ИСТИНА

Organization of coelomic system is traditionally used in phylogenetic analysis to establish the relationships between major taxa. Brachiopods and their closest relatives – phoronids and bryozoans (or ectoprocts) – were traditionally integrated into the single group called Lophophorata. During last two decades, existence of the lophophorates is being discussed and bryozoans are allocated as a separated clade, which is not relative to group Brachiozoa that now includes phoronids and brachiopods. On the other hand, Lophophorata is usually united with Deuterostomia due to data on embryonic development and coelomic organization: both Deuterostomia and Lophophorata have tripartite coelom. However, new data of molecular phylogeny testify that there is a relationship of brachiopods as a part of Brachiozoa and a currently protostomes, for last one the tripartite coelom is not a typical characteristic. Investigation of the coelomic organization in brachiopods let us understand the evolution of Lophophorata and also sheds light on fundamental questions about appearance of the last common bilaterian ancestor. The organization of the coelom in Lingula anatina was investigated by methods of histology, semi-thin sectioning, scanning electron microscopy, and 3D-reconstructions. The coelom of L. anatina consists of three main compartments – preoral coelom, lophophoral coelom, and trunk coelom (Fig. 1A). The preoral coelom is a narrow cavity, which extends at the base of the brachial fold (=epistome) along each brachium of the lophophore and above the mouth. It has its own epithelial lining, which is formed by ciliated cells. These cells are connected via desmosomes and tight junctions and underlined by basal lamina. Muscle cells are located under the coelomic epithelium in some parts of the preoral coelom, where numerous muscles, which are responsible for brachial fold’s mobility, take pass. The lophophoral coelom is divided into paired coelomic sacs: large and small sinuses (=canals) of the lophophore. The large canal of the lophophore is located along each brachium at its base. The small canal of the lophophore extends above the large canal and gives rise to coelomic channels into each tentacle. The lophophoral coelom exhibits great variety of types of the lining. The lining of both canals in the point of their connection with each other as well as with the trunk coelom is formed by podocyte-like cells. Those are large cells with big nucleus and numerous long basal projections (Fig. 1B, C). These basal projections are connected via desmosomes and form typical to this kind of cells profile (Fig. 1B). The major part of small and large canals is paved by epithelial and muscle cells, which are alternated (Fig. 1D). Muscle cells are located as separated bundles and connect basal lamina via hemidesmosomes. Between muscle cells there are numerous adhesive. Usually epithelial cells are located over the muscle cells. If there are no muscle cells, long projections of epithelial cells have a connection with a basal lamina. Adherens junctions, tight junctions, and septate junctions are found between epithelial cells. If there are no epithelial cells, muscle cells have a connection with body cavity (Fig. 1D). In those sites, epithelium cells and muscle cells are united by desmosomes. In the small canal of the lophophoral coelom, cells, which form the wall of the tentacular blood capillaries, present as flagellated myoepithelial cells. Their basal surfaces form long thin microvilli, which are jutting out into the lumen of the blood vessel. Myofilaments, which form circular muscle of the tentacle capillary, extend in the basal part of cell. The coelomic lining of the lateral sides of each tentacle is formed by flagellated epithelial cells. The trunk coelom has practically the same organization of the lining (Fig. 1C). The coelomic lining of L. anatina demonstrates huge variety of structure. We can find the most simple type of organization with only epithelial-muscle cells as well as the most complex type with alternated epithelial and muscle cells, and there are also specialized lining forming by podocyte-like cells. Such a big variety of structure of the colomic lining is firstly described for echinoderms podia (Rieger, Lombardi, 1987), and recently is shown in phoronids (Temereva, 2015). It is important to make a point that there are huge quantities of cellular junctions, which are used in coelomic lining structure. Those connections can afford a co-function of epithelial and muscle cell forming cytologically complex tissue. This research was supported in part by several grants. The collection of material was done with support from the Russian Foundation of Basic Research (#14-04-00238), the TEM and 3-D investigations were done with support from the Russian Scientific Fund (#14-04-262), and the processing of the paper was supported by Grants of the President of Russia (#MD-5812.2015.4).