In some systems of biological classification, the Protozoa are a diverse group of unicellular eukaryotic organisms. 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. The terms protozoa and protozoans are also used informally to designate single-celled, non-photosynthetic protists, such as ciliates, amoebae and flagellates.