Ultrastructure of the coelom in the brachiopod Lingula anatina
The organization of the coelomic system and the ultrastructure of the coelomic lining are used in
phylogenetic analysis to establish the relationships between major taxa. Investigation of the anatomy
and ultrastructure of the coelomic system in brachiopods, which are poorly studied, can
provide answers to fundamental questions about the evolution of the coelom in coelomic bilaterians.
In the current study, the organization of the coelom of the lophophore in the brachiopod
Lingula anatina was investigated using semithin sectioning, 3D reconstruction, and transmission
electron microscopy. The lophophore of L. anatina contains two main compartments: the preoral
coelom and the lophophoral coelom. The lining of the preoral coelom consists of ciliated cells. The
lophophoral coelom is subdivided into paired coelomic sacs: the large and small sinuses (5 canals).
The lining of the lophophoral coelom varies in structure and includes monociliate myoepithelium,
alternating epithelial and myoepithelial cells, specialized peritoneum and muscle cells, and
podocyte-like cells. Connections between cells of the coelomic lining are provided by adherens
junctions, tight-like junctions, septate junctions, adhesive junctions, and direct cytoplasmic bridges.
The structure of the coelomic lining varies greatly in both of the main stems of the Bilateria, that
is, in the Protostomia and Deuterostomia. Because of this great variety, the structure of the coelomic
lining cannot by itself be used in phylogenetic analysis. At the same time, the ciliated
myoepithelium can be considered as the ancestral type of coelomic lining. The many different
kinds of junctions between cells of the coelomic lining may help coordinate the functioning of epithelial
cells and muscle cells.