Some Responses to Light in a Specimen of Pelagohydra Mirabilis Dendy, 1902 (Coelenterata: Hydrozoa)

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Abstract
1. A single specimen of P. mirabilis was subjected to varying degrees of illumination. 2. In constant light occasional spontaneous reactions occurred, the tentacles flexing in an oral direction and moving the animal aborally a very insignificant distance each time. 3. When the level of illumination was reduced suddenly and almost completely there occurred a sequence of such reactions; successive reactions in the sequence occurred with increasing vigour and at diminishing frequency. 4. Readmission of light was shown to act as an inhibitory condition, since it interrupted the reaction sequence. 5. When the illumination was reduced in a series of steps, each step was followed by a sequence of reactions, the sequences showing increase in vigour and decrease in frequency. Each such sequence appeared to be freshly initiated by the stepwise reduction in light.

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