Abstract
Bryozoans are small colonial coelomates. They can be conceptualised as "origami-like" animals, composed of three complexly folded epithelial layers: epidermis of the zooidal/colonial body wall, gut epithelium and coelothelium. We investigated the general microanatomy and ultrastructure of the hornerid (Cyclostomatatida) body wall and polypide in four taxa, including three species of Hornera and one species belonging to an undescribed genus. We describe epithelia and their associated structures (e.g., ECM, cuticle) across all portions of the hornerid body wall, including the terminal membrane, vestibular wall, atrial sphincter, membranous sac and polypide-skeletal attachments. The classic coelomate body wall composition (epidermis-ECM-coelothelium) is only present in an unmodified form in the tentacle sheath. Deeper within a zooid it is retained exclusively in the attachment zones of the membranous sac: [skeleton]-tendon cell-ECM-coelothelium. A typical invertebrate pattern of epithelial organisation is a single, continuous sheet of polarised cells, connected by belt desmosomes and septate junctions, and resting on a collagenous extracellular matrix. Although previous studies demonstrated that polypide-specific epithelia of Horneridae follow this model, here we show that the body wall may show significant deviations. Cell layers can lose the basement membrane and/or continuity of cell cover and cell contacts. Moreover, in portions of the body wall, the cell layer appears to be missing altogether; the zooidal orifice is covered by a thin naked cuticle largely devoid of underlying cells. Since epithelium is a two-way barrier against entry and loss of materials, it is unclear how hornerids avoid substance loss, while maintaining intracolonial metabolite transport with imperfect, sometimes incomplete, cell layers along large portions of their outer body surface.