They are grouped together based on the presence of an organic shell or theca under the dorsal surface of the cell.

The name derives from the Ancient Greek words for footless (ἄπους) and animal (ζῷον).

[11] While some classification systems have placed Hemimastigida in Apusozoa,[12] 2018 research indicated that hemimastigotes (Hemimastix kukwesjijk [de]/Hemimastix/Spironematellidae) are their own supra-kingdom.

This characteristic was originally considered a good indicator of relationships, but is now known to vary even among close relatives.

As currently defined, Apusozoa is a paraphyletic grade that includes two classes: Thecomonadea (the apusomonad flagellates) and Breviatea (the breviate amoebae),[14] each of them monophyletic and sister groups to the Opisthokonta supergroup.