Plagiostomum lemani possesses extremely specialized intraepidermal sensory cells. These obvious photoreceptors, which are not visible with the light microscope, are ciliary aggregations located in an intracellular cavity. The numerous spiralled cilia have the classic 9 × 2 + 2 arrangement at their base and a modified pattern of microtubules apically. The discovered differentiations do not show a connection to the surface. Neither mantle cells nor pigment cells have been found. The structural similarities with other epidermal photoreceptors of species among the different taxa of free-living Plathelminthes are outlined. Besides the larval stages of the taxon Polycladida known so far, the same kind of light-sensing photoreceptive cell has never been described in any other species of the Plathelminthes.& b d y :
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