Cavalier-Smith 1991
Page & Blanton 1985 The Percolozoa are a group of colorless protists including many that can transform between amoeboid, flagellate, and encysted stages, collectively referred to as schizopyrenids or amoeboflagellates. They also include the acrasids, a small group of cellular slime moulds. The entire group is usually called the Heterolobosea, but this may be restricted to members with amoeboid stages. Most are bacterivores found in soil and freshwater environments, but there are a few marine and parasitic forms, including the species Naegleria fowleri which is pathogenic in humans.

In amoeboid form, cells are typically around 20-40 μm, and are roughly cylindrical with a single clear pseudopod at the front. Sometimes filose extensions are formed at the posterior, but these do not aid in locomotion. Usually this form is taken when food is plentiful, and the flagellate form is used for rapid locomotion. The latter is slightly smaller, with two or four anterior flagella associated with a ventral feeding groove. This and other features suggest close relationships to some other flagellates collectively known as excavates, and especially to the Euglenozoa, united with them by having mitochondria with discoid cristae.

Not all members are able to assume both forms. The genera Percolomonas, Lyromonas, and Psalteriomonas are known only from flagellate stages, and the genera Vahlkampfia and Pseudovahlkampfia lack flagellate stages, as do the acrasids. Under certain circumstances, acrasid amoebae come together to form stalked structures that produce spores. These are formed entirely from living cells, which aggregate as individuals or in small groups, in contrast to those of the superficially similar but unrelated dictyostelid slime molds.

The Heterolobosea were first defined by Page and Blanton in 1985 as a class of amoebae, and so only included those forms with amoeboid stages. Cavalier-Smith created the phylum Percolozoa for the extended group, together with the enigmatic flagellate Stephanopogon. He maintained the Heterolobosea as a class for amoeboid forms, but most others have expanded them to include the flagellates as well.