Why is euglena hard to classify?

Why is euglena hard to classify?
Euglena are single celled flagellate algae that can be found in the water, mud and sand of ponds and puddles. Although they contain chlorophyll, Euglena cannot survive solely by photosynthesis. Euglena vary in shape from cylindrical to oval, with flagella that are shorter than their bodies. They range in size from 35-45um and may be green or reddish in colour. They are regarded as difficult organisms to classify as they have both plant and animal characteristics
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