Invertebrate Zoology: A Study Guide - T. A. Dauda 2014

Protozoans
Sarcomastigophorans
Class Flagellata

Flagellates comprise over 6,000 species of protozoa inhabiting seas, freshwater bodies, soils, and the organisms of Multicellular animals. Their body shape varies, though it is most commonly oval, spherical, or spindle-shaped. The Organelles of locomotion are thread-like cytoplasmic extensions known as flagella (ranging from 1 to 100), typically located at the anterior end of the body and directed forward in the direction of movement (Fig. 2). The beating of the flagellum causes the protozoan's body to rotate, effectively screwing it through the Water.

In addition to flagella, some species may temporarily or permanently develop pseudopodia. The body of such flagellates is bounded from the external environment solely by a Cell membrane, but most species develop various protective envelopes that maintain a relatively constant body shape. In many flagellates, the ectoplasm condenses to form one of these exterior envelopes, known as the pellicle. A spherical Nucleus located in the endoplasm regulates the metabolic activity and reproduction of the protozoan. The contractile vacuole periodically fills with liquid, which is expelled upon contraction into a specialized vesicle—the reservoir—and subsequently discharged outside through a canal. As a rule, contractile vacuoles are absent in marine flagellates.

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Fig. 2

Green euglena (Euglena viridis):

1 — chromatophore; 2 — contractile vacuole reservoir; 3, 4 — flagellum; 5 — eyespot (stigma); 6 — contractile vacuole; 7 — nucleus.

The Nutritional types of flagellates are highly diverse. Some feed on the breakdown products of organic matter, which are absorbed across the entire cell surface. This mode of Nutrition is termed saprophytic. Others consume Bacteria, Algae, and other microorganisms using specialized feeding organelles—a cell Mouth, cell Pharynx, and digestive vacuoles. This mode of nutrition is referred to as animal (animalistic) or holozoic.

Saprophytes and holozoics are typical heterotrophic organisms, meaning they feed on preformed organic substances. A distinct position regarding nutrition is held by certain euglenoids, notably *Euglena viridis*. The Cytoplasm of these flagellates contains chlorophyll housed within chromatophores. In the presence of light, these structures synthesize Organic compounds, much like plants do. This type of nutrition is called plant-like or holophytic, and such organisms are autotrophic.

In the dark, euglenas lose their green color and switch to saprophytic nutrition. Thus, *Euglena viridis* exhibits a mixed, or mixotrophic, type of nutrition. The cytoplasm of mixotrophic flagellates may contain various inclusions, such as starch granules and lipid droplets, alongside a red or brown pigmented spot known as the eyespot. Flagellates possessing an eyespot typically migrate toward a light source, exhibiting positive phototaxis.

For the majority of flagellates, only asexual reproduction is known. It begins with the division of The Nucleus, which may be accompanied by the division of chromatophores. Subsequently, a longitudinal constriction forms, dividing The Cell into two. During this process, the existing flagellum may either be discarded or retained by one of the newly formed offspring, while a new flagellum develops in the other.

Flagellates survive unfavorable conditions in an encysted state. Within the cyst, they retain The ability to divide.

The class Flagellata comprises 20 orders. Mixotrophic flagellates are classified under the subclass Phytomastigina (plant flagellates), whereas heterotrophic free-living and parasitic flagellates belong to the subclass Zoomastigina (animal flagellates). Typical representatives of phytomastigotes include *Euglena viridis*, *Volvox* (which forms spherical colonies), peridinians, and others.

Autotrophic flagellates play a vital role in the global biogeochemical cycling of matter as producers of primary organic matter. Many serve as bioindicators assessing the organic pollution levels of aquatic ecosystems. Furthermore, certain colorless flagellates consume and assimilate organic compounds, playing a significant role in the biological purification of wastewater.

Among zoomastigotes, Blood parasites affecting numerous vertebrates—specifically trypanosomes—deserve special attention. These are relatively small flagellates, 20–70 µm in length, featuring a flattened, ribbon-like body tapered at both ends, a single flagellum, and an undulating membrane. The latter is formed by the fusion of the flagellum with the body surface. Trypanosomes inhabit the blood of vertebrates, with various blood-sucking insects serving as their vectors.

*Trypanosoma rhodesiense* causes human sleeping sickness in tropical Africa.

The natural reservoir for these trypanosomes consists of antelopes, while the vectors are blood-sucking tsetse flies (Fig. 3).

Several trypanosome species cause severe diseases in cattle and camels (*Trypanosoma evansi*, *T. brucei*).

Their vectors include horseflies and flies of the genus *Glossina*. In horses, particularly in southern regions, a trypanosomal disease known as dourine occurs. Its causative agent, *T. equiperdum*, is transmitted during mating. The pathology is associated with damage to The Nervous system.

Leishmanias are intracellular parasites of humans, transmitted by phlebotomine sandflies. In Central Asia and Transcaucasia, two leishmanial species pathogenic to humans are found: *Leishmania donovani*, which causes visceral leishmaniasis (predominantly in children), accompanied by hepatosplenomegaly, anemia, and emaciation.

Another species, *Leishmania tropica*, causes cutaneous lesions (Oriental sore) that persist for 1–2 years and leave scars upon healing. In nature, the natural reservoirs for human leishmaniasis pathogens are dogs, and additionally rodents in the case of cutaneous leishmaniasis.

Fig. 3

Trypanosome (*Trypanosoma sp.*):

1 — kinetoplast; 2 — nucleus; 3 — undulating membrane; 4 — flagellum.

Fig. 4

Trichomonas (diagram of Structure):

1 — free flagella; 2 — nucleus; 3 — nucleolus; 4 — supporting fibril; 5 — axostyle; 6 — recurrent flagellum running along the margin of the undulating membrane.

Other Trichomonas species also parasitize humans: Trichomonas hominis in the intestine, and T. vaginalis in the urogenital tract (Fig. 4).

Lamblia (Giardia) are common parasites of the human Bile ducts, duodenum, and Small Intestine. Heavy infection with Lamblia is accompanied by intestinal dysfunction.

Review and Self-Assessment Questions:

1. On The basis of which characteristics are some Protozoans classified under the phylum Sarcomastigophora?

2. What is The structure of an amoeba?

3. How do sarcodines carry out the processes of nutrition, Respiration, locomotion, water excretion, reproduction, and survival under unfavorable conditions?

4. What is The life cycle of the dysentery amoeba? What harm does it cause to humans?

5. Which Representatives of the Class Sarcodina do you know?

6. What is the structure of flagellates?

7. How do flagellates feed?

8. What types of plant flagellates do you know?

9. What parasitic flagellates are known to you?