Flagellates

mastigophorans, unicellular and colonial organisms having flagella as their organs of locomotion. Certain groups of flagellates, such as euglenoids, are assigned to the plant world by botanists and to the animal world by zoologists.

(1) In botany, flagellates (Flagellatae) were earlier regarded as a division (phylum) consisting of several classes, a number of which (Pantostomatinae, Protomastiginae, and Distomatinae) are not recognized as plant organisms today. Because of their great biochemical and morphological diversity, some of the remaining flagellates are separated into an independent division (euglenoid algae), and others (Peridiniales and Cryptomonadineae) are considered as classes of the division pyrophytic algae or other divisions (Chrysomonadineae of the division golden algae, Volvocineae of the division of green algae).

(2) In zoology, the flagellates (mastigophorans) are a class of protozoans consisting of two subclasses (Phytomastigina and Zoomastigina), which include 13 orders (approximately 3,000 species). The first subclass includes organisms containing pigments and feeding themselves principally by photo-synthesis. The second subclass includes heterotrophic organisms and organisms with a holozoic type of nutrition. Among the heterotrophic mastigophorans there are some that feed on liquid organic substances and inhabit putrefying fluids or parasitize the blood and tissues of animals and man. Mastigophorans that feed holozoically swallow solid food, principally bacteria and unicellular organisms.

Mastigophorans usually have from one to eight, and some-times more, flagella that proceed from the anterior end of the body; one of the flagella directed backward sometimes clings to the wall of the body, forming a curving wavelike membrane (for example, in trypanosomes). The body is clad in a thin external membrane, a pellicle, often with a compact chitinous armor or a casing of cellulose plates. The nucleus is usually single; sometimes there are a few score nuclei. Contractile vacuoles serve to regulate osmotic pressure and excretion. Mastigophorans equipped with chromatophores have a stigma (a light-sensitive organoid) located at the base of the flagellum; these mastigophorans are characterized by positive phototaxis. They reproduce most often by longitudinal binary fission; many are transformed during reproduction into cysts with thick membranes, after which they divide once or several times. In some mastigophorans, individuals produced by asexual reproduction do not separate but form colonies. Sexual reproduction is seldom observed, predominantly among representatives of the subclass Phytomastigina; the sexual process includes isogamy, heterogamy, or oogamy. One or several individuals, sometimes an entire colony, are obtained from a zygote. Free-living mastigophorans inhabit fresh waters and seas; parasitic ones are found in the bodies of animals and men, and many of them are causative agents of diseases of humans and domestic animals (for example, Trypanosoma, Leishmania, and Giardia).