Ultrastructure of the lophophoral coelomic lining in the brachiopod Hemithiris psittacea: functional and evolutionary significance
The ultrastructure of the lophophoral coelomic lining in the articulate brachiopod Hemithiris psittacea was studied using electron microscopy and confocal laser scanning microscopy. The coelomic system of the lophophore consists of large and small canals; both extend along each brachium. A small coelomic canal gives rise to a blind coelomic canal into each tentacle. The lophophoral coelothelium consists of two types of cells, epithelio-muscle and peritoneal cells, and exhibits different types of organization that is known in the Bilateria: from an epithelium consisting only of epithelio-muscle cells to a pseudostratified myoepithelium. The lophophoral coelothelium forms the musculature and blood vessel wall of the brachia and tentacles. The lophophoral blood vessel runs in the extracellular matrix of the septum that separates the large and small canals. In the lophophoral vessel, the upper wall consists of epithelio-muscle cells, whereas the lower wall is formed by peritoneal cells. The lophophoral vessel gives rise to blind branches into each tentacle. The lophophoral wall is highly muscular and contains both longitudinal and transverse muscles. Because the spirolophe lophophore of H. psittacea is supported only by short crura, the coelomic system of the lophophore is assumed to stabilize the lophophore by regulating the hydrostatic pressure of the coelomic fluid.