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The protozoa are more diverse than the protophyta, including ameboid, flagellated, ciliated, and symbiotic forms. The latter include some of the most devastating parasites of humans, such as the causative agents of malaria and sleeping sickness. A more critical review of the diversity of the protozoa starts with the realization that they, in all probability, arose from colorless protophyta. This also means that there has been a multiple or polyphyletic origin of the protozoa; they are derived, at least, from the Chrysophycophyta, the Euglenophycophyta, and the Pyrrophycophyta. This polyphyletic origin is the main reason for no longer using the term protozoa as a taxonomic designation. Within the protozoa, the symbiotic forms are polyphyletic, since there are ameboid, flagellated, and ciliated cells that live in or on host organisms. The association may (1) mutually benefit host and symbiont (mutualism), (2) damage the host but benefit the symbiont (parasitism), and (3) neither benefit nor damage either host or symbiont (commensalism). Regardless of whether the protozoa are free-living or symbiotic, they occur in three broad categories or types. They are flagellated, ameboid, or ciliated. However, the evolutionary trends are twofold, i.e., kinetidal and pseudopodial.

Pseudopodial evolution. The pseudopod, or false foot, a cytoplasmic extension of the protozoan cell, is used for locomotion or food capture or both. All ameboid protozoa have pseudopodia, but these can be quite different in different species. One species can have one blunt pseudopodium and another many. The pseudopodium can be thin, and hundreds can extend in all directions from a spherically symmetrical cell. The pseudopodia in some forms can interconnect to form net-like structures. The pseudopodial or ameboid cells are now viewed as all evolving from originally flagellated cells. In some cases, the flagellated form was a pigmented protophyte that showed ameboid tendencies. With loss of plastids it became an ameboid protozoan. Others lost their pigment, but remained flagellated, and then became ameboid.