ИСТИНА

The data on mesoderm formation are used to resolve the relationships of large taxa and may be useful for clarification of brachiopod phylogeny and evolution. Development of the larval coelomic system are studied in embryos and larvae of rhynchonelliform brachiopod Coptothyris grayi by confocal laser scanning and transmission electron microscopy. In the gastrula, the epithelium of the archenteron forms two pairs of the lateral enterocoelic diverticula. The first pair forms the anterior coelomic pouches, i.e., the anterior mesoderm. The second pair forms the posterior coelomic pouches, i.e., the posterior mesoderm. In development, gastrula transforms into the two-lobed larva consisting of the apical and body lobes. In two-lobed larva, the anterior coelomic pouches are located in the apical lobe, whereas the posterior coelomic pouches are divided into two pairs and extend into the body lobe. After that, the larva becomes three-lobed, i.e., the body lobe is divided into the mantle and the pedicle lobes. Two pairs of setal bundles start to form in the mantle lobe. In three-lobed larvae, all coelomic pouches start to partially fuse. Therefore, the united coelomic anlage that is subdivided into three pairs of interconnecting chambers is formed. In the competent larvae, the posterior mesoderm forms the anlage of the perivisceral coelom that gives rise to branches extending into the mantle anlage. After metamorphosis, the anterior mesoderm forms the lophophoral coelom. Anterior and posterior sources of the mesoderm, which are known in annelids, phoronids, arthropods, and deuterostomes, are described in brachiopod development for the first time. Brachiopods, which is phylum among Lophotrochozoa, are characterized by the enterocoelic formation of mesoderm that is similar to that of the deuterostomes. Besides, the metameric structure of coelom is found in the brachiopod larvae. This work is supported by RFBR (19-04-00449)and RSF (18-14-00082).