HOW DOES CELLARIA GET OUT OF ITS BOX - A NEW CHEILOSTOME HYDROSTATIC MECHANISM (BRYOZOA, CHEILOSTOMATA)

Accepted explanations cannot explain lophophore protrusion in Cellaria diffusa and C. bassleri, whose apertural structure differs markedly f rom that of most other cheilostomes. The sclerotized portion of the op erculum is overlain by a hypostegal coelom. Typical parietal muscles a re lacking. Lophophore protrusion is accomplished by muscular contract ion: opercular divaricators cause the operculum to pivot on proximally located hinge teeth (cardelles) and simultaneously pull down on a pai r of longitudinal cuticular ridges (depressor bars) on the epitheca. T he resulting depression of the epitheca causes fluid to pass from the hypostegal coelom to the perigastric cavity around the proximolateral edges of the operculum. Both species possess well-developed setigerous collars, so the location of the vestibule can be definitely establish ed. The vestibular roof is not involved in operculum formation. The op erculum appears not to have been derived from the operculum of simple cheilostomes such as Membranipora, but rather from a cuticular extensi on of the cryptocyst. The apparent homology of the divaricators to ope siular muscles suggests derivation from cheilostome ancestors with ext ensive cryptocysts and opesiules. Apertural morphologies are seldom st udied in adequate detail, so it is unclear how many cheilostomes share this mechanism or if Cellaria-like ancestors gave rise to any ascopho ran cheilostomes.