An order of the class Zoomastigophorea in the phylum Protozoa, also known as Protomastigida, containing a heterogeneous group of colorless flagellates possessing one or two flagella in some stage of their life cycle. These small organisms (5–89 micrometers in length) typically have pliable bodies. Some species are holozoic and ingest solid particles, while others are saprozoic and obtain their nutrition by absorption. Their life cycles are usually simple but some species have two or more recognizably distinct stages. The species may be either free-living or parasites of vertebrates, invertebrates, and plants. Reproduction is by longitudinal fission, although multiple fission occurs in some species. Several are important disease-causing parasites of humans and domestic and wild animals.