SISTER-GROUP RELATIONSHIP OF GNATHOSTOMULIDA AND ROTIFERA-ACANTHOCEPHALA

The trophi of rotifers resemble the sclerotized jaws of gnathostomulid s, but whether trophi are homologous to gnathostomulid jaws, and conse quently show phylogenetic relationship between Gnathostomulida and Rot ifera, has been unclear. We have found that the trophi of a rotifer in the genus Seison, which is ranked close to the ancestral stock of rot ifers, have an ultrastructural feature similar to that reported in the literature for jaws of both scleroperalian and filospermoidean gnatho stomulids, as well as for trophi in the more derived bdelloid rotifer genus Philodina. Specifically, these trophi and jaws have arrays of tu be-like support rods composed of lucent material surrounding a dense c ore. Jaw-like structures in other small vermiform animals (certain pol ychaetes and molluscs) lack this special feature. We propose that jaw substructure shows a homology, and thus a sister-group relationship, b etween Gnathostomulida and the clade containing Rotifera plus Acanthoc ephala.