What is the peculiarity of eating euglena green?

Euglena green is the simplest microorganism. The habitats are reservoirs and puddles.
Its body has an unchangeable shape due to its dense shell. There is a peculiarity in the diet of euglena that indicates the kinship of plants and animals.
The organism of this protozoan contains chloroplasts. Thanks to them, euglena green can, like all green plants, synthesize organic matter using solar energy.
But in case of falling into a dark, sunless place, chloroplasts disappear and euglena green begins to feed like all heterotrophs.