This paper describes the structure of the flagella, basal bodies, and some of the associated fibre systems in three genera of complex flagellates, Trichonympha, Pseudotrichonympha, and Holomastigotoides.

Three groups of longitudinal fibres occur in a flagellum: two central and nine outer fibres such as have been repeatedly described in other material, and an additional set of nine smaller secondary fibres not previously identified as such. Each central fibre shows a helical substructure; the pair of them are enveloped in a common sheath. Each outer fibre is a doublet with one subfibre bearing projections—called arms—that extend toward the adjacent outer fibre.

The basal body is formed by a cylinder of nine triplet outer fibres. Two subfibres of each triplet continue into the flagellum and constitute the doublets. The third subfibre terminates at the transition of basal body to flagellum, possibly giving rise to the nine radial transitional fibres that seem to attach the end of the basal body to the surface of the organism. The central and secondary flagellar fibres are not present in the lumen of the basal body, but other complex structures occur there. The form of these intraluminal structures differs from genus to genus.

The flagellar unit is highly asymmetrical. All the flagella examined have possessed the same one of the two possible enantiomorphic forms.

At least two systems of fibres are associated with the basal bodies of all three genera.

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