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In many algal divisions, the simplest representatives are motile, unicellular organisms, known collectively as ''flagellates''. The Cell of a typical flagellate, illustrated by ''Euglena'' (Figure 26.1 - Euglena gracilis), has a very marked polarity: it is elongated and leaf-shaped, the flagella usually being inserted at the anterior end. In the Euglenophyta, to which ''Euglena'' belongs, there are two flagella of unequal length, which originate from a small cavity at the anterior end of the Cell. Many chloroplasts and mitochondria are dispersed throughout the cytoplasm. Near the base of the flagellar apparatus is a specialized organelle, the ''eyespot'', which is red, owing to its content of special carotenoid pigments; the eyespot serves as a photoreceptor to govern the direction and intensity of illumination. The Cell of ''Euglena'', unlike that of many other flagellates, is not enclosed within a rigid wall; its outer layer is an elastic ''pellicle'', which permits considerable changes in shape. Cell division occurs by ''longitudinal fission'' [Figure 26.2(a)]. About the time of the onset of mitosis, there is a duplication of the organelles of the Cell, including the flagella and their basal apparatus; cleavage subsequently occurs through the long axis, so that the duplicated organelles are equally partitioned between the two daughter Cells. This mode of Cell division is characteristic of all flagellates except those belonging to the Chlorophyta, such as ''Chlamydomonas'', where each Cell undergoes ''two or more multiple fissions'' to produce four smaller daughter Cells, liberated by rupture of the parental Cell wall [Figure 26.2(b)]. Even in such cases, however, the internal divisions take place in the longitudinal plane. As we shall see in a subsequent section, longitudinal division also occurs in the nonphotosynthetic flagellate protozoa and is one of the primary characters that distinguish these organisms from the other major group of protozoa that possess flagellalike locomotor organisms, the ciliates.
 
In many algal divisions, the simplest representatives are motile, unicellular organisms, known collectively as ''flagellates''. The Cell of a typical flagellate, illustrated by ''Euglena'' (Figure 26.1 - Euglena gracilis), has a very marked polarity: it is elongated and leaf-shaped, the flagella usually being inserted at the anterior end. In the Euglenophyta, to which ''Euglena'' belongs, there are two flagella of unequal length, which originate from a small cavity at the anterior end of the Cell. Many chloroplasts and mitochondria are dispersed throughout the cytoplasm. Near the base of the flagellar apparatus is a specialized organelle, the ''eyespot'', which is red, owing to its content of special carotenoid pigments; the eyespot serves as a photoreceptor to govern the direction and intensity of illumination. The Cell of ''Euglena'', unlike that of many other flagellates, is not enclosed within a rigid wall; its outer layer is an elastic ''pellicle'', which permits considerable changes in shape. Cell division occurs by ''longitudinal fission'' [Figure 26.2(a)]. About the time of the onset of mitosis, there is a duplication of the organelles of the Cell, including the flagella and their basal apparatus; cleavage subsequently occurs through the long axis, so that the duplicated organelles are equally partitioned between the two daughter Cells. This mode of Cell division is characteristic of all flagellates except those belonging to the Chlorophyta, such as ''Chlamydomonas'', where each Cell undergoes ''two or more multiple fissions'' to produce four smaller daughter Cells, liberated by rupture of the parental Cell wall [Figure 26.2(b)]. Even in such cases, however, the internal divisions take place in the longitudinal plane. As we shall see in a subsequent section, longitudinal division also occurs in the nonphotosynthetic flagellate protozoa and is one of the primary characters that distinguish these organisms from the other major group of protozoa that possess flagellalike locomotor organisms, the ciliates.
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Most multicellular algae are immotile in the mature state. However, their reproduction frequently involves the formation and liberation of motile Cells, either asexual reproductive Cells (''zoo-spores'') or gametes. Figure 26.3 shows the liberation of zoospores from a Cell of a filamentous member of the Chlorophyta, ''Ulothrix''; it can be seen that these zoospores have a structure very similar to that of the ''Chlorogonium'' Cell, illustrated in Figure 26.2(c). The structure of the motile reproductive Cells of multicellular algae thus often reveals their relationship to a particular group of unicellular flagellates.
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Most multicellular algae are immotile in the mature state. However, their reproduction frequently involves the formation and liberation of motile Cells, either asexual reproductive Cells (''zoospores'') or gametes. Figure 26.3 shows the liberation of zoospores from a Cell of a filamentous member of the Chlorophyta, ''Ulothrix''; it can be seen that these zoospores have a structure very similar to that of the ''Chlorogonium'' Cell, illustrated in Figure 26.2(c). The structure of the motile reproductive Cells of multicellular algae thus often reveals their relationship to a particular group of unicellular flagellates.
    
== The Nonflagellate Unicellular Algae==
 
== The Nonflagellate Unicellular Algae==
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