The euglenid flagellates are a type of protist – single celled eukaryotic organisms. Some species are phagotrophs (they can eat food as lumps), osmotrophs (they absorb nutrients across the cell surface), and others carry out photosynthesis. We have met some of the heterotrophs, both phagotrophs or osmotrophs, among other flagellates. The green euglenids are the ones that can carry out photosynthesis. Their chloroplasts contain chlorophyll B, which gives them a bright green color. Most are 20 – 100 µm long (1 µm = 1/1000 mm), larger than many other flagellates. Yet, these examples have been magnified with a microscope. Various devices are used to create contrast within the images.
Euglenids typically have 2 flagella, but often only one emerges from the cell. The bases of the two flagella can be seen ending in a small flagellar pocket near the front of the upper right cell. The example at the upper left shows that euglenid flagella tend to flex with a broad looping behavior. The loops often pass along the flagellum and push against the water so that the cells move forward. Green euglenids have a red eyespot. It is associated with the flagellar apparatus, and is used to help orient swimming in relation to sunlight.
While it is most usual for green euglenids to have a single flagellum (upper left and lower right – Phacus and Trachelomonas), some have two (Eutreptiella), others have a single flagellum but is very short (Lepocinclis fusca), and some have no emergent flagellum at all (Euglena mutabilis).
Several images show large flat chloroplasts. It seems likely that in the far past some ancestral euglenids acquired an alga and it persisted inside the cytoplasm, becoming a plastid. As plastids in other organisms are themselves descendents of endosymbiotic cyanobacteria, this would be an example of secondary symbiosis. Many of the cyanobacterial features, except for the enclosing membrane, photosynthetic pigments, and some DNA, are no longer evident. The plastids in the cell second down on the right are unusual, as they seem to have a darker area. These dark structures are pyrenoids which are used in the metabolism of carbon. They are sometimes misidentified as starch, but starch is refractile. The Lepocinclis cells have very obvious large refractile starch granules. The larger Lepocinclis cells (bottom left), some of which can be almost a millimeter in length, reveal a folded cortex in the form of parallel ridges running along the length of the cell. Adjacent ridges can slide relative to each other, allowing many euglenids to squirm (also called metaboly). Lepocinclis fusca and Phacus have lumps, excrescences, on their outer surfaces; while Trachelomonas cells occupy a small vase.
One species, Euglena sanguinea, can make red pigment inside the cell. It is said to do this when there is too much sunlight and it wants to shade its plastids. These guys can occur in very larger number in ponds, so that the water seems green in dim light, but turns red when it becomes very sunny. These are called traffic-light ponds.