How does Cellaria get out of its box? A new cheilostome hydrostatic mechanism (Bryozoa: Cheilostomata)

The sclerotized portion of the operculum of Cellaria diffusa and C. bassleri is overlain by a hypostegal coelom. Typical parietals are lacking. Polypide extrusion is accomplished by contraction of the opercular divaricators, which cause the operculum to pivot on the proximally located cardelles and simultaneously pull down on a pair of longitudinal cuticular ridges ("depressor bars") on the epitheca. The resulting depression of the epitheca causes fluid to pass from the hypostegal to the perigastric cavities around the proximolateral edges of the operculum. Both species possess well-developed setigerous collars, which indicate the position of the vestibule and confirm that the vestibular roof is is not involved in the operculum formation. The operculum appears not to have been derived from the operculum of simple cheilostomes like Membranipora, but rather from a cuticular extension of the cryptocyst. The apparent homology of the divaricators to opesiular muscles suggests derivation from a coilostegan cheilostome ancestor. Apertural morphologies are seldom studied in adequate detail, so it is unclear how many cheilostomes share this mechanism or if Cellaria-like ancestors gave rise to any ascosphoran cheilostomes.