Although Trimastix was first described in 1880, new information has changed perceptions about where this genus fits in the "tree of life", and a stable classification has yet to emerge.
The International Code of Zoological Nomenclature is the applicable code.
Grassé (Traité de Zoologie, vol. 1, p. 704, 1952) observed, in Trimastix, several features that he thought were similar to those of the trichomonads, a group of parasitic flagellate protozoa. At the time, protozoa were treated as a phylum within the Kingdom Animalia, resulting in this classification:
Cavalier-Smith (Archiv für Protistenkunde 147: 237, 1996/97) accepted Grassé's conclusion, but incorporated it into his larger and more complex classification system. He also took into account what was then known about Trimastix ultrastructure, especially differences in Golgi architecture between Trimastix and trichomonads, and created a separate category (subphylum) for Trimastix within the phylum that includes the trichomonads.
The most recent version of Cavalier-Smith's classification (Biological Reviews 73: 203, 1998) is:
(The family does not appear in Cavalier-Smith's paper, but it stands as described by Kent (Manual of the Infusoria, vol. 1, p. 310, 1880) and emended by Grassé.)
Brugerolle and Patterson (European Journal of Protistology 33: 121, 1997) showed, however, that Trimastix actually has few ultrastructural similarities with the trichomonads. O'Kelly, Farmer and Nerad (Protist 150: 149, 1999) confirmed this finding, and contributed the idea that Trimastix may be closely related to the jakobids. Some of the evidence for these points of view are shown and discussed in PID.
Brugerolle and Patterson placed Trimastix and Trimastigidae sedis mutabilis ("position subject to change") among the protists. This view is accepted by O'Kelly et al., and here, pending further work especially on the possible relationship between Trimastix and the jakobids, most particularly on the apparent group including Trimastix, Malawimonas, and Carpediemonas.
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