There are thousands of species of protistans, some possessing cilia or flagellum, whereas others use pseudopodium for locomotion. Some protistans secrete shells of silica or lime, whereas others have combined individual cells to produce a colony (eg Volvox). The constituent cells of Volvox, however, are co-ordinated, for all the flagellum around the sphere beat in an organized way and give direction to locomotion.
Volvox is one of the best known genera of green algae, and is the culmination of the evolution of spherical colonies. Each Volvox is composed of on the order of a thousand cells, each a bi-flagellate similar to Chlamydomonas, interconnected and arranged in a hollow sphere (a Coenobia), with a distinct anterior and posterior. Asexual colonies consist of somatic or vegetative cells, which do not reproduce, and gonidia, which reproduce, the reproduction being a process of longitudinal division. Sexual or oogamous colonies contain, as well as somatic cells, ova (non-motile female cells) or spermatozoa (small, motile male cells) or a mixture of the two. These cells, near the back of the colony, develop into new colonies, initially with the flagella directed inwards and held within the parent. Eventually the parent bursts and the daughter colonies evert.