The Flagellates (Phylum: Sarcomastigophora; Subphylum: Mastigophora)

Abstract

An efficient means of motility is a decided asset to most living organisms, especially those that reproduce rapidly and must move about efficiently to invade new territory and reduce competition for resources. If the environment is nutritionally limited (oligotrophic), then the pressure toward survival by efficient and sustained locomotion is all the more critical. Both autotrophic and heterotrophic organisms are benefited by efficient locomotion, either to move to regions of appropriate illumination, or to increase the probability of finding external sources of food. Aqueous environments have been efficiently exploited by the flagella-bearing protozoa whose undulating, whiplike organelles provide an efficient and dexterous mode of moving through fluids. Each flagellum or undulipo-dium is a thin cylindrical organelle approximately 150 μm long and 0.2 μm in diameter. The surface membrane surrounding the flagellum is continuous with the cell plasma membrane at the point where the flagellum is attached to the cell. The internal structure (see Figure 10.2) is complex consisting of a central pair of cytoplasmic tubules surrounded by nine doublets. Each doublet consists of a pair of tubules connected along their length. A basal body (kinetosome), usually of somewhat denser refractile properties and staining more intensely than the distal part of the flagellum, anchors it within the cytoplasm. In some species, the basal body also has one or more fine fibrils attached to it that are clearly visible only with the electron microscope.