Euglenid

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{{Taxobox | color = khaki | name = Euglenids | regnum = Protista | phylum = Euglenozoa | classis = Euglenoidea | classis_authority = Bütschli 1884 | subdivision_ranks = Major groups | subdivision = Phototrophs
   Euglenales
   Eutreptiales
Osmotrophs
   Rhabdomonadales
Phagotrophs
   ?Heteronematales
   ?Sphenomonadales }} The euglenids (also spelled euglenoids) are one of the best-known groups of flagellates, commonly found in freshwater especially when it is rich in organic materials, with a few marine and endosymbiotic members. Many euglenids have chloroplasts and produce energy through photosynthesis, but others feed by phagocytosis or strictly by osmosis. They belong to the phylum Euglenozoa, and their cell structure is typical of that group.

Euglenids are distinguished mainly by the presence of a pellicle, which 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 the flagella.

Phagocytosis is the primitive mode of nutrition. Prey such as bacteria and smaller flagellates are 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.

Chloroplasts presumably originated from some ingested green alga. They are pigmented with chlorophylls a and b, giving them a bright green colour, and are bound by three membranes. Often they are associated with granules of paramylon, a storage carbohydrate that is unique to the group. Most coloured euglenids also have a stigma or eyespot, which is a small splotch of red pigment on one side of the flagellar pocket. This shades a collection of light sensitive crystals near the base of the leading flagellum, so the two together act as a sort of directional eye. The cytostome is vestigial, although nutrients may still be obtained by absorption.

A few coloured euglenids have two roughly equal flagella, such as Eutreptia, and some have four. In most, however, the trailing flagellum is shortened so that it does not emerge from the flagellar pocket. The emergent flagellum typically undergoes a complex looping motion that pulls the euglenid along a slightly helical path. These include the common genera Euglena, Phacus, and Trachelomonas, which produces an organic lorica that encases the cell. There is also one genus, Colacium, in which the mature cells are non-motile and form branched colonies supported by mucous stalks.

In many cases exposure to certain chemicals or prolonged absence of light may kill off the chloroplasts without otherwise harming the organism. There are a number of species where chloroplasts have been lost, formerly treated in separate genera such as Astasia (colourless Euglena) and Hyalophacus (colourless Phacus). Since they lack a developed cytostome, these forms feed exclusively by absorption. Some primitively colourless euglenids, such as Rhabdomonas and Distigma, are also strict absorption-feeders. On molecular trees they form a monophyletic group, as do the photosynthetic euglenids together with their colourless derivatives.

The euglenids were first defined by Otto Bütschli in 1884 as the flagellate order Euglenida. They were treated by botanists as the algal division Euglenophyta, a double-placement they retained until the flagellates were broken up. Both names are still used to refer to the group, though the formal taxon Euglenida may be restricted to Euglena and its close relatives. The classification is still variable, as groups are being revised to conform with the molecular phylogeny.

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