The protist kingdom is very diverse. All protists are eukaryotes that cannot be classified as animals, plants, or fungi. All live in moist surroundings. Most
are unicellular, but some are multicellular. Some are heterotrophs, some are autotrophs, and some are both. Protists can be divided into three categories:
animal-like, funguslike, and plantlike protists.
Like animals, animal-like protists are heterotrophs, and most are able to move from place to place to obtain food. Animal-like protists are also
called protozoans. Protozoans can be divided into four types: sarcodines, ciliates, flagellates, and those that are parasites. Sarcodines move and feed
by using pseudopods. Pseudopods are temporary bulges of the cell.
Pseudopods form when cytoplasm flows toward one location and the rest of the organism follows. Protozoans that live in fresh water, such as amoebas,
have a contractile vacuole, which collects the extra water and expels it from the cell. Ciliates have structures called cilia, which are hairlike projections
that move with a wavelike motion. Flagellates move using whiplike flagella.
Some flagellates live inside the bodies of other organisms in a state of symbiosis. Symbiosis is a close relationship between two species where at
least one of the species benefits. Sometimes, flagellates harm their hosts. In other cases, their relationship is one of mutualism, in which both partners benefit. Protozoans that are parasites feed on their hosts’ cells and body
fluids.
Plantlike protists are called algae. Like plants, algae are autotrophs.
Algae can exist in a variety of colors because they contain many types of pigments—chemicals that produce color. Plantlike protists include diatoms,
dinoflagellates, euglenoids, red algae, green algae, and brown algae.
Diatoms have beautiful, glasslike cell walls. Dinoflagellates are covered by stiff plates and move using two flagella. Euglenoids can be heterotrophs
when sunlight is not available. Red algae and brown algae live in the oceans.
Green algae live in fresh water, salt water, and moist places on land.
Like fungi, funguslike protists are heterotrophs, have cell walls, and use spores to reproduce. Spores are tiny cells that are able to grow into new
organisms. All funguslike protists are able to move at some point in their lives. The three types of funguslike protists are slime molds, water molds,
and downy mildews. Slime molds live in moist soil and on decaying plants.
Water molds and downy mildews grow as tiny threads in water or moist places.
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