Taxonomy and phylogeny of heliozoa. 1. The order desmothoracida Hertwig etLesser, 1874

The order Desmothoracida Hertwig et and Lesser, 1874 is reviewed. Five gene ra and 10 species are recognised: Clathrulina Cienkowski, 1867 [C. elegans Cienkowski, 1867 and C. smaragdea (Entz. 1877) comb, n.], Hedriocystis Hert wig et Lesser, 1874 [H. pellucida Hertwig et Lesser, 1874, H. minor Siemens ma, 1991 and H. zhadani sp. n.], Penardiophrys gen. n. [P. reticulata (Pena rd, 1904) comb. n. and P. spinifera (Brown, 1918) comb. n.], Cienkowskya Sc haudinn, 1896 [C. mereschkovckii (Cienkowski, 1881) Schaudinn, 1896 and C. brachypous (De Saedeleer, 1930) comb. n.] and Actinosphaeridium Zacharias, 1893 [A. pedatus Zacharias, 1893]. The generic names Elaster Grimm, 1872 an d Orbulinella Entz, 1877 are synonymised with Clathrulina, and Monomastigoc ystis De Saedeleer, 1930 with Cienkowskya. The genus Cienkowskya differs fr om Hedriocystis by an irregular shape of the capsule, and by its attachment to the substrate by a short conical basal region of the capsule rather tha n by a differentiated tubular stalk as in Hedriocystis. The 2 species of He driocystis having a capsule wall composed of polygonal facets are transferr ed to a new genus Penardiophrys. The genus Actinosphaeridium includes a sin gle freshwater species with branching granule-bearing filopods and the cell body surrounded by a mucus sheath located at the top of a tubular stalk; a nd is here regarded as a primitive desmothoracid. The first marine Hedriocy stis - H. zhadani sp. n. - is described. The stalked heliozoan genus Servet ia Poche, 1913 is not included within desmothoracids. Diagnoses of all desm othoracid taxa and a key to 10 species are included. We should look for phy logenetic roots of desmothoracids among heterotrohic biflagellates; cercomo nads and gymnosphaerid heliozoa are considered as the most closely related groups to them.