Protozoa means "first animals," a misnomer in the context of the five-kingdom system or any other current classification scheme. The term persists to refer informally to protists that live primarily by ingesting food, an animal-like mode of nutrition. These heterotrophs actively seek and consume bacteria, other protists, and detritus (dead organic matter). There are also symbiotic protozoa, including some parasites that cause human diseases. The protozoa are subdivided into phyla partly on the basis of how they feed and move. In this section, we examine six of the protozoan phyla. As systematists continue to reevaluate protistan taxonomy, they are splitting some of these phyla, combining others, and proposing different names to reflect these changes. What is important to take from this survey is not so much the formal taxonomy of protozoa as the diversity of the locomotory and feeding mechanisms.
 
 

Rhizopoda (Amoebas) Members of the phylum Rhizopoda, the amoebas and their relatives, are all unicellular. Some of them have shells. No stages in their life histories are flagellated. Instead, amoebas use cellular extension called pseudopodia to move and to feed. You have probably observed this mode of motility in Amoeba proteus in the laboratory. Pseudopodia may bulge from virtually anywhere on the cell surface. When an amoeba moves, it extends a pseudopodium and anchors its tip, and then more cytoplasm streams into the pseudopodium (rhizopoda means "rootlike feet"). The cytoskeleton, consisting of microtubules and microfilaments, functions in amoeboid movement. Pseudopodial activity may appear chaotic but in fact amoebas show taxis as they creep slowly toward a food surface.

 

Actinopoda (Heliozoans and Radiozoans) Actinopoda means "ray feet," a reference to the slender pseudopodia called axopodia that radiate from the beautiful protists that compose the phylum. Each axopodium is reinforced by a bundle of microtubules, covered by a thin layer of cytoplasm. The projections place an extensive area of cellular surface in contact with the surrounding water, help the organisms float, and function in feeding. Smaller protists and other microorganisms stick to the axopodia and are phagocytized by the thin layer of cytoplasm. Cytoplasmic streaming then carries the engulfed prey down to the main part of the cell. Actinopoda are components of plankton. Most heliozoans ("sun animals") live in fresh water, whereas radiozoans are primarily marine. Radiozoa have delicate shells, most commonly made of silica, the material of glass. After these organisms die, their shells settle to the seafloor, where they have accumulated as an ooze that is hundreds of meters thick in some locations.

 

Foraminifera (Forams) Foraminifera, or forams, are exclusively marine. The majority of members of this phylum live in the sand or attach themselves to rocks and algae, but some families are also abundant in plankton. They phylum is named for the porous shells of its members. The shells are generally multichambered and consist of organic material hardened with calcium carbonate. Strands of cytoplasm extend through the pores, functioning in swimming, shell formation, and feeding. Many forams also derive nourishment from the photosynthesis of symbiotic algae that live beneath the shells.

 

Apicomplexa (Sporozoans) All members of the phylum Apicomplexa, which were formerly called sporozoans, are parasites of animals. Some cause serious human diseases. The parasites disseminate as tiny infectious cells called sporozoites. As seen with the electron microscope, one end (the apex) of the sporozoite cell contains a complex of organelles specialized for penetrating host cells and tissues, thus the phylum name Apicomplexa. Most apicomplexans have intricate life cycles with both sexual and asexual stages, and these cycles often require two or more different host species for completion. An example is Plasmodium, the parasite that causes malaria. The incidence of malaria was greatly diminished in the 1960s by the use of insecticides that reduced the population of Anophleles mosquitoes, which spread the disease, and by drugs that killed the parasites in humans. However, the multiplication of resistant varieties of both the mosquitoes and Plasmodium species have caused a resurgence of the disease. Each year, about 300 million people are infected in the tropics, and up to 2 million die from the disease.

 

Zoomastigophora (Zooflagellates) Phylum Zoomastigophora is named for the whiplike flagella these protozoa use to propel themselves. Also called zooflagellates, these heterotrophs absorb organic molecules from the surrounding medium or engulf prey by phagocytosis. Most live as solitary cells, but some form colonies of cells. There are both free-living and symbiotic zooflagellates. Living within the gut of a termite, for instance, are symbiotic flagellates that digest cellulose in the wood eaten by the host. At the opposite end of the spectrum of symbiotic relationships are parasitic flagellates, some of which are pathogenic to humans. For example, species of Trypanosoma cause African sleeping sickness, which is spread by the bite of the tsetse fly. Molecular systematists have recently confirmed that zooflagellates are closely related to the group of flagellated protists that includes the photosynthetic Euglena.

 

Ciliophora (Ciliates) The diverse protists of the phylum Ciliophora are characterized by their use of cilia to move and feed. Most members of Ciliophora, or ciliates, live as solitary cells in fresh water. In contrast to most flagella, cilia are relatively short. They are associated with a submembrane system of microtubules that may coordinate the movement of thousands of cilia.