The reactions of Planarians to light . ns enclosed fivesides. It will be seen at a glance that the great majority of the wormsplaced in directive light come to rest as far from the light as pos-sible. That this is due to the directive power of light is at onceapparent by comparing A, B and C with D, where the light wasnon-directive. The darkened area was selected whenever thedirective force of the light did not prevent, as in A, C and D. 84 Herbert Eugene Walter The five worms coming to rest on the lighter side of D werecarefully examined and found to be mutilated or fragmentedindividuals, whi
The reactions of Planarians to light . ns enclosed fivesides. It will be seen at a glance that the great majority of the wormsplaced in directive light come to rest as far from the light as pos-sible. That this is due to the directive power of light is at onceapparent by comparing A, B and C with D, where the light wasnon-directive. The darkened area was selected whenever thedirective force of the light did not prevent, as in A, C and D. 84 Herbert Eugene Walter The five worms coming to rest on the lighter side of D werecarefully examined and found to be mutilated or fragmentedindividuals, while the same was not true of the others. The reason why the worms in B failed to arrive in the darkenedarea is probably that, being started near the middle boundaryline, their first movements were normal, i. <?., away from the light,and carried them into the area of greatest intensity, whence theywere unable to escape. In this case the effect of the directivelight seems to have more than counterbalanced the locomotive Light vertical. ?????? 1 6 :..??3\ ?.? . - 2 .I ? • ; 1 . ? 2-i ?:.7. , 2I I * ?in ?mm B D Fig. 8 Planaria gonocephala. The arrows represent the direction of the light. The dotted areaswere surrounded by black backgrounds, except for a space on the side toward the light, and the clearareas similarly by white backgrounds. The figures represent the number of planarians that came torest in any particular locality. energy exerted by the worms. Had the species experimentedupon been Phagocata gracilis, instead of Planaria gonocephala,the result might have been different, for in the former species, asalready shown (p. 78), the phototactic response is secondary tothe tendency to a general wandering. It was frequently observed that worms when fatigued after aperiod of activity apparently lost their phototaxis, with the resultthat the final movements of a tired worm would sometimes bemade toward the light. Such behavior is probably not to be con-sidered as a revers
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