The Arcellinida are distinguished by their tests, comprising a single aperture and composed of either secreted proteinaceous material or agglutinated. Proteinaceous tests can be flexible, with rigid sheets of fibrous material or with regularly arranged hollow building units that form an areolate surface. Agglutinated tests can be either calcareous or siliceous (Odgen 1990; Ogden and Ellison 1988). Dictyosomes are involved in the secretion of the organic building units and the cement. Mitochodria have banched tubular cristae. Contractile vacuoles are present. Arcellinida have either endolobopodia that are granular or completely hyaline, or ectolobopodia that are generally fingerlike and in some species can anastomose (reticulolobopodia) (Bonnet 1961, 1963). Under unfavorable environmental conditions, most testate lobose amoebae produce cysts.
Classification of the Arcellinida is based mainly on characters of the test. Kudo (1954) presents the arcellinids as two unrelated families, Arcellinidae (with a membranous shell) and Difflugiidae (shell with foregin bodies, platelets, or scales). A more recent classification (Meisterfeld 2002) considers Arcellinida as a monophyletic order divided into three suborders: Arcellinina (membranous test, digitate pseudopodia), Difflugiina (test rigid with mineral particles, digitate pseudopodia), and Phryganellina (test with siliceous material, pseudopodia conical). Testate lobose amoebae were for a long time considered as part of the Testacea (Rhizopoda), a taxon uniting all amoeboid protists that are enveloped by a single-chambered shell (Kudo 1954). In the first edition of the "Illustrated Guide of Protozoa" (Bovee 1985), the Testacea are not treated as a monophyletic taxon. The shape of pseudopodia (lobose or filose) is considered as an important taxonomic feature and testate lobose amoebae are placed within the class Lobosea (Carpenter, 1861; while the testate filose amoebae are placed within the class Filosea Leidy, 1879. In the second edition of the "Illustrated Guide to the Protozoa" (Meisterfeld 2002), the Arcellinida are treated as a group of amoebae of uncertain affinities, because their evolutionary origins are unclear and molecular data are awaited to solve the problem. Molecular data have already helped to resolve the phylogenetic status of testate filose amoebae which form at least three independent lineages within the recently defined super-group Rhizaria (Burki et al. 2002; Nikolaev et al. 2003, 2004; Wylezich et al. 2002). In contrast, the Arcellinida is one of the last widespread and well-known groups of eukaryotes that is not represented in molecular databases.