- Euglenoids are Euglena-like unicellular flagellates that possess pellicle instead of cell walls.
- They belong to the division Euglenophyta and class Euglenophyceae.
- They were previously placed under class Phytomastigophora of Protozoa by zoologists.
Characteristic Features of Euglenoids
I. Habitat and Habits
- Euglenoids occur in freshwater habitats and damp soils.
Freshwater habitat of Euglena
- 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.
- Nutrition is holophytic (photoautotrophic), saprobic & 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).
Structure of Euglena
- Euglenoids are unicellular flagellate protists without cell walls. .
- The body is covered by a thin and flexible pellicle (periplast) with oblique parallel stripes called myonemes.
- The pellicle is composed of fibrous elastic protein, a small amount of lipid and carbohydrates and maintains a definite shape and also helps in temporary changes in the body shape.
- 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.
- The apical end bears an invagination having three parts cytostome, cytopharynx and reservoir. The cytostome is generally eccentric.
- Just in the area of the union of two roots, the flagellum bears a swelling called paraflagellar body.
- An orange-red eyespot or stigma occurs attached to the membrane of the reservoir at the level of the paraflagellar body. Eyespot contains red pigment astaxanthin, found only in crustacea.
- Both paraflagellar bodies and eyespot perceive the stimulus of light. They help in directing the organism toward the optimum light.
- 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.
- The photoautotrophs and holophytic forms possess chloroplasts with or without pyrenoids. Photosynthetic pigments include chlorophyll a and chlorophyll b.
- A single large nucleus lies roughly in the middle. The nuclear envelope persists during division. The nucleolus also persists and divides into two.
III. Reserve Food
- They store their carbohydrates as paramylon bodies.
Euglena showing scattered Paramylon in the Cytoplasm
- The latter are formed outside the chloroplasts.
- Paramylon bodies are scattered throughout the cytoplasm.
- Paramylon is chemically different from starch and glycogen and does not stain with iodine.
- Euglena reproduces asexually by binary fission, they divide longitudinally. Sexual reproduction has not yet been definitely proved.
- Under unfavourable conditions, some euglenoids form reproductive cysts. Many Euglenoids gather, lose their flagella, and get enclosed in a gelatinous substance. Individual Euglena cells generate a reproductive cyst, which divides into daughter cells in a binary fission process. These progeny cells get flagellated and emerge from the mass under favourable conditions. The palmelloid stage of the lifecycle is named after this.
Longitudinal asexual reproduction in Euglena
- Examples: Euglena, Phacus, Eutreptia, Trachelomonas, Peranema
A. Euglena - The Spindle Organism
- Euglena is a large genus having 152 species (Gojdics, 1953). The common species is E. Viridis. It is found in freshwater ponds and pools. It also occurs on moist mud (Lackey, 1968).
- Euglena is a free-living solitary and unicellular flagellate.
- Holozoic or phagotrophic forms are absent in Euglena.
- Asexual reproduction occurs by longitudinal binary fission.
- Perennation occurs through cyst formation.
- The pellicle is made up of proteins (about 80%), carbohydrates and lipids.
- Besides swimming, Euglena can also perform creeping movements or metaboly.
- 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 the 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 (eyespot).
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 paramylon bodies.
- They have polysaccharides called paramylon (β-1,3-glucan).
Euglena is studied as a plant as well as an 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.
B. Consumer-Decomposer Protists - Slime Moulds
- Slime moulds possess the characters of both animals and fungi and, therefore, they are commonly called fungus-animals.
- Modem biologists include slime moulds under the kingdom Protista and call them protistan fungi.
General Characters of Slime Moulds
- The slime moulds live usually amongst decaying vegetation. They are quite common on lawns and moist fields.
- They do not have chlorophyll.
- They are surrounded by the plasma membrane only (somatic parts are without cell walls). However, the spores have cell walls made up of cellulose.
- At one stage of the life cycle, they have an amoeboid structure.
- They exhibit a wide range of colouration.
- They have phagotrophic or saprotrophic nutrition.
- Both asexual and sexual modes of reproduction are found. They produce spores within sporangia. A spore possesses a cell wall of cellulose.
Sporangia in Slime Moulds
- The slime moulds resemble both protozoa and true fungi. They are like protozoa in their amoeboid plasmodial stage and similar to true fungi in spore formation.
Life Cycle of Slime Moulds
Protozoans are single-celled eukaryotes that can live either free or as parasites. They occur in a variety of sizes and shapes. They can be found in a range of habitats, including freshwater and marine. In nature, they are heterotrophic. Protozoa have a complicated internal structure and are microscopic.
Microscopic image of Protozoa
- Protozoans were first studied by Leeuwenhoek (1677). The name "Protozoa" was coined by Goldfuss (1817).
- The branch of biology which deals with protozoa is called protozoology.
Classification of Protozoans
On the basis of locomotion, the protozoans are divided into four groups:
- Flagellated Protozoans
- Amoeboid Protozoans
- Ciliated Protozoans
Group 1. Flagellated Protozoans
- They possess flagella for locomotion.
- They may be free-living aquatics, parasites, commensals or symbionts.
- Zooflagellates are generally uninucleate, occasionally multinucleate.
Structure of a Protozoa
- The body is covered by a firm pellicle.
- Nutrition is holozoic, saprobic and parasitic.
- Asexual reproduction is by binary fission.
Binary fission in Protozoa
- Sexual reproduction is recorded in some forms only.
Examples: Trichonympha, Lophomonas, Giardia, Trypanosoma, Leishmania and Trichomonas.
Trichonympha and Lophomonas are cellulose digesting symbionts in the body of termites and wood roaches respectively.
Some of the Protozoans are parasites present in human beings:
- The parasite of sleeping sickness. It was first observed by Forde in 1901.
- Fruce discovered that the parasite of sleeping sickness is transmitted by tse-tse fly.
- It causes Gambian sleeping sickness.
- The disease, also called Gambian trypanosomiasis, is found in western and central parts of Africa.
- The parasite is transmitted by blood-sucking tse-tse fly, Glossina palpalis. The reserve host is antelope.
- The parasite does not affect the antelope and the fly.
- Mouth and contractile vacuole are absent.
- Food is absorbed through the body's surface.
- In human beings, the parasite lives in the blood plasma.
- Later the parasite enters cerebrospinal fluid and damages the brain.
- It makes the patient lethargic and unconscious.
Group 2. Amoeboid Protozoans
- They develop pseudopodia which are temporary protoplasmic out growths. They are of four types- lobopodia (broad and blunt), filopodia (slender, unsupported, independent), axopodia (slender with axial support) and reticulopodia (slender, reticulate).
- Pseudopodia are used for locomotion and engulfing food articles.
- Sarcodines (largest Phylum of Protozoans) are mostly free-living, found in fresh water, sea water and damp soil. Only a few are parasitic.
- The body may be covered with plasmalemma or a shell.
- Nutrition is commonly holozoic.
- Sarcodines are generally uninucleate, binucleate (e.g., Ardellä) and multinucleate (e.g., Pelomyxa) types also occur but the nuclei are monomorphic.
- Asexual reproduction takes place by binary fission, multiple fission, budding and spores.
- Sexual reproduction occurs through syngamy.
Examples: Amoeba, Pelomyxa, Entamoeba, Radiolarians, Foraminiferans, Heliozoans.
(a) Amoeba proteus: The Proteus Animalcule.
- Amoeba was discovered by Russel von Rosenhoff in 1755. H.I. Hirschfied (1962) has given a detailed account of the biology of Amoeba.
- It is found in freshwater.
- Types of pseudopodia are lobopodia.
- A contractile vacuole is present for osmoregulation.
- Mitochondria are often seen aggregated around the contractile vacuole of the Amoeba.
- The cytoplasm is differentiated into endoplasm and ectoplasm.
- Endoplasm is further differentiated into plasma gel and plasmasol.
- The body is covered by plasmalemma.
- Nutrition is holozoic.
- The process of obtaining food by Amoeba is called phagocytosis.
1- Ingestion; 2- Digestion; 3- Absorption; 4- Assimilation; 5- Egestion
- 1. Ingestion: When a food particle is near the Amoeba, it forms temporary finger-like projections called pseudopodia around the food particle and engulfs it.
2. Digestion: The food is digested in the food vacuole with the help of enzymes.
3. Absorption: It is then absorbed in the cytoplasm of the Amoeba by diffusion.
4. Assimilation: The absorbed food provides energy and a part of nutrition is used for growth.
5. Egestion: The undigested and the waste food particles are thrown out.
- Sol-gel theory of amoeboid movement was given by Hyman and supported by Pantin and Mast.
(b) Entamoeba histolytica
- Lamble (1859) discovered Entamoeba histolytica.
- Losch (1875) discovered its pathogenic nature.
- The life cycle of Entamoeba histolytica is monogenetic (single host life cycle).
- It resides in the upper part of the human large intestine and repeatedly causes the disease known as amoebic dysentery or amoebiasis.
E. histolytica feeding in human large intestine
- The symptoms of the disease are abdominal pain and motions with blood and mucus.
- The parasite is generally one pseudopodium.
- The contractile vacuole is absent as there is no need for osmoregulation.
- It feeds on red blood corpuscles by damaging the wall of the large intestine and reaching the blood capillaries. It produces ulcers.
- Multiplication is by binary fission.
- Entamoeba histolytica has two forms:
(i) Magna (trophozoite) pathogenic form found in the mucosa and submucosa of intestine forming ulcers.
(ii) Minuta-nonpathogenic form found in the lumen of the intestine. Minuta from encysts.
- A mature cyst is called a tetra-nucleate cyst. It has four nuclei and two chromatoid bodies.
- Tetranucleate cyst is the infective stage.
- It is important to note that only one young amoeba with four nuclei hatches out from a cyst of E.histolytica.
- However, a single cyst of E. histolytica produces eight amoebae.
- The most effective medicine for amoebiasis is Metragyl or Flagyl.
Group 3. Sporozoans
- All sporozoans are endoparasites.
- Some sporozoans such as Eimeria cause severe diseases like coccidiosis in the birds.
- Locomotory organelles (cilia, pseudopodia, etc.) are absent.
- Nutrition is heterotrophic & Saprozoic. Phagotrophy is rare.
- The body is covered with an elastic pellicle or cuticle.
Structure of Sporozoa
- Contractile vacuoles are absent.
- Asexual reproduction occurs through fission.
- Sexual reproduction takes place through syngamy.
- Life cycle consists of two distinct asexual and sexual phases. They may be passed in one (monogenetic) or two different hosts (digenetic).
Life cycle of PlasmodiumExamples: Plasmodium, Monocystis. Eimeria. Monocytes.
Group 4. Ciliated Protozoans
- Kinety system is present in ciliates which comprise cilia, kinetosomes (basal bodies) and kinetodesoma (bundle of fibrils).
- Cilia are used for locomotion and obtaining food.
- There is a high degree of morphological and physiological specialization.
- Most ciliates are free-living individuals in fresh marine waters. A few are parasites.
- The body is covered by a pellicle.
- Nutrition is holozoic except in parasitic forms.
- There are definite regions for ingestion and egestion. The region of ingestion consists of an oral groove, cytostome (mouth) and gullet.
- Ciliates show nuclear dimorphism or two types of nuclei, larger macronucleus (meganucleus) and smaller micronucleus. The macronucleus controls metabolic activities and growth. It is also called vegetative nucleus. Micro nucleus takes part in reproduction. Hence, it is termed the reproductive nucleus.
- Ciliates often possess minute ejectable trichocysts for defence.
- They have contractile vacuoles for osmoregulation.
- Asexual reproduction takes place by transverse binary fission or budding.
- Cyst formation occurs under unfavourable conditions.
- Sexual reproduction is by means of conjugation.
Examples: Paramecium, Vorticella, Opalina, Balantidium.
(a) Paramecium- The Slipper Organism or Slipper Animalcule:
- Paramecium is a free-living ciliate that is found in freshwater.
Structure of Paramecium
- The most widely distributed species are Paramecium caudatum and Paramecium aurelia.
- Nutrition is microphageal.
- Bacteria are its chief food.
- Paramecium is a surface feeder.
- Pellicle maintains the shape.
- The cilia of the extreme posterior end are longer and form a bunch called a caudal tuft.
- The discharged trichocysts serve for anchoring or defence, or they may be a reaction to injury.
- Feeding Apparatus consists of peristome (oral groove), vestibule, buccal cavity, cytostome (cell mouth) and cytopharynx.
- The latter opens into the endoplasm.
- A temporary opening called cytopyge (cytoproct or cell anus), is present a little behind the cytostome. Undigested food is passed out through cytopyge.
- Paramecium caudatum contains a single large macronucleus and one small micronucleus.
- Paramecium aurelia has one macronucleus and two micronuclei.
- Paramecium contains two contractile vacuoles surrounded by 12 radial (feeding) canals.
- The contractile vacuoles and radial canals are for osmoregulation.
- Several non-contractile food vacuoles (gastrioles) are seen moving along a definite course (cyclosis) within the streaming endoplasm.
- The food vacuoles are meant for intracellular digestion.
- Both asexual and sexual reproduction are found in Paramecium.
Asexual & Sexual Reproduction in Paramecium
- Asexual reproduction occurs through transverse binary fission.
- Various methods of sexual reproduction in Paramecium are:
(i) Conjugation & Autogamy: It corresponds to self-fertilization.
(ii) Endomixis it corresponds to parthenogenesis.
(iii) Cytogamy: It shows the characters of both conjugation and autogamy.
(iv) Hemixis only the macronucleus takes part in hemixis. In hemixis, the macronucleus first breaks into a few irregular pieces; later some pieces reunite to form the macronucleus.
- The remaining pieces disintegrate in the endoplasm. Thus hemixis is called the "Purification Act".