Euglenoids: Structure and Reproduction | Protists

In this article we will discuss about:- 1. Habitat and Habits of Euglenoids 2. Structure of Euglenoids 3. Reproduction 4. Euglena—The Spindle Organism.

Habitat and Habits of Euglenoids:

(i) Euglenoids occur in fresh water habitats and damp soils.

(ii) Euglenoids swim actively in a liquid medium with the help of their long flagellum. They can also perform creeping movements by expansion and contraction of their body. This phenom­enon is called metaboly or euglenoid movements.

(iii) Nutrition is holophytic (photoautotrophic), saprobic or holozoic. In dark even photosynthetic forms can behave like het­erotrophic, predating on smaller organisms (holozoic) or feeding on organic remains (saprobic). Such a mode of nutrition is called mixotrophic (e.g., holophytic + saprobic or holozoic).

Structure of Euglenoids:

(i) Euglenoids are unicellular flagellate protists. They are without cellulose cell wall. The body is covered by thin and flexible pellicle (= periplast). The pellicle has oblique but parallel stripes called myonemes. The pellicle is composed of fibrous elastic protein, small amount of lipid or/and carbohydrates and maintains a definite shape. It is flexible enough to permit temporary changes in the body shape.

(ii) The euglenoids have two flagella, usually one long and one short. Each flagellum arises from a basal granule (= blepharoplast). The flagella bear hair (= tinsels). So the flagella are tinsel type.

(iii) The apical end bears an invagination having three parts — cytostome, cytopharynx and reservoir. The cytostome is generally eccentric.

(iv) Just in the area of union of two roots, the flagellum bears a swelling called paraflagellar body. An orange-red eye spot or stigma occurs attached to the membrane of reservoir at the level of paraflagellar body. Eye spot contains red pigment astaxanthin, found elsewhere only in crustacea. Both paraflagellar body and eye spot perceive the stimulus of light. They help in directing the organism toward the optimum light.

(v) An osmoregulatory contractile vacuole occurs in the anterior part of the cell below the reservoir. It is fed by a number of canals. The contractile vacuole discharges its contents into the reservoir.

(vi) The photoautotrophs or holophytic forms possess chloroplasts with or without pyrenoids. Photosynthetic pigments include chlorophyll a and chlorophyll b.

(vii) A single large nucleus lies roughly in the middle. The nuclear envelope persists during division. The nucleolus also persists and divides into two.

Reserve Food:

They store their carbohydrates as paramylon or paramylum bodies. The latter are formed outside the chloroplasts. Paramylum bodies are scattered throughout the cytoplasm. Paramylum is chemically different from starch and glycogen and does not stain with iodine.

Reproduction of Euglenoids:

Sexual reproduction has not yet been definitely proved. Under favourable conditions, euglenoids multiply by longitudinal binary fission. The palmella stage is found during unfavourable conditions.

Examples:

Euglena, Phacus, Eutreptia, Trachelomonas, Peranema.

Euglena— The Spindle Organism:

Euglena having 152 species. The com­mon species is E. viridis. It is found in fresh water ponds and pools. It also occurs on moist mud. Euglena is a free living solitary and unicellular flagellate. Euglena is mixotrophic (holophytic + saprobic) in nutrition.

Holozoic or phagotrophic forms are absent. Asexual reproduction occurs by longitudinal binary fis­sion. Sexual reproduction has not yet been re­corded. Perennation occurs through cyst forma­tion. The body is covered by a plasma membrane followed by periplast or pellicle. The pellicle is made up of proteins (about 80%), carbohydrates and lipids.

Besides swimming Euglena can also perform creeping movements or metaboly. At the place of union of the two branches, the fla­gellum bears a swelling called paraflagellar body (photoreceptor). The posterior end is pointed.

The anterior end of the cell is blunt and bears an eccentric cytostome (mouth). The cytostome leads into a tubular canal, also called cytopharynx (gullet). The latter expands at the base to form a large rounded reservoir. At one end of the reservoir, the cytoplasm contains an orange red stigma (eye spot).

The latter is photosensitive. Just below the reservoir is found a contractile vacuole having many feeding canals (= accessory vacuoles). The contractile vacuole takes part in osmoregulation. It expands and pumps its fluid contents in the reservoir. Chloroplasts are numerous, discoid shaped or ribbon-like.

Pyrenoids (proteinaceous bodies) may be present in the chloroplasts. Chlorophyll a and chlorophyll b are present. The endoplasm contains several paramvlum bodies. They have polysaccharide called paramylon or paramylum (β-1, 3 glucan). Euglena is studied as plant as well as animal. It is called plant animal.

Plant Characters of Euglena:

(i) Presence of chloroplasts with chlorophyll.

(ii) Holophytic (photosynthetic) nutrition.

Animal Characters of Euglena:

(i) Presence of pellicle which is made up of proteins and not of cellulose.

(ii) Presence of stigma and paraflagellar body (photosensitive structures).

(iii) Presence of contractile vacuole (not found in plants),

(iv) Presence of longitudinal binary fission.