Opisphokonts are a large group of eukaryotic organisms. Their common link is that they all had a single unicellular, flagellated ancestor that existed roughly 1-1.5 billion years ago. As time passed, divergent lineages arose that led to the ancestors of animals and fungi. It's likely that as these unicellular organisms gradually evolved into multicellular animals and fungi, their flagella disappeared. The classification of opisphokont stays, despite their lack of flagella, because the term opisphokont only signifies that all members of the clade claim a common flagellated ancestor.
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This information comes from the "The Origin of Fungi" section on page 653. The clade in Figure 31.8 is evidence of the divergent evolution that eventually would do away with the flagellum.