Image from www.micropia.nl/en/discover/microbiology/volvox/

Micropia, the museum of microbes in Amsterdam, has a page devoted to Volvox:

Ponds and ditches are not only home to unicellular green algae, but also to multicellular forms.

Some ‘colonies’ are nothing more than a mass of single cells all doing exactly the same thing, but with the spherical volvox it’s a slightly different story. Here different cells have specialised and work together. All the cells are located on the outside of the sphere. There are cells with flagella (whip-like hairs) to help the colony move around and cells which are responsible for reproduction.

Inside the mother sphere

During good times, reproductive cells grow directly into daughter colonies, which float inside the mother sphere. They continue on their own when the mother colony ruptures and then dies. As winter gets closer, sex comes into the picture. The reproductive cells either transform into a packet of sperm cells or enlarge to form an egg cell. The sperm cells are released and swim to the egg cells to fertilise them. The fertilised egg cell develops a tough, thick shell. Inside this capsule, the volvox is able to survive the winter in one piece.