The Arcellinida are the largest group of the testacea (shelled amoebae) containing about three quarters of the known species of these amoebae. All Arcellinida have a test or tectum. This key character that binds the Arcellinida together may not be homologous, but at moment the ultrastructural and molecular data do not clarify the relations within this group or with other sarcodina. They are distinguished by their tests, tectum, or other external membrane with single aperture and composed of either organic material or inorganic material in an organic cement.