A new epidermal ciliary photoreceptive cell type in Plagiostomum lemani (Plathelminthes, Prolecithophora)

Abstract

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 :

First evidence of unpigmented rhabdomeric photoreceptors in a Microstomum species (Plathelminthes, Macrostomorpha, Microstomidae) and ultrastructure of photoreceptor-like ciliary aggregations

“…Besides unpigmented rhabdomeric photoreceptors, electron microscopic research revealed the existence of another type of unpigmented light-perceiving organs in species of the Plathelminthes, not traceable when viewed with the light microscope, the so-called ciliary aggregations. Meanwhile this type has also been found in several taxa (for references see Piskurek et al 1998).…”

“…The existence of ciliary aggregates could be shown for several taxa of the Proseriata (see Ehlers and Ehlers 1977;Sopott-Ehlers 1982, 1991. Furthermore, this kind of presumptive photoreceptor has been found in the epidermis of Müller's larvae of the polyclad Pseudoceros canadensis Hyman, 1953 (see Lacalli 1983), in an undescribed species of the 'Typhloplanoida' (see Sopott-Ehlers 1991), in the epidermis of Plagiostomum lemani (Du Plessis, 1874), a representative of the Prolecithophora (see Piskurek et al 1998), and most recently in Ciliopharyngiella intermedia Ax, 1952, a neoophoran species with uncertain systematic position (see Sopott-Ehlers 1999).…”