Why is Volvox referred to as single-celled organisms?

Volvox colonial flagellates. They belong to green unicellular algae, which are interconnected with the help of the thinnest filaments of protoplasm and create colonies that look like a spherical structure filled with mucus. Each cell has two flagella and a light-sensitive ocellus. The cells in the colony are divided by function, which later led to the formation of multicellular organisms.