Unusual crystalline structures in photoreceptors of Lonchoplanella axi (Plathelminthes, Rhabdocoela, 'Typhloplanoida'): functional aspects and phylogenetic implications

Lonchoplanella axi has a pair of small dot-like pigment-cup ocelli. Each eye is composed of a single cup cell and two sensory cells of the rhabdomeric type. The most conspicuous differentiations in the sensory cells are spindle-shaped crystalline structures accompanying the nuclei. These structures flank the dorsolateral, respectively ventrolateral side of the nucleus. It is supposed that these 'spindles' serve rather as an additional shading device than as dioptrics. Since such structures in photoreceptors have hitherto not been reported for representatives of the Plathelminthes, it is concluded that the spindle-shaped crystalline bodies in the eyes of Lonchoplanella axi are an autapomorphic feature of this species or even an autapomorphy of the taxon Mariplanellinae.