Flagellata

Flagellata, or flagellates, is a historic and informal grouping of mostly unicellular eukaryotes defined by the presence of one or more flagella used for locomotion. The concept arose in early protistology to describe a diverse set of organisms that move with whip-like appendages, without implying a single common ancestry. Consequently, Flagellata is not considered a valid clade in modern taxonomy and many organisms once placed there belong to multiple, distinct lineages.

Organisms commonly associated with flagellates include photosynthetic forms such as Euglena and a range of non-photosynthetic, often parasitic, groups such as Trypanosoma and Giardia. Flagellates exhibit diverse lifestyles, including free-living heterotrophs, autotrophs, and mixotrophs that combine photosynthesis with ingestion. They inhabit a variety of environments, including freshwater and marine systems, soils, and animal or insect hosts; some contribute to ecological processes like algal blooms, while others are notable pathogens.

Morphologically, flagellates vary from slender cells to more complex or amoeboid shapes. Locomotion is typically accomplished by flagella, though some lineages possess additional structures such as undulating membranes or specialized flagellar arrangements. Reproduction is usually asexual via binary fission; certain groups also display sexual stages or genetic exchange.

Because Flagellata is not monophyletic, modern classifications favor lineage-based definitions rather than a unified taxon. The term remains useful for historical description of flagellated protists but is seldom used in formal taxonomy today.