Stentor is a heterolrich ciliate which often forms lawn-like covers on the bottom and/or blooms in the pelagial of lakes worldwide. The species involved in these spectacular events were usually either not determined or misidentified because the keys are outdated and incomplete. Thus, we have revised the nominal species described since the first major revision by Ehrenberg (1838). Main species characteristics are the presence/absence of symbiotic algae, the shape of the macronucleus and the colour of the cortical pigment granules. The last character mentioned must be studied in live cells because the pigment bleaches in chemically fixed specimens. Nineteen valid species are recognized and dichotomously keyed according to these characteristics. Twenty-seven other species and varieties, described after Ehrenberg's revision, are synonyms or species indeterminata A new species. S.araucanus , is described from South American lakes. It is a small, broadly trumpet-shaped Stentor with symbiotic algae, vermiform macronucleus and blue-green cortical granules. Stentor araucanus is probably euplanktic and restricted to the southern hemisphere. S tentor auriculalus Kahl. 1932 sensu Wang (1934) is recognized as a new species, Condylostoma wangi , and transferred to the genus Condylostoma . New nomenclatural corrections: Stentor baicalius nom. nov. (pro S.pygmaeus , preoccupied). S loricatiis nom. corr. (for S.loricata ), S.ruber nom. corr. (for S.ruhra ).

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