flagellid

Flagellid is a term that has historically been used in biology to refer to a flagellum or, more loosely, to a flagellated organism. In contemporary usage, flagellum is the preferred term for the locomotive appendage, while flagellates describe a broad and diverse set of single‑celled organisms that bear one or more flagella. The distinction is important because flagella occur in a variety of cellular lineages, including bacteria, archaea, and eukaryotes, and their structure and function differ markedly across groups.

In eukaryotes, flagella are complex, whip-like structures driven by dynein motors within a 9+2 arrangement of microtubules (the axoneme). They are anchored to the cell by a basal body. Eukaryotic flagella typically move in waves that propel the cell and can also serve sensory functions. In bacteria, flagella are helical filaments that rotate like propellers, powered by a motor at the base; bacterial flagella are built from flagellin proteins and differ structurally from eukaryotic flagella.

Flagellates—organisms that bear flagella—are found in many protist lineages and occupy a wide range of habitats, from freshwater and marine environments to the tissues of animals. They may be free-living or parasitic. Reproduction among flagellates varies and can include asexual binary fission or complex life cycles with sexual stages. Well-known examples include photosynthetic Euglena, as well as parasitic groups such as Giardia and Trypanosoma, which rely on flagella for movement at different life stages.

Taxonomically, the term flagellid or flagellata arises from historical classifications that grouped flagellate protists together. Modern systematics place flagellated organisms across several clades, reflecting their diverse evolutionary origins.