ИСТИНА

Polychaetes of the Order Sabellida have well delineated thoracic and abdominal regions marked by chaetal inversion, the arrangement where uncini become dorsal (notopodial) and setae become ventral (neuropodial). Also fecal groove (ciliated path) shifts its position from dorsal to ventral side of the body. In its classical form the inversion is present in Sabellidae /sensu lato/ (including Fabricinae and Sabellinae) and Serpulidae (including Spirorbinae). It has been suggested that chaetal inversion is also present in Sabellariidae, thus being a synapopomorphy uniting sabellariids with serpulids and sabellids, but this view is debated now. Siboglinidae and Oweniidae that were previously included in Sabellida lack chaetal inversion. Although the inversion is a very conservative feature of Sabellida /sensu strictu/, its functional significance and evolutionary driving force remain unclear.