Novel Protonephridial Filtration Apparatus in Cylindrostoma fingalianum, Allostoma sp. and Pseudostomum quadrioculatum (Platyhelminthes: Prolecithophora)

Abstract

The diverse types of protonephridial filtration apparatus in the Platyhelminthes provide valuable characters for phylogenetic resolution, yet only one species from the order Prolecithophora has previously been studied. We examined three further species, two belonging to the family Cylindrostomidae and one from the Pseudostomidae, and found a novel arrangement consisting of scattered, short filtration slits in the cytoplasmic cylinder of the terminal cell surrounding the flame of cilia. In these species there are no regular, longitudinal ‘ribs’, such as are found in many other platyhelminth taxa, nor bundles of supporting microtubules in the cylinder wall, and cilia arise at various levels throughout the long terminal cell column rather than in a group at the base of the flame, as is found in most other taxa. The perikaryon lies adjacent to the flame, the wall surrounding the lumen is strengthened by long, cross-striated ciliary rootlets, and the terminal cell is joined to the proximal canal by a septate junction. This simple type of filtration structure bears some resemblance to that found in Tricladida, but is distinctly different from that described in another prolecithophoran, Archimonotresis limophila (Protomonotresidae). This suggests that there may be a fundamental division within the Prolecithophora with regard to protonephridial filtration structures.