A group of colorless protists including many members that can transform between amoeboid, flagellate, and encysted stages, collectively referred to as Schizopyrenida or amoeboflagellates. They also include a small group of cellular slime molds, the Acrasida, related by ultrastructural and biochemical characteristics. Most are bacterivores found in soil or freshwater environments, but the group includes a few marine and parasitic forms, including one (Naegleria fowleri) which is pathenogenic 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. The flagellate form is slightly smaller and has either two or four flagella, usually associated with an ingestion region supported by microtubules. Usually the amoeboid form is taken when feeding, and then the flagellate form is used for rapid locomotion between food sources. Two genera, Vahlkampfia and Pseudovahlkampfia, as well as the Acrasids, lack flagellate stages entirely. The flagellate Percolomonas may be a member of this group that has lost the amoeboid stage instead.
Under certain conditions, Acrasid amoebae will come together to form stalked structures, which produce spores. Unlike the similar but unrelated slime molds of the Dictyosteliida, the stalk is composed entirely of living cells, and is formed by the aggregation of individual amoebae or small groups of amoebae rather than by the conglomeration of streams. Acrasis and Guttulina are probably the best known forms.