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Amoebas are single-celled protozoans of the order Amoebida. They consist of a mass of cellular fluid surrounded by a membrane, and containing one or more nuclei (depending upon the species), as well as other cell organelles, such as food vacuoles.

The word amoeba is derived from the Greek word ameibein (to change), which describes the amoeba's most easily distinguishable feature, the continuous changing of shape by repeated formation of pseudopods (Greek: false feet).

Pseudopodal movement is based on a continual change in the state of protoplasm flowing into the foot-like appendage. An interior fluid (endoplasm), under pressure from an exterior gel (ectoplasm), flows forward in the cell. When the endoplasm reaches the tip of a developing pseudopod, the fluid is forced backward against the ectoplasm, and is turned into a gel. After returning to the body of the cell, the newly formed ectoplasm gel is converted back to fluid endoplasm, and again flows forward under the pressure of the exterior gel.

Pseudopods serve two important functions—locomotion and food capture, activities that are often interrelated. Amoebas use their pseudopods to ingest food by a method called phagocytosis (Greek: phagein, to eat).

The streaming of protoplasm inside the pseudopods moves the amoeba forward. When the organism contacts a food particle, the pseudopods surround the particle. After the food is corralled by the amoeba, an opening in the membrane allows the food particle to pass into the cell. Inside the cell, the food is enclosed within food vacuoles, digested by enzymes, and assimilated by the amoeba. The amoeba expels particles that are not acceptable as food.

The organisms generally implied by the term "amoe ba" belong to the phylum Protozoa, class Mastigophora, which includes organisms with flagellae (whip-like organs of locomotion) such as Chlamydomonas angulosa, as well as those with pseudopods. The class Sarcodina, which has as its principle distinguishing feature the almost universal presence of pseudopods, includes Amoeba proteus, the best-known protozoan.

The Rhizopoda (in some classifications a subclass of Sarcodina) contains all common "naked amoebas," which are either tubular or somewhat flattened. They move by means of protoplasmic flow, by producing pseudopodia, or by advancing as a single mass. Rhizopoda also includes sarcodinids known as giant amoebas and testaceous forms (those with tests, or shells). Some apparently "naked" amoebas have coatings of various kinds, such as scales, mucoid layers called glycocalyces, or complex filaments much smaller than scales.

In addition to the naked forms, many species of amoeba have tests (hard coverings), and are referred to as shelled amoebas. Most of these shelled amoebas are classified in the order Arcellinida. They have a test with a single opening, and are predominantly freshwater organisms. Shelled amoebas feed on a variety of organisms, such as bacteria, algae, and other protozoans.

Most members of the order of Amoebida are free-living in fresh or salt water or soil, and ingest bacteria. Larger members also feed on algae and other protozoans. Several amoebas of this group are pathogenic to humans.

The family Amoebidae includes mostly freshwater species, whose pseudopodal movement is either monopodial (the entire protoplasmic mass moves forward) or polypodial (several pseudopods advance simultaneously). One member, Amoeba proteus is commonly used for teaching and cell biology research. Chaos carolinense, one of the larger species, has multiple nuclei and can reach a length of 0.12 in (3 mm).

The Hartmannellidae family includes small and medium-sized amoebas that move forward monopodially, advancing by means of a steady flow. They feed on bacteria, although some species of the genus Saccamoeba also feed on unicellular algae.

The family Entamoebidae includes most of the obligately endozoic (parasitic inside a host) Amoebida organisms, including Entamoeba histolytica. Amebiasis (infection with E. histolytica) is a serious intestinal disease also called amoebic dysentery. It is characterized by diarrhea, fever, and dehydration. Although amebiasis is usually limited to the intestine, it can spread to other areas of the body, especially the liver.

E. histolytica exists as either a trophozoite or cyst. The trophozoite is motile, possesses a single nucleus, and lives in the intestine. It is passed from the body in diarrhea, but cannot survive outside the host. The cyst form, consisting of condensed protoplasm surrounded by a protective wall, is produced in the intestine, can survive outside the host, and even withstands the acid of the stomach when it is ingested with food or contaminated water. Once inside the intestine, E. histolytica multiplies by means of binary fission.

Another family, Acanthamoebidae (in the Amoebida suborder Acanthopodina), includes the genus Acanthamoeba genera, which are often isolated from fresh water and soil. Acanthamoeba cause primary amebic meningoencephalitis (PAM, inflammation of the brain and its protective membranes), especially in individuals who are ill and whose immune systems are weakened. Acanthamoeba infections have been traced to fresh water, hot tubs, soil, and homemade contact lens solutions. In the latter case, contamination of contact lens solution with the organism has caused keratitis, an inflammation of the cornea accompanied by pain and blurred vision. Severe cases can require a corneal transplant or even removal of the eye.

A member of the order Schizopyrenida, Naegleria fowleri is an especially dangerous human parasite, causing rapidly fatal PAM in people swimming in heated water, or warm, freshwater ponds and lakes, mainly in the southern United States. Both Naegleria and Acanthamoeba enter through the nasal mucosa and spread to the brain along nerves.

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