Euglenid

Euglenoids are distinguished mainly by the presence of a type of cell covering called a pellicle. Within its taxon, the pellicle is one of the euglenoids' most diverse morphological features.[7] The pellicle is composed of proteinaceous strips underneath the cell membrane, supported by dorsal and ventral microtubules. This varies from rigid to flexible, and gives the cell its shape, often giving it distinctive striations. In many euglenids, the strips can slide past one another, causing an inching motion called metaboly. Otherwise, they move using their flagella.

Diagnostic drawings of euglenids

1—2. Ascoglena sp. (Euglenales);
3—4. Cryptoglena sp. (idem);
5—9, 14—15, 24—25, 27—29. Trachelomonas spp. (id.);
10. Eutreptia sp. (Eutreptiales);
11, 20. Astasia spp. (Euglenales);
12. Distigma sp. (Eutreptiales);
13. Menoid[i]um sp. (Rhabdomonadales);
16—18. Colacium sp. (Euglenales);
19, 26. Petalomonas spp. (Sphenomonadales);
21. Sphenomonas sp. (id.);
22—23. Euglenopsis sp. (Euglenales);
30. Peranema sp. (Heteronematales)

The euglenids were first defined by Otto Bütschli in 1884 as the flagellate order Euglenida, as an animal. Botanists subsequently created the algal division Euglenophyta; thus, they were classified as both animals and plants, as they share characteristics with both. Conflicts of this nature are exemplary of why the kingdom Protista was adopted. However, they retained their double-placement until the flagellates were split up, and both names are still used to refer to the group. Their chlorophylls are not masked with accessory pigments.

The classification of euglenids is still variable, as groups are being revised to conform with their molecular phylogeny. Classifications have fallen in line with the traditional groups based on differences in nutrition and number of flagella; these provide a starting point for considering euglenid diversity. Different characteristics of the euglenids' pellicles can provide insight into their modes of movement and nutrition.[8]

As with other Euglenozoa, the primitive mode of nutrition is phagocytosis. Prey such as bacteria and smaller flagellates is ingested through a cytostome, supported by microtubules. These are often packed together to form two or more rods, which function in ingestion, and in Entosiphon form an extendable siphon. Most phagotrophic euglenids have two flagella, one leading and one trailing. The latter is used for gliding along the substrate. In some, such as Peranema, the leading flagellum is rigid and beats only at its tip.

Although euglenids share several common characteristics with animals, which is why they were originally classified as so, no evidence has been found of euglenids ever using sexual reproduction. This is one of the reasons they could no longer be classified as animals.

For euglenids to reproduce, asexual reproduction takes place in the form of binary fission, and the cells replicate and divide during mitosis and cytokinesis. This process occurs in a very distinct order. First, the basal bodies and flagella replicate, then the cytostome and microtubules (the feeding apparatus), and finally the nucleus and remaining cytoskeleton. Once this occurs, the organism begins to cleave at the basal bodies, and this cleavage line moves towards the center of the organism until two separate euglenids are evident.[10] Because of the way that this reproduction takes place and the axis of separation, it is called longitudinal cell division or longitudinal binary fission.[11]

• 
  Euglena sp. (Euglenales)
• 
  Phacus sp. (Euglenales)
• 
  Trachelomonas sp. (Euglenales)
• 
  Euglenoid cultures in Petri dishes
• 
  Cell diagram
• 
  Astasia sp. (Euglenales)
• 
  Euglena, Astasia and Phacus spp. (Euglenales)
• 
  Euglena, Phacus and Lepocinclis spp. (Euglenales)
• 
  Anisonema, Petalomonas, Notosolenus, Scytomonas and Tropidoscyphus spp. (Sphenomonadales); Heteronema, Dinema and Entosiphon spp. (Heteronematales)

• Ciugulea, I.; Triemer, R. E. (2010). A Color Atlas of Photosynthetic Euglenoids. East Lansing, MI: Michigan State University Press. ISBN 9780870138799.
• Leander, B. S.; Triemer, R. E.; Farmer, M. A. (2001). "Character evolution in heterotrophic euglenids" (PDF). European Journal of Protistology. 37 (3): 337–356. S2CID 4181281.
• Leander, B.S.; Lax, G.; Karnkowska, A.; Simpson, A.G.B. (2017). "Euglenida". In Archibald, J.M.; Simpson, A.G.B.; Slamovits, C. (eds.). Handbook of the Protists. Springer. pp. 1–42. doi:10.1007/978-3-319-32669-6_13-1. ISBN 978-3-319-32669-6.
• Leedale, G. F. (1978). "Phylogenetic criteria in euglenoid flagellates". BioSystems. 10: 183–187.
• Wołowski, K; Hindák, F (2005). Atlas of Euglenophytes. Krakow: VEDA Publishing House of the Slovak Academy of Sciences. ISBN 9788022408363.

• Data related to Euglenoidea at Wikispecies
• The Euglenoid Project
• Tree of Life: Euglenida