Euglenoids are unicellular flagellate protists. Here I’ll be explaining the characteristic features of Euglenoids in detail including their habitat, structure, mode of nutrition, and examples.
It’s an integral part of the Kingdom Protista in the second chapter “Biological Classification” of Class 11 NCERT Biology from which questions have been repeatedly asked, both in the NEET exam as well as boards.
Previously euglenoids were placed in the plant kingdom due to their photosynthetic ability. But due to the absence of cell walls and animal-like nutrition, some scientists placed them in the animal kingdom. But now according to five-kingdom classification, they are included in protista.
General characteristic features of Euglenoids:
- Belong to Kingdom Protista (unicellular eukaryotes).
- The majority of them are freshwater organisms found in stagnant water (ponds and lakes). Sometimes they are also found in damp soil and brackish water.
- Euglenoids are devoid of cellulose cell walls. They are surrounded by a cell membrane that is made up of lipoprotein and this cell membrane is covered with pellicle (thin protein-rich layer) which makes their body flexible. The pellicle has oblique but parallel stripes called myonemes. The pellicle is composed of fibrous protein (elastic in nature) and a small amount of lipid or carbohydrates.
- Euglenoids have chloroplasts that have pigments – Chlorophyll a, Chlorophyll b, and Xanthophyll (Zeaxanthin). These pigments of euglenoids are identical to those present in higher plants.
- Euglenoids have two flagella, a long one and a short one. Each flagellum arises from a basal granule (blepharoplast). Out of the two flagellums, only one is functional. The functional flagellum has numerous tiny hairs along one side and helps in motility. This flagellum originates in the reservoir. The smaller flagellum (non-functional) is contained within the reservoir but does not protrude.
- At the anterior end of Euglenoids, a cavity called reservoir is present. Flagellum originates from the base of the reservoir.
- One eyespot is also present at the anterior position.
- The two flagella join with each other at a swelling called the paraflagellar body. An orange-red coloured eyespot or stigma is located at the base of the flagellum attached to the membrane of the reservoir at the level of the paraflagellar body. They contain red pigment astaxanthin. Both the paraflagellar body and eyespot act as photoreceptors and direct the organism towards the optimum light.
- They can undergo creeping through expansion and contraction of the body called metaboly.
- The apical end bears an invagination having three parts – cytostome (mouth), cytopharynx (gullet or canal), and reservoir. The cytostome is generally eccentric.
- They have a contractile vacuole (in the anterior part of the cell below the reservoir). The contractile vacuole helps in osmoregulation by acting as a pump and removing the excess water. The excess water is pumped out of the cell through the reservoir and a new contractile vacuole is formed subsequently after each discharge.
- Nutrition is holophytic (photoautotrophic), saprobic (Rhabdomonas), or holozoic (Peronema). Even holophytic forms can pick up organic compounds from the outside medium. Such a mode of nutrition is called mixotrophic.
- The product of photosynthesis is paramylon (carbohydrate) which is stored in the form of paramylon or paramylum granules in the paramylum bodies in the cytoplasm.
- Under favourable conditions, euglenoids multiply by longitudinal binary fission.
- They perennate during unfavourable conditions as cysts.
Examples of Euglenoids: Euglena, Phacus, Eutreptia, Trachelomonas, Peranema
Features of Euglena:
Euglena is a connecting link between animals and plants. It resembles the ancestral form from which the plants and animals evolved.
The body is covered with pellicle, a small cytostome (cell mouth) and cytopharynx is present at the anterior end of the body.
Euglena is a free-swimming flagellate with two methods of movement:
(i) Euglenoid movement by contraction and expansion of the body (wriggling movement).
(ii) Flagellar movement by stichonematic type flagellum.
Nutrition in Euglena is mixotrophic. When sunlight is available, it is photosynthetic. When sunlight isn’t available (in darkness), it is saprophytic, absorbing food from the surrounding water.
Euglena contains chlorophyll, yet it resembles animals because it feeds like animals in the absence of sunlight.
Reserve food is stored in the form of paramylum or paramylon. Asexual reproduction occurs by longitudinal binary fission, there is no sexual reproduction.
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