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). In its classical form the inversion is present in Sabellidae sensu lato 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, the functional significance and evolutionary driving force of the chaetal inversion remains unclear. As a step to solve this mystery, we discuss our data on formation of chaetal arrangement in Sabellidae during regeneration and compare them with existing similar data on Serpulidae. We also review the hypotheses of the chaetal arrangement evolution and argue that understanding of the chaetal inversion phenomenon requires further detailed analysis of larval and juvenile development of the Sabellida.