Characteristics of protonephridial terminal organs and the eyes of four species of Trigonostominae (Platyhelminthes : Rhabdocoela)

Two complex, phylogenetically informative structures were: investigated usi ng transmission electron microscopy in four species within a small taxonomi c unit of rhabdocoel turbellarians. A synapomorphic modification of the str ucture of the protonephridial terminal organs, viz. reduced definition of t he filtration ribs, was found in three of the species, Ceratopera sp., Prox enetes fasciger and Trigonostomum setigerum. Modifications of the mitochond rial lensing elements found in the eyes of Beklemischeviella contorts impli ed that it may join with Ptychopera westbladi to form a branch within the s ubfamily, while the distribution of lensing from giant mitochondria illustr ated the plasticity of this feature. The study demonstrates the potential o f such characters to assist phylogenetic resolution, but highlights the dan ger of using the characteristics of single species to represent larger taxa .