Dark field light micrograph of the surface of a colony of the freshwater green alga Volvox sp. The colony is a hollow sphere made up from thousands of cells connected by cytoplasmic threads and arranged in a single layer. Each cell has two flagella, which point outwards, and are used for locomotion. Each cell also contains a green chloroplast and a red eyespot. Magnification: x220 when printed 10 centimetres wide.