Euglena | Structure, Diagram, Characteristics, Classification
Euglenoid (Euglena-like) : It is a group of chlorophyllous and non chlorophyllous flagellate protists. Largest genera being Euglena amongst them.
- Euglenoids are unicellular, flagellate protists found in water or damp soil. Majority of them are fresh water organisms found in stagnant water.
- Body is spindle shaped with blunt anterior end and pointed posterior end.
- Cell wall is absent but a covering periplast or pellicle is present which is proteinaceous (elastic) in structure.
- Locomotory organs are flagella.
- The cell bears a single long tinsel type flagellum (stichonematic) arising at the anterior end. Actually, there are two flagella but one of these is reduced. The longer flagellum has two branches at the base each having its own basal granule. In the area of union of two flagella is present a photosensitive paraflagellar body.
- Myonemes are oblique but parallely arranged strips in pellicle. Euglenoids perform creeping movement of contraction and expansion with the help of myonemes which is called metaboly or euglenoid movement.
- The apical end of the cell bears an invagination with three distinct parts, i.e., mouth (cytostome), canal (gullet or cytopharynx) and reservoir. It helps in the ingestion of solid food particles.
- Stigma or an eye spot is attached to the membrane of the reservoir at the level of paraflagellar body and along with it seems to be involved in perception of light stimulus. It contains photosensitive red orange pigment called astaxanthin.
- A contractile vacuole occurs in the anterior end of the cell just below the reservoir, meant for Osmoregulation and excretion.
- Single large nucleus lies near the centre of the protoplast.
- Nutrition in Euglena viridis is photoautotrophic. However, it is capable of getting nourishment from dead and decaying organic matter in the substrate by secreting digestive enzymes (saprophytic nutrition) in the absence of light. This dual mode of nutrition is termed as mixotrophic. Holozoic nutrition is absent in Euglena. Some forms are holozoic (Paranema) or saprobic (Rhabdomonas).
- Photosynthetic pigments are chlorophyll a, chlorophyll b, xanthophyll and B-carotene.
- Reserve food material is paramylon, stored in cytoplasm in the fornm of paramylum granules. They are chemically B-1, 3-glucans.
- Under favourable conditions, they mainly reproduce by longitudinal binary fission. During unfavourable conditions, palmella stage and cysts are formed for perennation. Sexual reproduction is not known to occur in euglenoids, e.g., Euglena and Paranema.