Broad-scale classifications have previously grouped Sabellidae and Serpulidae under names such a Serpulacea, Sabelliformia and Serpulimorpha. Dales (1962) proposed that Serpulidae and Sabellidae be grouped as the order Sabellida and this name has subsequently become the most commonly used (e.g., Fauchald 1977; Pettibone 1982), although the membership of and hierarchical groupings within, have varied. Fauchald (1977) included five major taxa in Sabellida: Caobangiidae, Sabellongidae, Sabellidae, Serpulidae and Spirorbidae. Fitzhugh (1989) revised this to contain Sabellariidae, Sabellidae and Serpulidae. The apomorphy for Sabellida, as formulated by Rouse and Fauchald (1997), was the fusion of the prostomium with the peristomium, hence it is weakly supported. Sabellariidae, Sabellidae Serpulidae and Oweniidae (Fig 12.1) were included by Rouse and Fauchald (1997) in Sabellida. Also, in a dramatic shift compared with traditional systematics Rouse and Fauchald (1997) placed Siboglinidae (Fig 12.IF) formerly outside polychaetes as Pogonophora and Vestimentifera, as part of Sabellida. In further analyses by Rouse (1999c; 2000), Oweniidae did not group with the remaining Sabellida, and Chaetopteridae (included here in as part of Spionida) did. Clearly further study is required. In addition, similarities between Sabellariidae and Pectinariidae (in Terebellida) also deserve further study. Sabellariidae has previously been considered as part of Terebellida (e.g., Fauchald 1977). Rousset et al. (2004) most recently assessed the placement of Siboglinidae, and so the composition of Sabellida, using a combination of morphological and molecular data. They found that sabellids and serpulids formed a clade, though the serpulids may actually belong inside Sabellidae. They also showed Oweniidae as sister group to Siboglinidae but neither this clade, nor Sabellariidae, formed a clade with sabellids and

serpulids. Thus, the formulation of Sabellida used in this volume may well be artificial. The support values for the Rousset et al. (2004) study were quite low in general and further data are clearly needed.