Light microscopy of colonies of Volvox, a freshwater green alga. Each colony is a hollow sphere comprising a single layer of hundreds or thousands of cells, joined by a glycoprotein matrix. Each cell has two flagella, which are beaten in a co-ordinated fashion to propel the colony towards light, which is detected using eyespots. The colonies contain several daughter colonies, smaller spheres within the parent, which are produced asexually and released when large enough. Volvox colonies may reach around a millimetre in diameter. Filmed with differential interference contrast illumination.