Eyes covered by mitochondrial lenses in Petaliella spiracauda and Ptychopera purasjokii (Plathelminthes, Rhabdocoela, Trigonostominae). Ultrastructural features and phylogenetic implications.

The submicroscopic anatomy of the eyes in Petaliella spiracauda and Ptychopera purasjokii is described. These eyes correspond in general to the basic pattern of rhabdomeric pigment-cup ocelli. This, however, does not apply to modifications of the cup cell such as the differentiation of mitochondrial lenses. Corresponding with two sensory cells two extensions of the cup cell capping the eye aperture are crowded with small unmodified mitochondria in the eyes of P. spiracauda. The eyes of P. purasjokii have three sensory cells and the lenticular element is formed by a trifoil-shaped differentiation of three giant mitochondrial derivatives. These derivatives show peripheral appendages of various configurations, all of which resemble the profiles of small mitochondria. The implication of the existence of such appendages is that the lenses in P. purasjokii are derived from many fused mitochondria, rather than from a single enlarged one. It is concluded that the unmodified or modified mitochondrial differentiations in proliferations of the pigment cell capping the opening of the eye cup serve to focalize incoming light. The evolution of mitochondrial lenses in Plathelminthes is considered.

Evidence for the mitochondrial origin of the eye lenses in embryos of Entobdella soleae (Plathelminthes, Monogenea).

The lens associated with each of the four pigmented eyes of the oncomiracidium of Entobdella soleae (Plathelminthes, Monogenea, Capsalidae) develops in a special region of the pigment cup cell of the eye. It is confirmed that the inner of the two membranes enclosing each lens bears short, inwardly projecting, membranous profiles identical to mitochondrial cristae. Studies of embryos incubated for 19 days at 12 degrees C (hatching begins at 28 days at this temperature) revealed that the matrix of the developing lens of each anterior eye contains many mitochondrial membrane compartments, some having the configuration of separate, small mitochondria. The implication is that the lens is derived from many fused mitochondria, rather than from a single large one. The anterior eyes of 19-day-old embryos are less well developed than the posterior eyes. Pigment granules in the anterior eyes appear to be at the premelanosome stage and contain dispersed dense particles lacking an obvious orderly arrangement. The posterior eyes mostly contain mature melanosomes. Membranous compartments in the matrix of the posterior eye lenses are rare. Apart from longer peripheral cristae, lenses of 22-day-old embryos are identical with those of oncomiracidia. The evolution of mitochondrial lenses in Plathelminthes is considered.

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.