The morphology and some morphogenetic details of the marine hypotrich Amphisiella annulata (Kahl, 1928) were investigated using live observation and protargol impregnation. The population from the Adriatic Sea matches almost perfectly the authoritative redescription by Kahl (1932). Characteristic features of A. annulata are (i) several ring-shaped structures (lithosomes?) 4-8 µm across scattered throughout the cytoplasm; (ii) the very narrowly spaced and rather wide cirri of the amphisiellid median cirral row; and (iii) the formation of an additional cirral anlage between the ordinary anlagen IV and V. This additional anlage produces only a transverse cirrus so that A. annulata has six transverse cirri. In addition, the oral primordium is formed from several roundish anlagen pits which originate left of the middle and rear portion of the amphisiellid median cirral row, resembling the situation in A. marioni, type of the genus. The literature on A. annulata is reviewed, showing that this conspicuous and thus easy to identify species has been recorded only about 11 times since its discovery before 75 years. The population from the Italian coast of the northern Adriatic Sea is designated as neotype because (i) no preparations are available of the original type population from saltwater in North Germany; (ii) synonymy with an older species was proposed in the revision by Hemberger (1982); and (iii) the descriptions available so far do not agree very well.
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