Introduction
Euglenoids are unicellular, flagellated protists that show both plant-like and animal-like characteristics. The most common example is Euglena. They are usually found in freshwater, especially in ponds and stagnant water.
1. Habitat and General Nature
- They are mostly freshwater organisms, commonly found in stagnant or slow-moving water.
- Euglenoids are unicellular and microscopic, belonging to the kingdom Protista.
- They often form a green scum on the surface of nutrient-rich waters due to their chlorophyll.
2. Body Covering and Shape
- Pellicle instead of cell wall: Euglenoids do not have a typical cellulose cell wall. Their body is covered by a thin, flexible, protein-rich layer called the pellicle.
- The pellicle provides a definite shape and at the same time allows flexibility and wriggling movements.
- The body is usually elongated, spindle-shaped or somewhat leaf-like in form.
3. Locomotion and Sensory Structures
- Flagella: They possess flagella for locomotion. Typically there are two flagella inserted at the anterior end, but in many forms only one long flagellum is externally visible.
- Eyespot (stigma): A small, reddish eyespot or stigma is present near the base of the flagellum. It is sensitive to light and helps in phototactic movement (movement towards light).
- A contractile vacuole is present for osmoregulation, helping to remove excess water from the cell.
4. Nutrition and Pigments
- Euglenoids contain chloroplasts with chlorophyll (mainly chlorophyll \(a\) and \(b\)), so in the presence of sunlight they can carry out photosynthesis and behave like autotrophs (plant-like nutrition).
- In the absence of light, many euglenoids switch to heterotrophic nutrition, absorbing dissolved organic substances or ingesting small organisms. Thus, they show mixotrophic nutrition (both autotrophic and heterotrophic).
- Reserve food is often stored in the form of paramylon granules, which is a carbohydrate different from starch.
5. Reproduction
- Reproduction is mainly asexual, usually by longitudinal binary fission, in which the cell divides along its length.
- Sexual reproduction is either absent or not clearly known in typical euglenoids like Euglena.
6. “Borderline” Nature
- Plant-like features: Presence of chloroplasts, chlorophyll and photosynthesis are similar to those of algae and plants.
- Animal-like features: Absence of a cellulose cell wall, presence of pellicle, active movement with flagella, contractile vacuole and ability to ingest food are animal-like characters.
- Due to this combination of plant-like and animal-like traits, euglenoids are often considered a link between plants and animals.
Short Exam-style Answer
Euglenoids are unicellular, freshwater protists with a flexible protein-rich pellicle instead of a cell wall. They possess one or two flagella for locomotion and have a red eyespot that is sensitive to light. Their chloroplasts with chlorophyll enable photosynthesis in sunlight, whereas in darkness they can ingest or absorb food, showing mixotrophic nutrition. A contractile vacuole helps in osmoregulation. Because they have both plant-like (chloroplasts, photosynthesis) and animal-like (no cell wall, active movement, heterotrophy) features, euglenoids form a connecting link between plants and animals.