The kingdom Protista comprises organisms which are unicellular or unicellular-colonial and which form no tissues. ~ Robert Whittaker in 1969
The mishmash multiformity of life is well exemplified by protists, which represent a failure of classification by dint of diversity. Protists are a catchall category of eukaryotic microbes. Most are unicellular, typically defined by what they are not: fungus, plant, or animal; though their similarities to these other life forms are striking. Among the protists are algae, plankton, and protozoa.
Protists are a disparate grouping of 30–40 phyla, with little in common except relative simplicity, notably their lacking tissue differentiation. Owing to their oddities and variety, protist classification has been an ongoing controversy.
Trophically, various protists may be photosynthetic autotrophs or heterotrophic. Protists are either plant-like, as with algae, or animal-like, such as protozoa.
Euglena go both ways. They have chloroplasts for photosynthesis, but will hunt prey, typically smaller protists, when they can’t get enough light energy.
Individually, protists are microbial; such as diatoms, which are an algal form with 110,000 species. Numerous protists, including some diatoms, form visible colonies.