Euglena is a classic textbook organism, exhibiting properties of both an alga (photosynthesis) and a protozoan (flagellated movement).
The chloroplast of Euglena is of secondary endosymbiotic origin. They also have a distinctive red eyespot that gives it sensitivity to light stimuli. Like other euglenids, their cell membrane is covered with a pellicle formed of proteinaceous strips, but this is not easily visible with light microscopy. The cells are filled with carbohydrate grains made of a starch-like polymer called paramylon.
A well-illustrated review of euglenid biology by Brian Leander is available on the Tree of Life website.
In their classic spindle-shaped form, the flagella emerge from a pocket in the anterior of the cell, where the red eyespot (“stigma”) is also located. The rear end of the cell tapers into a point. The main flagellum flails in a whip-like motion that propels the cell forward. It spins rapidly as it does so. See the animated gif below.
Euglenids in general also exhibit a form of movement called metaboly, or “euglenoid motion”. The spindle flattens to a squat bulb on one end, and this bulb passes as a wave through the cell, allowing it to squeeze and squirm across a surface or through an obstruction.
Euglena at rest
This sample of pond water contained a large amount of cyanobacterial scum – a gelatinous mass embedded with cyanobacterial cells that secreted them, along with other organisms. A large proportion of the cells were balled-up Euglena, recognizable by the glassy paramylon grains, green coloration, and red eyespot.
Diversity of Euglena
Other Euglena species aside from the one described above. Euglena are diverse in pattern and shape. Some have warty or ornamented pellicles, whereas others are less strongly armored. Their exact shapes also vary, though the spindle-shaped swimming form is common to most. They all also have eyespots and whiplash-type flagella.