Presented below is a photomicrograph of the amoeba Chaos carolinensis, pseudopodia extended in search of food. This digital image was captured with the DXM 1200 ACT-1 control software in single-image acquisition mode utilizing DIC illumination.

Amoebas are primitive organisms characterized by their flowing movements, extending cytoplasm outward to form pseudopodia (false feet); this type of movement is considered to be the most primitive form of animal locomotion. Pseudopodia are also used for feeding. In response to chemical stimuli from smaller organisms, pseudopodia are induced to envelop the organism, at the same time forming a vacuole (cavity). Digestive enzymes are secreted into the cavity and the nutrients diffuse from the vacuole into the cytoplasm. Undigested food and wastes are eliminated through the ectoplasm. The ectoplasm also performs respiration, absorbing oxygen from the water and eliminating carbon dioxide. Amoebas reproduce asexually by binary fission.

Many species that belong to the order Amoebida are free living, but some are well-known parasites of plants and animals. At least six forms of amoeba are parasitic in humans The most well known of these is Entamoeba histolytica, which causes amebic dysentery. This often occurs in epidemics when raw sewage contaminates water supplies or when soil for crops is fertilized with untreated human wastes.