ИСТИНА

Brachiopoda is a relict group of marine invertebrate animals with a rich paleontological history. Over 30000 species of Brachiopods are known in fossils while only about 300 species are described in a present-day fauna (Zezina, 1979). There is lack of information in Brachiopod’s morphology and anatomy as most of the data describing living Brachiopods was collected more than a hundred years ago without the use of modern techniques. Brachiopods are traditionally united with phoronids and bryozoans into the Lophophorata. The lophophorates have a tentacle organ called lophophore, a polyfunctional apparatus with a highly variable structure (Kuzmina, Malakhov, 2011). Taxonomic position of Lophophorata is being now a subject of intense discussion. Lophophorata were claimed as the closest relatives of the Deuterostomia due to the similarities of the coelom organization in the lophophorates and deuterostomes. This division of the phoronid coelom into three compartments that appears similarly to the tripartite coelomic cavities of ambulacral deuterostomes was the basis of this homology inference of the unity of the lophophorata and deuterostomia, which form a clade called archicoelomata. Coelomic system of Brachiopoda was described as bipartite, including two parts - the lophophore coelom and trunk coelom (Lüter, 1996, 2011). Literature data mention the existence of some cavities in the base of epistome in brachiopod lophophore, but the nature and structure of these cavities had not been studied (Pross, 1981). In the present report, the organization of the coelomic system of the lophophore in the inarticulate brachiopod Lingula anatina is studied by methods of histology, semi-thin sectioning, scanning electron microscopy, and 3D-reconstructions (Fig. 1, 2). According to present data, in L. anatina brachial fold contains a system of cavities. These cavities have large volume at the base of the brachial fold, and along its ventral side form numerous small chambers (Fig. 1A, B). Three-dimensional reconstruction evidences that cavities, which are located at the base of brachial fold, form single cavity that extends along each brachium and above the mouth and is isolated from canals of the lophophore coelom (Fig. 2A, B). TEM investigation reveals that the cavity has its own lining, which consists of non-muscular monociliated epithelial cells connected via desmosomes and tight junctions. The isolation and presence of own lining allow regard the epistomal cavities of L. anatina as a true coelom. Presence of the isolated coelom in the epistome of L. anatina demonstrates that Brachiopoda might have both bipartite and tripartite coelom. An analysis of the coelomic system organization reveals that tripartite coelom is a plesiomorphy for all lophophorates. The tripartite coelom is also typical for all deuterostomes. Because the tripartite coelom is found in two main lineages of bilateria – protostomia and deuterostomia, we can suggest that this type of the coelom might be inherited from the last common bilaterian ancestor. This suggestion is consistent with published gene expression studies, in which Hox genesare never expressed in the two first segments of the body, whereas Otx genes are expressed in the mostanterior segments of some bilaterians. Interesting, phoronids, brachiopods, and some deuterostomes exhibit the metameric organization of the trunk coelom. Such combination of trimery and metamery might be also regarded as plesiomorphy of all bilaterians. This research is 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 report was supported by Grants of the President of Russia (#MD-5812.2015.4; # NSH-1801.2014.4).