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Nicidion Kinberg, 1865, and Paramarphysa Ehlers, 1887, were previously recognized as abranchiate groups of Eunice Cuvier, 1817, and Marphysa de Quatrefages, 1865, respectively. However, recent studies have demonstratedNicidion is monophyletic, and the genus was redefined. Likewise, the character presence/absence of peristomial cirri, which traditionally was used to differentiate between Nicidion and Paramarphysa, is now considered non-diagnostic. Subsequently, the type species of Paramarphysa was recently transferred to Nicidion, implying that they are synonyms. Despite the above, some abranchiate species are still classified in Marphysa, raising the question of their positions within the genus. In the present study, we re-examined seven abranchiate species, studying their type material, and reviewing the literature, intending to disclose their taxonomic status. We concluded that only four of them belong to Nicidion: N. parvipes (Benham, 1927) n. comb., N. posteriobranchia (Day, 1962) n. comb., N. proppi (Averincev, 1972) n. comb., and N. saxicolas (Langerhans, 1881) n. comb. Two species, Marphysa simplex Langerhans, 1884, and Marphysa orientalis(Willey, 1905), were considered indeterminable. And one, ?Palola teres(Treadwell, 1922), is now considered incertae sedis because of insufficient information available. Additionally, we provided redescriptions of N. cincta Kinberg, 1865, type species of Nicidion, and N. hentscheli (Augener, 1931). Further character analysis on all Nicidion species suggested the genus consists of two groups (G1, G2) based on the distribution and presence and form of branchiae, the architecture of maxilla II, and the shape of the body’s posterior region. Also, the study of these and other characters such as the distribution of the swollen base of the ventral cirri and the coloration of the aciculae throughout the body, allow us to redefine the diagnosis of Nicidion. Inglés