A loose association of unicellular organisms that live in a colony and may be surrounded by a common membrane. Often the cells are held together in a jelly excreted by all individuals in the colony. Both algae and bacteria form coenobia. For example, Volvox (a green alga) forms a hollow sphere in which 20 000 cells may live; some of these are reproductive and others are concerned with photosynthesis. The cells in this coenobium are interconnected by protoplasmic strands.