Euglenozoa

Euglenozoa are a sizable collection of flagellated organisms within the Discoba. This group contains many common species that live freely, along with a handful of notable parasites, some of which can infect people. Four main subgroups make up the Euglenozoa: Kinetoplastea, Diplonemea, Euglenida, and Symbiontida.

These are single-celled life forms, typically measuring between 15 and 40 micrometers (0.00059 to 0.00157 inches) across, though certain euglenids can reach lengths of up to 500 micrometers (0.020 inches).

Most euglenozoans possess two flagella. These flagella are inserted parallel to each other inside an apical or subapical pocket. In some species, these flagella are linked to a cytostome, or mouth, which is used to consume bacteria or other small organisms. This feeding structure is supported by one of three sets of microtubules that originate from the flagellar bases; the other two sets support the cell’s dorsal and ventral surfaces.

Other euglenozoans obtain nutrients by absorption. Many euglenids also contain chloroplasts, making them the only eukaryotes outside the Diaphoretickes group to do so without relying on kleptoplasty. These chloroplasts allow them to generate energy through photosynthesis. The chloroplasts are enclosed by three membranes and contain chlorophylls A and B, along with other pigments. This suggests they were acquired long ago from a green alga through an endosymbiotic event in an early euglenozoan. Reproduction happens solely through cell division. During mitosis, the nuclear membrane stays intact, and the spindle microtubules form inside it.

The group is defined by the ultrastructure of its flagella. Besides the usual supporting microtubules, or axoneme, each flagellum contains a rod called a paraxonemal rod. This rod has a tubular structure in one flagellum and a latticed structure in the other. Based on this feature, two smaller groups—the diplonemids and *Postgaardi*—have been included here.

Historically, euglenozoans have been classified as either plants or animals, depending on whether they belong to largely photosynthetic groups. As a result, they have names under either the International Code of Nomenclature for algae, fungi, and plants (ICNafp) or the International Code of Zoological Nomenclature (ICZN). For instance, one family is called Euglenaceae under the ICNafp and Euglenidae under the ICZN. Similarly, the genus name *Dinema* is acceptable under the ICZN but illegitimate under the ICNafp, since it is a later homonym of an orchid genus; the synonym *Dinematomonas* must be used instead.

Euglenozoa are generally accepted as a monophyletic group. They are related to Percolozoa, and both groups share mitochondria with disk-shaped cristae, a feature found in only a few other lineages. Both likely belong to a larger eukaryotic grouping called the Excavata, though this classification has been challenged.