Polypide anatomy of hornerid bryozoans (Stenolaemata: Cyclostomatida)

Bryozoans are small colonial coelomates whose colonies are made of individual modules (zooids). Like most coelomate animals, bryozoans have a characteristic body wall composition, including epidermis, extracellular matrix (ECM) and coelothelium, all pressed together. The order Cyclostomatida, however, presents the most striking deviation, in which the ECM and the corresponding coelothelium underlying major parts of the skeletal wall epidermis are ";peeled off" to form an independent membranous sac. The polypide anatomy and ultrastructure of this group is best known from one family, the Crisiidae (Articulata). Here we examined four species from the phylogenetically and ecologically contrasting family Horneridae (Cancellata) from New Zealand. Here we provide the first detailed ultrastructural examination of the hornerid polypide, including tentacles, mouth region, digestive system and the funiculus. We were able to trace continuity and transitions of cell and ECM layers throughout the whole polypide. In addition we identified that the funiculus is a lumen-free ECM cord with two associated muscles, disconnected from interzooidal pores. While agreeing with the general cyclostomate body plan, hornerids have some unique traits that make them worthy of additional study. HighlightsHornerids share a general cyclostomate body plan. The frontal tentacle ECM transitions into oral sphincter ECM, the abfrontal lophophore ECM becomes a septum between coelomic compartments, and the funuculus is a solid ECM cord supplied with muscles.