and if, moreover, the eyes, as Hesse (97)maintains, are the principal organs which, when 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 certai
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