Euglenids or euglenoids are one of the best-known groups of Eukaryote : single-celled organisms with flagella, or whip-like tails. They are classified in the phylum Euglenozoa, 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 Endosymbiont members. Many euglenids feed by phagocytosis, or strictly by diffusion. A monophyletic subgroup known as Euglenophyceae have 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 (membranous organelles) in all extant photosynthetic species result from secondary endosymbiosis between a euglenid and a green alga.
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 (Euglenophyceae), the (mainly the 'primary osmotrophs' known as Aphagea), and the , 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:
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 Phagocytosis 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.
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.
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.
==Gallery==
|
|