A large order of green algae (Chlorophyceae) comprising all forms that normally are flagellate and motile. In zoological classification, it is called Volvocida and placed in the class Phytomastigophora. The cells are solitary or united into colonies of definite structure (coenobia), often with morphological and functional differentiation among the component cells. Some taxonomists place the unicellular forms in a separate order, Chlamydomonadales. Unicells that have volvocalean cytological features but are nonflagellate and sedentary in their vegetative phase are considered here to constitute the order Tetrasporales. Alternatively, these sedentary forms may be retained in the Chlamydomonadales or Volvocales.