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Type Sarcomastigophora. Movement. Reproduction. Phylum Sarcomastigophora
These protozoa possess flagella or pseudopodia as locomotor or feeding organelles and a single type of nucleus. The 18,000 described species constitute the largest phylum of protozoa, which is composed of two principal groups subphyla), the flagellates and the sarcodines.
The flagellates, or mastigophorans, include those protozoa that possess flagella as adult organelles. They can be conveniently divided into phytoflagellates and zooflagellates. The phytoflagellates usually bear one or two flagella and typically possess chloroplasts. These organism s are thus plantlike, and phycologists treat most species in this division as algae. The phytoflagellate division contains most of the free-living members of the class and includes such common forms as Euglena, Chlamydomonas, Volvox, and Peranema. The zooflagellates possess one to many flagella, lack chloroplasts, and are either holozoic or saprozoic. Some are free living, but the majority are commensal, symbiotic, or parasitic in other animals, particularly arthropods and vertebrates. Many groups have become highly specialized. It is generally agreed that this division does not represent a closely related phylogenetic unit.
The presence of flagella is the distinguishing feature of flagellates. The phytoflagellates usually have one or two flagella, the zooflagellates one to many. When two or many flagella occur, they may be of equal or unequal length, and one may be leading and one trailing, as in Peranem and the dinoflagellates.
The ultrastructures of flagella and cilia are fundamentally similar in all eukaryote organisms. A single flagellum (or cilium) is constructed very much like a cable. Two central microtubules form a core that is in turn encircled by nine double outer microtubules. One microtubule of each doublet bears two rows of projections (arms) directed toward the adjacent doublet. The entire bundle is enclosed within a sheath that is continuous with the cell membrane. The flagellum always arises from a basal body, sometimes called a blepharoplast in flagellates, that lies just beneath the surface. Like a centriole, the basal body has an ultrastructure somew hat like a flagellum, except that the central fibrils are absent and the nine fibrils in the outer circle are in triplets, two of the three being continuous with the doublets of the flagellum. Arms are absent and the inner microtubule of each triplet is connected by a radial strand to a central ring for part of its length.
A fibrillar root system connecting the basal body with various organelles, especially the nucleus, characterizes m any flagellates. In some species the basal body functions as a centriole in mitosis.
Flagellar propulsion in most astigophorans essentially follow s the same principle as that of a propeller, the flagellum undergoing undulations in one or two planes that either push or pull. The undulatory waves pass from base to tip and drive the organism in the opposite direction, or the undulations pass from tip to base and pull the organism. In many species with two flagella, the actual path of movement is determined by the flagellar orientation. Other types of beat have been described for flagella besides undulatory.
The relationship of flagellar (or ciliary) ultrastructure to movement has received much attention, and the sliding tubule model is now widely accepted. According to this theory, the microtubules do not change length but adjacent doublets slide past each other, causing the entire organelle to bend. Sliding involves the establishment of cross bridges and utilization of adenosine triphosphate (ATP), as in muscle contraction.
Mastigophorans that have thin, flexible pellicles are often capable of am eboid m ovem ent; some forms, such as chrysomonads, may cast off their flagella and assume an ameboid type of locomotion entirely.
Phytoflagellates are primarily autotrophic and contain chlorophyll. When the chlorophyll is not masked by other pigments, a flagellate appears green in color, like the phytom onads and euglenids. If the xanthophylls dominate, the color is red, orange, yellow, or brown.
Strict heterotrophic nutrition occurs in zooflagellates as well as some other groups, and there are many parasitic species. The mechanism s of food capture and ingestion vary greatly, and the methods employed by some of the better known groups will be described in the following sections Phytoflagellates store reserve foods, such as oils or fats, or they may store carbohydrates as typical plant starch or in other forms. In zooflagellates, glycogen is the usual reserve food product.