ULTRASTRUCTURAL FEATURES OF THE PECULIAR FILTER-FEEDING HYPOTRICH CILIATE URONYCHIA-TRANSFUGA

In this report, dealing with Uronychia transfuga, ultrastructural deta ils of a member of the genus Uronychia are provided for the first time . The most interesting features appear to be: (1) the lack of an alveo lar, electrondense material in the cortex. This trait distinguishes Ur onychia from the members of the family Euplotidae. (2) The peculiar ci liary arrangement of the massive caudal cirri. (3) The oral membrane ( OM) delimited by cytoplasmic laminae of different length, it consists of two components each one formed by semicircular sheets of cilia. The length of the cilia progressively increases from the outermost sheet to the internal one. The kinetosomes are interconnected with each othe r by a dense material that, proximally, forms peculiar crescent-like s tructures connecting the kinetosomes to the innermost cytoplasmic lami na. The two components of the OM partially overlap at the apical end;, at this level their kinetosomes are interconnected by infraciliary str uctures. The OM components are able to close the peristomial cavity by flexing toward it and completely overlapping each other. The filter f eeding mechanism of Uronychia differs from those of other ciliates and allows the ingestion of large active prey. (4) The endoral membrane, probably stichomonad, is limited to the terminal part of the oral cavi ty. On the whole, our observations appear consistent with the hypothes is that Uronychia should be placed in its own family. (C) 1997 Elsevie r Science Ltd.