ИСТИНА

Sedentary invertebrates, obligately living in tubes, face the problem of removing of feces. Such a lifestyle causes the funny body plan with the anterior anus placed dorsally near the head, for example, as in Phoronids or Kamptozoa. Sabelliform polychaetes (Sabellidae, Fabriciidae, Serpulidae) have ciliary fecal groove using for washing feces away. The groove runs from the anus anteriorly along the ventral side of the abdomen, turns to the dorsal side of the thorax and ends close to the base of the branchial crown. The position of ciliary groove is associated with the chaetal inversion - a very conservative feature of Serpulomorph polychaetes, where dorsal thoracic chaetae and ventral uncini reverse these positions in the abdomen. It has been suggested that chaetal inversion is also present in Sabellariidae, thus being a synapomorphy uniting sabellariids with serpulids and sabellids, but this view is debated now. Although the inversion is 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 Sabellidae larval development and regeneration and compare them with existing similar data on Serpulidae and Sabellariidae larval development.