Volovox is an Algae, but it moves with the help of Flagella. Protozoa has members that move by Flagella. Why is Volovox taxonomically in the Algae instead of Protozoa?
Protozoa is an archaic term that is no longer taxonomically valid because it is paraphyletic. A "protozoan" may be used as a generalized descriptive term of a motile single celled eukaryote, but Protozoa is not a taxon.