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Order Arcellinida Kent 1880 Diagnosis: Cell covered with hard or highly rigid organic or mineral extracellular test consisting of either self-secreted elements (calcareous, siliceous or chitinoid), a sheet-like chitinoid structure, or recycled organic or mineral particles bound together, with a single main opening. Remarks: Arcellinida comprises all testate amoebae with lobose pseudopodia. It is the largest group of testate amoebae with 687 nominal species (Kosakyan et al, 2016). However, the major part of these species needs comprehensive nomenclature and taxonomic revision. Arcellinida exhibit a wide array of the test shapes composed of different material, which was the key characteristic for their classification. Meisterfeld (2000) organized them into three suborders: Arcellina, Difflugina and Phryganellina. Recent molecular investigations has challenged the system, and the positions of many species and genera were switched between families. These molecular reconstructions have demonstrated the limitations of the use of test composition as a classification character. A number of modifications were made on high as well as low-taxonomic levels. Higher-level taxonomy of Arcellinida is uncertain, while lower-level taxonomy is constantly changing (Kosakyan et al, 2016; Gomaa et al., 2017). Further reading: Kosakyan et al. (2016): Current and future perspectives on the systematics, taxonomy and nomenclature of testate amoebae – European Journal of Protistology Gomaa et al. (2017): A contribution to the phylogeny of agglutinating Arcellinida (Amoebozoa) based on SSU rRNA gene sequences – European Journal of Protistology 59: 99-107
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