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.

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Here we describe a new hornerid, Hornera currieae n. sp. (Bryozoa: Cyclostomatida) from bathyal depths across the New Zealand region. Colonies are irregular, finely branched fans attaining ~40 mm or more in height. Key characters include: (1) thick, semi-hyaline porcellanous skeleton; (2) loss or reduction of nervi (longitudinal striae) away from growing tips; (3) sparse, threadlike cancelli; and (4) small (61–87 µm), widely spaced autozooidal apertures. Diagnostic hornerid traits possessed by H. currieae n. sp. include vertical ancestrular tube, periancestrular budding of daughter zooids, and skeletal ultrastructure dominated by hexagonal semi-nacre grading to pseudofoliated fabric. The abfrontal incubation chamber develops from a cryptic tube arising from the frontally positioned aperture of the fertile zooid. We used SEM, micro-CT and electron backscatter diffractometry (EBSD) to investigate the ultrastructure and internal architecture of H. currieae n. sp. EBSD reveals that crystalline c-axes of laminated crystallites are perpendicular to skeletal walls. Threadlike cancelli, which traverse secondary calcification, connect autozooidal chambers to the colony-wide hypostegal cavity. Micro-CT reveals that abfrontal cancelli usually bend proximally towards the base, but turn distally towards reproductively active regions of the colony in synchrony with gonozooid development. The zone of affected cancelli extends for 4–7 branch internodes below the gonozooid. We assessed whether skeletal ultrastructure was similarly affected, but neither cancellus direction, nor gonozooid proximity, were predictive of the crystallite imbrication direction. We hypothesise that (1) hornerid cancelli are active conduits for colonial metabolite transport and (2) that changes in gradients of metabolites and/or reproductive morphogens within the hypostegal cavity affect cancellus morphogenesis. Potentially, H. currieae n. sp. skeletons may preserve a record of intra-colony metabolite translocation dynamics over time.

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.

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