[6] Previously members of this class were thought to be heterotrichs because of similarities in morphology, most notably a characteristic dense arrangement of cilia surrounding their oral structures.

[7] However, the development of genetic tools and subsequent incorporation of DNA sequence information has led to major revisions in the evolutionary relationships of many protists, including ciliates.

[8] Metopids, clevelandellids, and armophorids were grouped into this class based on similarities in their small subunit rRNA sequences, making them one of two so-called "riboclasses" of ciliates, however, recent analyses suggest that Armophorida may not be related to the other two orders.

[10] It is hypothesized that these symbionts consume the hydrogen that is produced as an end-product of fermentation, making it a more favorable reaction and increasing its energetic yield.

[11] Marine armophoreans also harbor ectosymbiotic bacteria that are sulfate reducers, which are thought to play a similar role as the endosymbiotic methanogens in that they also consume hydrogen as a metabolite of host fermentation.