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These beautiful colonies of VOLVOX are hollow spheres of tiny flagellated cells that can collectively swim. How they got this way is fascinating: They evolved from a common green alga, CHLAMYDOMONAS, which also forms a hollow ball of cells during cell division. In the ancestral colonies, all had their flagella on the inside where they would be no use for swimming; this didn't matter as the balls of cells broke up, enabling the cells to swim off. VOLVOX inherited this mode of
