A long, thin and thread like flagellum formed of delicate protoplasm is found at the blunt anterior of the body. It moves actively in water like a whip. The function of locomotion and knowledge of environment is brought about with the help of sensory organ. The flagellum of euglena is also called tractelium because its locomotion its movements push the animals from behind forwards, rotating it at some time.
The flagellum is cylinderical in shape in axial fibrillar structure called axxoneme is present in its middle. The axoneme is surrounded by aa protoplasmic sheath which is continued with the protoplasm of the boy. The axoneme is formed of 11 fibres, two of these are placed in the centre and nine along the periphery. Each of the peripheral fibres is formed of two fibils. The protoplasmic sheath present around these fibres consists of two layers.