Historically, protozoa were defined as single-celled organisms with animal-like behaviors, such as motility and predation. The group was regarded as the zoological counterpart to the  protophyta , which were considered to be plant-like, as they are capable of photosynthesis.he definition of Protozoa as a phylum or sub-kingdom made up of unicellular animals was adopted by the zoologist Otto Buetschli —bütschli celebrated at his centenary as the architect of —protozoology and the term came into wide. use