Protozoans are heterotrophs that live as predators or parasites. They are considered primitive relatives of animals. There are four major groups of protozoans.

Amoeboid protozoans live in fresh water, sea water or moist soil. They move and capture their prey by putting out pseudopodia (false feet) as in Amoeba. Marine forms have silica shells on their surface. Some of them such as Entamoeba are parasites.

Flagellated protozoans include members that are either free-living or parasitic. They possess flagella for locomotion. The parasitic forms cause diseases such as sleeping sickness. Example: Trypanosoma.

Ciliated protozoans are aquatic, actively moving organisms due to the presence of thousands of cilia. They have a cavity called gullet that opens to the outside of the cell surface. The coordinated movement of rows of cilia causes water laden with food to be steered into the gullet. Example: Paramoecium.

Sporozoans include diverse organisms that have an infectious spore-like stage in their life cycle. The most notorious is Plasmodium (malarial parasite) which causes malaria, a disease with a staggering effect on human population.