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.