PROBLEM TO BE SOLVED: To easily and accurately measure planktons.;SOLUTION: Planktons having phototaxis are made to flow in a cell 1 for plankton observation, having a retaining region in which test water is retained, and light is emitted to the cell 1 by a light-emitting means 10, to make the planktons migrate in the incident direction of light. By making the planktons stay in the cell 1 for a prescribed time and photographing the state of the cell by an imaging means 11 for this duration, the planktons having phototaxis is observed.;COPYRIGHT: (C)2008,JPO&INPIT
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