This protist, which length is 45 to 65 microns to 14 to 20 microns has always captured my attention. I remember when I received my first microscope; it was a gift from my parents when I was almost seven years old. I spent many days observing drops of muddy water, looking for those evasive and small euglenas (Euglena viridis). At those times, Euglena was classified as a unicellular plant pertaining to the Division Algae (seaweeds).
To see the complexity of Euglenas, let us consider some of their main characteristics:
Euglenas’ locomotion is performed by flagella, the same as protozoans do.
However, Euglena is a mixotrophic organism (it is a holophytic organism because it produces her own food through photosynthesis in chloroplasts, and it is also a heterotrophic organism because it absorbs elaborated food, i.e. biotic products).
Euglenas have a receptor to perceive light (a light sensor) and an eyespot.
Euglena's eye-spot can detect light. It is situated in the frontal region, assumed from the direction of its movements, which looks like a red or brown dot.
Although the eye-spot cannot be considered an eye, it works like a sensorial organ that is capable of reacting to small changes of the intensity of light. The intensity of visible light determines the area towards which Euglenas swim, which always is towards the source of light, i.e. towards the place where the light is more intense. Euglena's stigma is composed by thirty to fifty granules of beta-carotene.
Euglena ejects its excreta through the cytopharinx and releases them to the surroundings through the cytostome.
As we can see, the complexity of these microorganisms was big enough as for being classified into an exclusive phylum, i.e. the phyllum Euglenoidea.