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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 μm in length) typically have pliable bodies. Some species are holozoic and ingest solid particles, while others are saprozoic and obtain their nutrition by absorption.
There is disagreement on the division of the order into families. However, the five or more families can be divided into two general groups (see illustration). The first group contains simple organisms with no distinctive features save one or two flagella of equal or unequal length (includes the families Oikomonadidae, Amphimonadidae, Monadidae, and Bodonidae). The second group contains organisms which have an undulating membrane in addition to one or two flagella (includes Trypanosomatidae and Cryptobiidae). The most important family is the Trypanosomatidae, since it includes several species that infect humans and domestic animals with serious diseases, such as African sleeping sickness. See Cilia and flagella