Euglenozoa

Euglenozoa are a large group of single-celled, flagellated organisms within the Discoba. They include many common free-living species and a few notable parasites that can infect humans. The group is divided into four main categories: Kinetoplastea, Diplonemea, Euglenida, and Symbiontida. Most euglenozoans are between 15 and 40 micrometers long, though some euglenids can reach up to 500 micrometers.

Most members have two flagella that run parallel to each other and are set in an apical or subapical pocket. In some species, these flagella are linked to a mouth-like structure called a cytostome, which is used to ingest bacteria or other small organisms. This feeding apparatus 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 euglenozoa absorb nutrients directly. Many euglenids have chloroplasts—the only eukaryotes outside Diaphoretickes to do so without stealing them from other organisms—and thus get energy through photosynthesis. These chloroplasts are wrapped in three membranes and contain chlorophylls A and B along with other pigments, suggesting they came from a green alga that was engulfed long ago by an early euglenozoan. Reproduction is always through cell division. During mitosis, the nuclear membrane stays intact, and the spindle microtubules form inside it.

The group is defined by the fine structure of its flagella. Besides the usual supporting microtubules (the axoneme), each flagellum contains a rod called the 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—are also included here.

Historically, euglenozoans have been classified as either plants or animals, depending on whether they are photosynthetic. As a result, they have names under both the International Code of Nomenclature for algae, fungi, and plants (ICNafp) and the International Code of Zoological Nomenclature (ICZN). For example, one family is called Euglenaceae under the ICNafp and Euglenidae under the ICZN. The genus name Dinema is valid under the ICZN but not under the ICNafp, because it is a later homonym of an orchid genus; the synonym Dinematomonas is used instead.

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