Divergent protonephridial architecture within the Kalyptorhynchia (Platyhelminthes) and implications for the phylogeny of the Rhabdocoela

Transmission electron microscopy of the protonephridial filtration structures in nine species of platyhelminths from five families in Kalyptorhynchia revealed three different types of architecture. Type A, found in representatives of Schizorhynchia and in two species of Cicerinidae (Eukalyptorhynchia), shows a terminal perikaryon associated with each flame bulb, and the bulb is connected to the proximal canal with a septate junction. Type B, found in two other species of Cicerinidae, also has a junction between the flame bulb and the proximal canal but does not have a nucleus in close association with the flame. Type C, found in the representatives of two other families of Eukalyptorhynchia, has no terminal perikaryon and the flame bulb is continuous with the proximal canal without the intervention of a junction. Type A may be considered the most plesiomorphic arrangement described for any rhabdocoel taxon while Type C is highly derived. Type C resembles that found in other orders of Rhabdocoela, namely Typhloplanida, Dalyelliida and Temnocephalida. The distribution of flame bulb types within Kalyptorhynchia, in conjunction with spermiogenesis data, suggests that the Cicerinidae is paraphyletic and that a subtaxon including the genera Toia, Nannorhynchides and (probably) Pocillorhynchus forms a sister group with the Schizorhynchia, while a subtaxon including at least the genera Cicer ina and Ptyalorhynchus forms a sister group with the remaining Eukalyptorhynchia.