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      Euglenids or euglenoids are one of the best-known groups of eukaryotic flagellates: single-celled organisms with flagella, or whip-like tails. They are classified in the phylum Euglenophyta, class Euglenida or Euglenoidea. Euglenids are commonly found in fresh water, especially when it is rich in organic materials, but they have a few marine and endosymbiotic members. Many euglenids feed by phagocytosis, or strictly by diffusion. A monophyletic subgroup known as Euglenophyceae have chloroplasts and produce their own food through photosynthesis. This group contains the carbohydrate paramylon.
      Euglenids split from other Euglenozoa (a larger group of flagellates) more than a billion years ago. The plastids (membranous organelles) in all extant photosynthetic species result from secondary endosymbiosis between a euglenid and a green alga.


      Structure


      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. 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.


      Classification



      The first attempt at classifying euglenids was done by Ehrenberg in 1830, when he described the genus Euglena and placed it in the Polygastrica of family Astasiae, containing other creatures of variable body shape and lacking pseudopods or lorica. Later, various biologists described additional characteristics for Euglena and established different classification systems for euglenids based on nutrition modes, the presence and number of flagella, and the degree of metaboly. The 1942 revision by A. Hollande distinguished three groups, Peranemoidées (flexible phagotrophs), Petalomonadinées (rigid phagotrophs) and Euglenidinées (phototrophs), and was widely accepted as the best reflection of the natural relationships between euglenids, adopted by many other authors. Gordon F. Leedale expanded on Hollande's system, establishing six orders (Eutreptiales, Euglenales, Rhabdomonadales, Sphenomonadales, Heteronematales and Euglenamorphales) and taking into account new data on their physiology and ultrastructure. This scheme endured until 1986, with the sequencing of the SSU rRNA gene from Euglena gracilis.
      Euglenids are currently regarded as a highly diverse clade within Euglenozoa, in the eukaryotic supergroup Discoba. They are traditionally organized into three categories based on modes of nutrition: the phototrophs (Euglenophyceae), the osmotrophs (mainly the 'primary osmotrophs' known as Aphagea), and the phagotrophs, from which the first two groups have evolved. The phagotrophs, although paraphyletic, have historically been classified under the name of Heteronematina.
      In addition, euglenids can be divided into inflexible or rigid euglenids, and flexible or metabolic euglenids which are capable of 'metaboly' or 'euglenid motion'. Only those with more than 18 protein strips in their pellicle gain this flexibility. Phylogenetic studies show that various clades of rigid phagotrophic euglenids compose the base of the euglenid tree, namely Petalomonadida and the paraphyletic 'Ploeotiida'. In contrast, all flexible euglenids belong to a monophyletic group known as Spirocuta, which includes Euglenophyceae, Aphagea and various phagotrophs (Peranemidae, Anisonemidae and Neometanemidae). The current classification of class Euglenida, as a result of these studies, is as follows:

      Euglenida incertae sedis: Atraktomonas, Calycimonas, Dolium, Dylakosoma, Tropidoscyphus, Michajlowastasia, Parastasiella, Dinemula, Paradinemula, Mononema, Ovicola, Naupliicola, Embryocola, Copromonas.
      Order Petalomonadida Cavalier-Smith 1993
      Order "Ploeotiida" Cavalier-Smith 1993 (paraphyletic)
      Clade Alistosa Lax & Simpson 2021
      Entosiphon Cavalier-Smith, Chao & Vickerman 2016
      Gaulosia Lax, Cho & Keeling 2023
      Clade Karavia Lax, Cho & Keeling 2023
      Chelandium Lax, Cho & Keeling 2023
      Olkasia Lax, Lee, Eglit & Simpson 2019
      Clade Spirocuta Cavalier-Smith, Chao & Vickerman 2016 [Helicales Perschke et al. 2017]
      Clade Anisonemia Cavalier-Smith 2016
      Order Anisonemida Cavalier-Smith 2016
      Family Anisonemidae Kent 1880 emend. Cavalier-Smith 2016
      Order Natomonadida Cavalier-Smith 2016
      Suborder Metanemina Cavalier-Smith 2016
      Family Neometanemidae Cavalier-Smith 2016
      Suborder Aphagea Cavalier-Smith 1993 emend. Busse & Preisfeld 2003 [Rhabdomonadina Leedale 1967 emend. Cavalier-Smith 1993]
      Family Astasiidae Kent 1884
      Family Distigmidae Hollande 1942
      Order Peranemida Bütschli 1884
      Family Peranemidae Bütschli 1884
      Clade Euglenophyceae Schoenichen 1925, emend. Marin & Melkonian 2003 [Euglenea Butschli 1884, emend. Busse & Preisfeld 2002]
      Euglenophyceae incertae sedis: Ascoglena, Euglenamorpha, Euglenopsis, Glenoclosteroium, Hegneria, Klebsina, Euglenocapsa.
      Order Rapazida Cavalier-Smith 2016
      Family Rapazidae Cavalier-Smith 2016
      Order Eutreptiales Leedale 1967, emend. Marin & Melkonian 2003
      Family Eutreptiaceae Hollande 1942
      Order Euglenales Leedale 1967, emend. Marin & Melkonian 2003
      Family Phacaceae Kim et al. 2010
      Family Euglenaceae Dujardin 1841, emend. Kim et al. 2010


      Nutrition


      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.
      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.


      = Osmotrophic euglenoids

      =
      Osmotrophic euglenids are euglenids which have undergone osmotrophy.
      Due to a lack of characteristics that are useful for taxonomical purposes, the origin of osmotrophic euglenids is unclear, though certain morphological characteristics reveal a small fraction of osmotrophic euglenids are derived from phototrophic and phagotrophic ancestors.
      A prolonged absence of light or exposure to harmful chemicals may cause atrophy and absorption of the chloroplasts without otherwise harming the organism. A number of species exists where a chloroplast's absence was formerly marked with separate genera such as Astasia (colourless Euglena) and Hyalophacus (colourless Phacus). Due to the lack of a developed cytostome, these forms feed exclusively by osmotrophic absorption.


      Reproduction


      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. Because of the way that this reproduction takes place and the axis of separation, it is called longitudinal cell division or longitudinal binary fission.


      Evolution


      The earliest fossil of euglenids is attributed to Moyeria, which is interpreted as possessing a pellicle composed of proteinaceous strips, the defining characteristic of euglenids. It is found in Middle Ordovician and Silurian rocks, making it the oldest fossil evidence of euglenids.


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      References




      Bibliography


      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". European Journal of Protistology. 37 (3): 337–356. doi:10.1078/0932-4739-00842. 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 (1–2): 183–187. Bibcode:1978BiSys..10..183L. doi:10.1016/0303-2647(78)90040-0. PMID 656566.
      Wołowski, K; Hindák, F (2005). Atlas of Euglenophytes. Krakow: VEDA Publishing House of the Slovak Academy of Sciences. ISBN 9788022408363.


      External links


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

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    euglenid

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    Euglenid - Wikipedia

    In addition, euglenids can be divided into inflexible or rigid euglenids, and flexible or metabolic euglenids which are capable of 'metaboly' or 'euglenid motion'. Only those with more than 18 protein strips in their pellicle gain this flexibility.

    Euglena | Definition, Diagram, Movement, Classification, & Facts ...

    Dec 24, 2024 · Euglena are characterized by an elongated cell (15–500 micrometres [1 micrometre = 10 −6 metre], or 0.0006–0.02 inch) with one nucleus, numerous chloroplasts (cell organelles that contain chlorophyll and are the site of photosynthesis), a contractile vacuole (organelle that regulates the cytoplasm), an eyespot, and one or two flagella.Certain species …

    Euglena - Wikipedia

    Euglena is a genus of single cell flagellate eukaryotes.It is the best known and most widely studied member of the class Euglenoidea, a diverse group containing some 54 genera and at least 200 species. [1] [2] Species of Euglena are found in fresh water and salt water.They are often abundant in quiet inland waters where they may bloom in numbers sufficient to color the …

    Euglena: Definition, Structure, & Characteristics with Diagram

    Jul 4, 2020 · The internal structures found in a typical photosynthetic Euglena are as follows:. Pellicle: A thin, flexible membrane that supports the plasma membrane and helps them to change shape; Plasma Membrane: The selectively permeable membrane that protects the inner content of the cell from the outside environment; Cytoplasm: A jelly-like substance distributed …

    Euglenida - an overview | ScienceDirect Topics

    E. gracilis is a photosynthetic autotroph whereas most euglenid genera are either bacteriotrophs/eukaryotrophs, which actively consume prey by means of phagocytosis, or osmotrophs, which meet their nutritional needs by absorbing …

    Phylogeny and Classification of Euglenophyceae: A Brief Review

    Mar 15, 2016 · Eutreptia and Eutreptiella, both containing marine species, correspond to the basal lineage of the euglenid phototrophic clade, thus supporting the hypothesis of a marine origin of the photosynthetic euglenids (Leander et al., 2001; Hrdá et al., 2012).

    Euglena Central Metabolic Pathways and Their Subcellular …

    We propose that this is due to the requirement of the heterotrophic ancestor to synthesise all necessary cellular components before the acquisition of the secondary plastid. In this context, it seems likely that the plastid pathways are replicating pathways that were originally present in the euglenid progenitor. 5. Materials and Methods 5.1.

    Aug 1, 2017 · Cells with 4–12 strips are rigid; most of those with more strips (typically ~20–40) have them arranged helically and exhibit active cell deformation called “euglenid motion” or “metaboly.”

    Euglena: Structure, Diagram, Characteristics, Classification

    Euglena Classification. Euglena is a genus of euglenoids. The c lassification of Euglena is contentious. They are kept in the phylum Euglenozoa or in the phylum Euglenophyta with algae due to the presence of chlorophyll.

    Euglenida - an overview | ScienceDirect Topics

    There are many different species of Euglenid algae, displaying a great diversity of shapes, number of chloroplasts, pigmentation and behavior (Triemer and Ciugulea, 2010), with the species Euglena gracilis being the most heavily studied.