Protozoa/Definition

A subkingdom consisting of unicellular organisms that are the simplest in the animal kingdom. Most are free living. They range in size from submicroscopic to macroscopic. Protozoa are divided into seven phyla: Sarcomastigophora, Labyrinthomorpha, Apicomplexia, Microspora, Ascetospora, Myxozoa, and Ciliophora.(National Library of Medicine)