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 an epidermis, an extracellular matrix (ECM) and a 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 detached to form an independent membranous sac. It forms a separate, much smaller compartment, suspended in the zooid body cavity and working as an important element of the cyclostome lophophore protrusion mechanism. The polypide anatomy and ultrastructure of this group is best known from studies of one family, the Crisiidae (Articulata). Here, we examined four species from the phylogenetically and ecologically contrasting family Horneridae (Cancellata) from New Zealand, and provide the first detailed ultrastructural description 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. Except for funicular core composition, the polypide anatomy of hornerids agrees well with the general cyclostomate body plan.