Volvox Linnaeus
	From Latin volvo, "to roll"
The hexagonal cells fit closely together, and each bear two flagella.   The cells are connected by cytoplasmic extensions.	Volvox is a motile green algal colony composed of 500-50,000 cells arranged along the periphery of a hollow mucilaginous sphere. These large colonies are often visible with the naked eye. Each cell has its own individual mucilage that may be distinct or inconspicuous. Strands of protoplasm hold the cells together and are more readily seen in young colonies of certain species. The colonial mucilage is surrounded by a boundary layer that has a similar chemical composition to the cell wall of Chlamydomonas.   Like Chlamydomonas, each spherical or ovoid cell has two equal flagella with a pair of contractile vacuoles at their base and a large cup-shaped chloroplast with a single pyrenoid. The colonies coordinate the flagellar movements of cells on different sides of the sphere in order to swim with a rolling motion and change direction as necessary. Volvox is phototactic and able to swim towards moderate light.   Volvox colonies demonstrate polar organization. The anterior cells have larger eyespots and greater phototactic abilities, while the posterior cells are involved with reproduction. Volvox is widely used by researchers as a genetic and developmental model to further understand cellular differentiation.   The 20 Volvox species are distinguished mainly by the characteristics of the mucilage and the cytoplasmic strands connecting adjacent cells. DNA sequencing has shown that the genus is polyphyletic and that the various species have evolved from four ancestral lineages. Researchers may eventually reclassify species into different genera to reflect this relationship.