Protists were traditionally subdivided into several groups based on similarities to the higher kingdoms: the unicellular animal-like protozoa, the plant-like protophyta (mostly unicellular algae), and the fungus-like slime molds and water molds.mong the pioneers in the study of the protists, which were almost ignored by Linnaeus except for some algae, were Leeuwenhoek, O. F. Mueller, Müller. C. G ehrenberg And Felix. Félix dujardin the first groups used to classify microscopic organism Were the animalcules And. the infusoria