Ultrastructure of Gnathostomaria lutheri Ax (Gnathostomulida : Scleroperalia). I. An hypothesis about the origin of micropodia in scleroperalian sperm

Initially described as an aberrant taxon of turbellarian flatworms, the Gna thostomulida were recognized as a distinct phylum by Rupert Riedl already i n 1969. Yet these microscopic, non-segmented marine worms remain organisms of enigmatic phylogenetic affiliation, and although they are often the domi nant invertebrate taxon in detritus-rich, oxygen-poor sands, our knowledge of their biology is still very scanty. About 90 gnathostomulid species are known to date; they are grouped in two orders, one with two suborders. Sper m structure, often a useful instrument for clarifying evolutionary pathways in metazoan taxa, differs so widely between the orders and suborders that sperm phylogeny within the Gnathostomulida also remains uncertain. We here report on the ultrastructural features of a peculiar type of sperm, the afl agellar "megasperm" of Gnathostomaria lutheri, and discuss new and older in terpretations of sperm cytology of the taxon.