Structure of the compensation sac in two ascophoran bryozoans

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Abstract
Harmer established two groups of ascophoran bryozoans on structural differences in the compensation sac (ascus) and the frontal wall, although both he and Silen believed that the frontal wall of both groups is deposited within a simple ventral fold of 'frontal membrane'. Such a fold would be expected to secrete a wall consisting of mineral and/or organic successions disposed symmetrically on either side of the first-formed layer. A study of Schizoporella and Umbonula, typifying the two ascophoran groups, showed asymmetrical frontal walls and other deviations from the Harmer-Silen growth model. In both genera, ascus development is determined by the formation and rapid migration of generative zones which result from the fusion of epithelia secreting periostracum and carbonate respectively. Differences in the location and timing of these processes influence subsequent events. In Schizoporella, the first-formed layer of the frontal wall is secreted within an epithelial fold which is separated from the overlying ventral periostracum and associated epithelium by an extra-zooecial body cavity. Next follows a fusion of the carbonate- and periostracum-secreting epithelia just proximal to the orifice, and the migration of the more proximal part of the junction to the zooecial walls. Cell proliferation in the wake of this retreat leads to the formation of the ascus floor. Exposed primary calcite on the dorsal surface of the frontal wall constitutes the roof of the ascus and, contrary to the Harmer-Silen model, shows no trace of periostracum or associated epithelium. In Umbonula, epithelial fusion occurs just within the periphery of the ventral surface, and the roof of the ascus is secreted by cells of a generative zone as it advances towards a submedial position near the orifice. The roof consists of a thin but complete periostracum continuous with that forming both the floor of the ascus and the ventral cover of the zooid. The calcareous frontal wall is subsequently deposited on the roof periostracum by the dorsal or carbonate-secreting layer of the doubled epithelium.

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