The ultrastructure of Trimastix marina Kent, 1880 (eukaryota), an excavateflagellate

The ultrastructure of Trimastix marina is documented for the first time. Th is protist has four flagella which insert at the head of a longitudinal fee ding groove. The basal bodies are elongate and arranged in a cruciate patte rn. The anterior and posterior basal bodies lack cartwheels. The posterior flagellum bears two vanes. The anterior flagellum is thickened. The cell ha s a poorly organised Golgi-like region and small hydrogenosome-like organel les. The flagellar apparatus includes left and right major microtubular roo ts which support the margins of the groove, and a third, major anterior roo t associated with a fan of microtubules which supports the dorsal side of t he cell, There is also a singlet 'root' associated with the groove, a minor anterior root and a suite of non-microtubular structures (including the A, B, C and I fibres). Arrays of microtubules diverge from both left and righ t roots to support the floor of the groove, Some microtubules from the righ t array converge to form an inner right root, Halfway down the cell the lef t root ends, while the right root reduces and associates with a sheet-like striated/dense composite fibre, The groove ends as a cytopharynx, with a 't ongue' structure associated with its opening. Trimastix marina is most simi lar to T: pyriformis, the only previously studied member of Trimastix, Amon g the other 'excavate taxa', Trimastix is most similar to 'core jakobids', Malawimonas, Carpediemonas and retortamonads but also shares several unusua l features with some heteroloboseids. The complexities and detail of struct ural similarities amongst various excavate taxa support the 'excavate hypot hesis' (i,e. that the excavate taxa have a common excavate ancestor) but su ggest that understanding the relationships amongst the excavate taxa throug h morphological data will be a complex task.