Single-celled critters are microscopic organisms that consist of only one cell. Two examples of single-celled critters are amoebas and amebae. Amoebas are a type of protozoan that moves by extending its cytoplasmic extensions, while amebae are a type of protozoan that moves by changing its shape. Both amoebas and amebae are found in aquatic environments and can be observed under a microscope.