1. Nematodinium possesses an eyespot which consists of a lens closely apposed to a pigment cup containing a supposedly light-sensitive retinoid.

2. The shape of the lens is described and its refractive index measured. From these data it is concluded that light from sources outside the cell will be brought to focus on the retinoid. The quality of the image formed is assessed.

3. A variety of functions are possible to the organelle. It is suggested that functions involving image formation may be realized, even in the absence of a true retina and nervous system.

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