Neuromuscular system of the larva of Fredericella sultana (Bryozoa: Phylactolaemata)

Phylactolaemate Bryozoa exhibit sexually produced larval stages, which bear one or more fully developed polypides. Following settlement, the polypides evert and start feeding immediately. This is in sharp contrast to the so-called “catastrophic” metamorphosis of gymnolaemate and stenolaemate larvae, during which almost all larval tissues are resorbed and adult tissues formed de novo. Because of these striking differences, homology relationships between larval structures as well as the larvae themselves have remained uncertain. The musculature of Fredericella sultana larvae mainly consists of adult elements, which have functions in the polypide and persist through metamorphosis. Two independent serotonergic nervous systems are found: one in the polypide and one in the apical hemisphere of the ciliated body wall, which degrades during metamorphosis. The latter system includes a basiepithelial nerve net and a concentration of serotonergic cells in the region of the apical plate. FMRFamidergic nerves are found in the entire ciliated larval surface, but not in the polypide. The separate origin of larval and adult serotonergic nervous systems, which remain unconnected is most likely ancestral and apomorphic for Bryozoa and argues for the homology of bryozoan larval forms. The pattern of serotonergic cells in the phylactolaemate apical plate does not show similarities to the apical organ in gymnolaemate bryozoans. Instead, these cells might be homologous to serotonergic intercoronal cells present in gymnolaemate larvae. This, along with the occurrence of gland cells in the apical plate supports the view that the apical pole of phylactolaemate larvae corresponds to oral pole of gymnolaemate larve.