Protozoa: The Animal Like Protists

Protozoa literally means "first animals." Existing as microscopic, one-celled heterotrophs, these tiny life forms exist just as much (if not more) diversity than Kingdom Animalia itself; species live in fresh water, salt water, dry sand, and moist soil... as parasites, as commensalists, or as mutualists. While scientists originally believed that protozoans only propagated their species by asexual means, new research shows that some species may augment a sexual cycle as well as normal mitotic divsion. Even though they are diverse in many ways, protozoa are classified into four phyla based upon their means of locomotion:

Mastigophora - Protists with flagella. Some species are free-swimming; others sessile. Mastigophora are unique for the symbiotic relationships they develop with organisms of other species. A representative species of the Mastigophorans is Trypanosoma gambiense , the zooflagellate that causes African sleeping sickness. It is carried by the tsetse fly.

Sarcodina- The most common stereotype of the protozoans (as immortalized by Gary Larson) is the Amoeba , this species is a Sarcodine. Sarcodines are known for moving by means of pseudopods , which are basically cytoplasmic false "feet" that extend and take the sarcodine where it needs to go. Two neat species, the Foraminifera and the Radiolaria , have hard shells of limestone and chalk; when they die, the shells build up among the ocean floor.

Sporozoa- These are among the bad guys that give both bacteria and protists their centuries-long ill reputation. True to their name, Sporozoans are parasitic spore formers. The lazy critters don't even move as adults....they are merely carried in the bodies of their hosts! Plasmodium vivax - that is, malaria, is a type of nasty sporozoan that is transmitted by the Anopheles mosquito.

Ciliata- has the greatest number of species out of ALL of the protozoans. Most species move by means of little hair like extensions called cilia. Their representative species is the Paramecium . They've got some rather unique organelles- a pellicle exists as a rigid ectoplasm, and they have two nuclei- a larger macronucleus to control metabolism, and a smaller micronucleus that controls mitosis and genetic information.

Pro`to*zo"a (?), n. pl. [NL., fr. Gr. first + an animal.] Zool.

The lowest of the grand divisions of the animal kingdom.

⇒ The entire animal consists of a single cell which is variously modified; but in many species a number of these simple zooids are united together so as to form a compound body or organism, as in the Foraminifera and Vorticellae. The reproduction takes place by fission, or by the breaking up of the contents of the body after encystment, each portion becoming a distinct animal, or in other ways, but never by true eggs. The principal divisions are Rhizopoda, Gregarinae, and Infusoria. See also Foraminifera, Heliozoa, Protoplasta, Radiolaria, Flagellata, Ciliata.


© Webster 1913.

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