INVERTEBRATE ZOOLOGY IN THREE BOOKS - BOOK 1 - H.I. Shcherbak - 1995

SUBKINGDOM PROTOZOA, OR UNICELLULAR ORGANISMS (PROTOZOA)

PHYLUM SARCOMASTIGOPHORA

Subphylum Flagellata, or Mastigophora

Representatives of this subphylum are widespread in fresh and saltwater bodies, and a significant number of them have adopted a parasitic lifestyle. Over 8,000 species of flagellates are known in total. Different groups of flagellates may have independent evolutionary origins (polyphyletic) and are united into a single subphylum based on a single shared trait—the presence of flagella. In addition to flagella, some forms may permanently or temporarily develop pseudopodia. Structurally, the eukaryotic flagellum is identical to those found in plants and Multicellular animals, though it is considerably more complex than the flagella of Bacteria.

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Fig. 3. The flagellar apparatus:

a — diversity of its types; b — diversity of external Structure; c — Cytology/practical/72.html">Cross section of a flagellum

Results of electron microscopic studies show that the flagellum is externally covered by Cell/30.html">The Plasma Membrane and contains 11 microtubules internally: nine peripheral doublets forming a tube, and two single microtubules running along the central axis (Fig. 3). The part of the flagellum embedded in the Cytoplasm is called the kinetosome (basal body). In Kinetoplastida, a kinetoplast—a specialized modified portion of the mitochondrion—is located near the Base of the flagellum. Flagella on a single individual may vary in length, with either a smooth surface or hairy projections (see Fig. 3). Furthermore, flagellates possess a specialized Cytoskeleton composed of microtubules and microfilaments connected to the base of the flagellum.

In plant-like flagellates, the constancy of body shape is maintained by a Cell wall located outside the Plasmalemma, composed of Cellulose, Glycoproteins (order Volvocida), or chitinous or siliceous scales, and sometimes even loricae (order Chrysomonadina). In some groups, flattened vesicles—alveolites containing cellulose plates (order Dinoflagellida) or proteinaceous plates (order Euglenida)—are located beneath the plasmalemma. In the latter case, this condensed layer of cytoplasm is called the pellicle. In animal-like flagellates, the pellicle is formed by a network of microtubules within the outer cytoplasm and membrane.

Flagellates exhibit diverse modes of Nutrition. The vast majority of plant-like flagellates possess chromatophores and feed autotrophically via Photosynthesis, whereas animal-like flagellates are heterotrophic. Some species (such as Euglena) exhibit mixed nutrition, or mixotrophy (undergoing photosynthesis in the light and heterotrophic nutrition in the dark). Heterotrophic flagellates feed by phagocytosis or pinocytosis.

Most flagellates reproduce exclusively asexually, with nuclear division occurring mitotically and cytoplasmic division proceeding along the longitudinal axis of the body. Division can be monotomic, syntomic, or palintomic.

A sexual process is characteristic primarily of photosynthetic species—both solitary (Chlamydomonas) and palintomic colonial forms (Volvox). Known processes include the fusion of two identical Gametes (isogamy) or gametes that differ in structure and size (anisogamy), accompanied by a life cycle involving gametic or zygotic Meiosis. The subphylum comprises the classes Phytomastigophorea (Plant Flagellates) and Zoomastigophorea (Animal Flagellates).