A new species of the genus Aeolosoma (Annelida: Aphanoneura: Aeolosomatidae) with peculiar sigmoid chaetae from fresh waters of Vietnam.

A new species of the Aeolosomatidae, Aeolosoma vietnamicum sp. n., found in freshwater bodies of Vietnam, is illustrated and described. The new species differs from its congeners in the presence of specific sigmoid chaetae in the dorsal and ventral bundles of all segments, beginning from II. These chaetae are characterized by tips with two apical teeth and an additional row of three to nine distinct denticles on the concave side of the chaetaes distal part. Aeolosoma vietnamicum sp. n. also differs from closely related species in the length, pattern and number of hair chaetae per bundle.

The ultrastructure of the zoospores of the parasitic dinoflagellate Ichthyodinium chabelardi Hollande et J. Cachon, 1952 (Alveolata: Dinoflagellata).

This is the first study on the ultrastructure of the zoospores of Ichthyodinium chabelardi, a parasitoid of the fish egg and early larval stages. The zoospores were characterized by the cell structure specific for dinoflagellates; particularly, cells contained large trichocysts and the "dinokaryon"-type nucleus. An unusual large electron-transparent zone was the only significant difference from the "classical" cell structure in Dinoflagellata. We did not find cell structures for the penetration to the host cell (microtubular basket, conoid, or secretory organelles such as rhoptries). The data on the fine structure of the zoospores of I. chabelardi agree with the results of molecular phylogeny; this allows us argue that excluding this species from Dinoflagellata and assigning it to Protalveolata was a mistake.