| Summary: | The main descriptive characters of sabellid polychaetes are presented. Easily misunderstood morphological features are clarified by a series of schematic drawings. The importance of scale is emphasised and some aspects of population ecology and reproduction are addressed. An introduction to Euchone Malmgren (Polychaeta: Sabellidae: Sabellinae) is given and the diagnostic character is discussed. A new taxon, Euchone x, is described from the North Sea and Northern Norwegian Sea. Jasmineira candela Grube from the Adriatic Sea is redescribed and Jasmineira oculata Langerhans from Atlantic waters is reinstated. Some little-known records of Chone in Northern Atlantic waters are presented and one anomalous taxon is described. A historical overview of sabellid systematics is given. Because the binomial system of taxonomic nomenclature still used today was devised more than a century before evolutionary thinking, finds of organisms that do not 'fit' into the traditional systematic hierarchy are not unexpected. An introduction to phylogenetic systematics is given and some current issues are discussed. The concept of phylogenetic taxonomy is introduced. Phylogenetic analyses were carried out of taxa mainly within Euchone with some members of Chone, Jasmineira and Amphicorina. Euchone sensu Malmgren is revealed as being an artificial taxon. Five taxa previously assigned to Euchone are recognised as a discrete sister group to Chone, Euchone and Jasmineira. The data set is further used to demonstrate how phylogenetic taxonomy might be applied to the study group of organisms. Some behavioural observations were made on captive live specimens of Euchone x and the ecological implications are discussed. Some observations are provided on its reproduction and an enigmatic parasitic organism found within the radiolar crown is investigated. Although several important aspects of the taxonomy of the group were resolved, there remain many future challenges in the taxonomy and systematics of soft-bottom sabellin fan worms.
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