familiar of the microscopic green algae, including many model organisms. The unicellular flagellate Chlamydomonas has been used to study flagellar motion and swimming (Mitchell, 2000 ? ), photosynthesis mutations (Niyogi, 1999 ? ), and plastid genome modifications in secondarily nonphotosynthetic taxa (Vernon et al., 2001 ? ). Colonial green algae such as Volvox have been models for the evolution of multicellularity, cell differentiation, and colony motility (Hoops, 1997 ? ; Kirk, 2003 ? ). Chlorophycean algae Scenedesmus and Pediastrum are also important paleoecological or limnological indicators (Nielsen and Sorensen, 1992 ? ; Komarek and Jankovska, 2001 ? ). Green algae in this class have a great range of vegetative morphology, from coccoid to swimming unicells, colonies, and simple flattened thalli to unbranched and branched filaments. All have a haplobiontic life cycle (zygotic meiosis). Orders previously recognized were defined on their vegetative morphology, form of sexual reproduction (either isogamy, anisogamy, or oogamy), and mode of asexual reproduction (zoosporic or autosporic). During cell division, mitosis is closed and cytokinesis involves a phycoplast system of microtubules, sometimes combined with furrowing. Swimming cells are vegetative cells, zoospores (asexual), or gametes, with two, four, or hundreds of flagella. Cells with two or four flagella have cruciate (X-2-X-2) rootlets and flagella that are displaced in a "clockwise" (CW, 1 -- 7 o'clock) direction or are "directly opposed" (DO, 12 -- 6 o'clock). In some swimming colonial forms such as Volvox , the flagellar apparatus undergoes developmental modification and the flagella are reoriented for colony swimming (Hoops,