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An approach towards a systematic revision of Marionina Michaelsen, 1890 is made through an historical overview of its nomenclature and definition, and a thorough characterization of its type species, the South Georgian marine littoral Pachydrilus georgianus Michaelsen, 1888. Relevant sections of early enchytraeid literature provide the background for appreciating the complex taxonomic history of the genus and giving a final word as to the controversial validity of its name, authority and date. Marionina in its current acceptation comprises about 100 nominal species, but the paper documents how, since its establishment, the genus has been an artificial assemblage of unrelated taxa, whose taxonomy cannot be sorted out (1) without finding new morphological characters and improving the standard of descriptions, and (2) without using a total evidence approach (morphology and molecules) within a phylogenetic framework. Confusion about the identity of the type species, originally briefly described and only partially figured, is unravelled upon examination of the syntypes available in Hamburg and Berlin. Since neither series proved to be monospecific, a lectotype is designated to assure correct and consistent application of the name in the future.

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Heterodrilus is a group of marine Naididae, common worldwide in subtropical and tropical areas, and unique among the oligochaetes by their tridentate chaetae. The phylogenetic relationships within the group are assessed from the nuclear 18S rDNA gene, and the mitochondrial cytochrome c oxidase subunit I (COI) and 16S rDNA genes. Sequence data were obtained from 16 Heterodrilus species and 13 out‐group taxa; 48 sequences are new for this study. The data were analysed by Bayesian inference. Monophyly of the genus is corroborated by the resulting tree, with Heterodrilus ersei (a taxon representing a small group of species with aberrant male genitalia) proposed to be outside all other sampled species. Although earlier regarded as a member of the subfamily Rhyacodrilinae, both molecular and morphological data seem to support that Heterodrilus is closely related to Phallodrilinae. However, the results are not conclusive as to whether the genus is the sister group of, or a group nested inside, or separate from this latter subfamily. The studied sample of species suggests at least two major clades in Heterodrilus with different geographical distributions, in one of the clades, most species are from the Indo‐West Pacific Ocean, while in the other, the majority are from the Western Atlantic Ocean. Morphological characters traditionally used in Heterodrilus taxonomy are optimized on the phylogenetic tree, revealing a high degree of homoplasy.