Heterolobosea

The Heterolobosea are a group of colorless protists including many members that can transform between amoeboid, flagellate, and encysted stages, collectively referred to as schizopyrenids or amoeboflagellates. They also include a small group of cellular slime molds, the acrasids. 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, while the flagellate form is used for rapid locomotion. The latter is slightly smaller, with either two or four anterior flagella associated with a ventral feeding groove. This suggests a close relationship to certain other groups, collectively known as excavates, among which the Heterolobosea are one of the lines with mitochondria, and typically have a single nucleus with a nucleolus at its center.

There is one genus, Percolomonas, which lacks an amoeboid stage, while Vahlkampfia, Pseudovahlkampfia, and the acrasids generally lack flagellate stages. Under certain conditions acrasids will come together and produce stalked structures, which function as sporangia. These are composed entirely of living cells and are assembled from individual or small groups of amoebae, rather than streams as in the superficially similar dictyosteliids. The best known such genera are Acrasis and Guttulina.