Protozoa are heterotrophic and live as parasites and predators. They are divided into following four groups:
(1) Amoeboid protozoans:These live in fresh water, sea water or moist soil. They use pseudopodia (false feet) to  move and capture prey. The marine forms have silica in their shells on their surface. For example - amoeba.

(2) Flagellated protozoans:
They are free-living or parasitic. They have flagella and are non-photosynthetic flagellates without a cell wall. For example - parasitic forms such as Trypanosoma that cause sleeping sickness in human beings.

(3) Ciliated protozoans:
They are aquatic individuals that form a large group of protozoa. Their characteristic features are the presence of numerous cilia on the entire body surface and the presence of two types of nuclei. They are actively moving. They posses a cavity called gullet that opens to the outside of the cell surface.  All the cilia beat in the same direction to move the water laden food inside a cavity called gullet. They include organisms such as Paramoecium, Vorticella,etc.

(4) Sporozoans: They include disease causing endoparasites and other pathogens. They have infectious spore like stage in their life cycle. They are uni-nucleate and their body is covered by a pellicle. They do not possess cilia or flagella. For example-malaria causing parasite Plasmodium.