The main distinguishing feature of the pellicle is that it lacks cellulose and, in some species, is flexible enough for self-propelled motion of the cell. The analogous structure in plants, by contrast, is much stiffer, contains cellulose and is incapable of motion.
The pellicle is the outer envelope surrounding the euglena like a skin. It is thick and provides some structure, much like the cell wall in plants, but it is made up of protein instead of complex sugars.
Euglenoids are considered neither plants nor animals, even though they do contain chloroplasts and create sugar from sunlight, as do plants. Euglena with pellicle layers that are arranged in a spiral around the cell are able to swim by twisting through the water. Any kind of locomotion is very uncharacteristic of plants.
Furthermore, the presence of a cell wall made up of cellulose and other complex sugars is an identifying characteristic of plants. Despite popular belief, plants are not the only creatures with cell walls. Fungi have them too, but they use chitin rather than cellulose for strength and structural support.