The anatomy and ultrastructure of the proboscis in Zonorhynchus-species and implications for phylogenetic relationships within the Eukalyptorhynchia Meixner, 1928 (Platyhelminthes, Rhabdocoela)

Abstract

The ultrastructural morphology of the proboscis in species of the genus Zonorhynchus Karling, 1952 is investigated. The proboscis epithelia are organized in four belts. The apical cone epithelium has intra-epithelial nuclei, and the distal belt of the sheath epithelium is formed by numerous cells lacking a distinct pattern. Multiciliary receptors are lacking. The diversity in ultrastructural data on the proboscis is used to elucidate the phylogenetic relationships within the Cicerinidae and Eukalyptorhynchia. A bipartite cone epithelium as found in Zonorhynchus-species is a common feature for all eukalyptorhynch species investigated. A syncytial basal cone epithelium is a synapomorphic character for all species except Toia calceformis Brunet, 1973 and Nannorhynchides herdlaensis Karling, 1956. A syncytial proximal belt of the sheath epithelium, and intra-epithelial nucleus(i) in the apical cone epithelium constitute apomorphic features for species of the genus Zonorhynchus. The absence of multiciliary receptors is regarded as the plesiomorphic state and distinguishes Zonorhynchus-species, I: calceformis and N. herdlaensis from other Eukalyptorhynchia investigated. Cicerina remanei Meixner, 1928, Psammorhynchus tubulipenis Meixner, 1938 and Cytocystis clitellatus Karling, 1953 have two insunk sensory organs with multiciliary receptors. The presence of four epithelial belts in Zonorhynchus-species as in C. remanei, P. tubulipenis and C. clitellatus is considered a shared plesiomorphic state within the Eukalyptorhynchia The presence of a third belt in the sheath epithelium is a synapomorphic character of all Koinocystididae, Cystiplanidae and Polycystididae.