ИСТИНА

The lophophore is specific tentacular apparatus, in which tentacles surround the mouth but not the anus (Emig, 1976). Based on their possession of a lophophore, three phyla of invertebrates have been traditionally regarded as lophophorates: phoronids, brachiopods, and bryozoans. Although the monophyly of the lophophorates is actively discussed (Temereva, 2017) and is not supported by recent molecular results, the organization of the lophophore in all three groups exhibits tremendous morphological similarity. In brachiopods, the lophophore consists of brachial axis, which bears two rows of tentacles (inner and outer) and the brachial fold. On the distal ends of brachial axis, the zone, where new tentacles arise, is located. Two coelomic canals (large and small) extend at the base of the brachial axis. Small canal gives rise to the coelomic protrusion into each tentacle. Among brachiopods, brachial axis can transform in different ways (twist, bend, coil, ect) forming complex structures. In brachiopods, there is tendency to complication of the lophophore morphology from the simplest trocholophe lophophore to schizolophe lophophore, which gave rise to three stems of the lophophore organization: spirolophe, ptycholophe, and zygolophe that led to plectolophe (Kuzmina, Malakhov, 2007) (Fig. 1). According to our new results, which are obtained by µCT and histology, deep sea linguliform Pelagodiscus atlanticus has specific organization of the lophophore. The lophophore consists of two arms, each of which is formed by bended brachial axis. In each brachial arm, the large canals of two halves of the brachial axis do not fuse each other and do not form united cavity at the base of the arm as it is in typical zygolophe lophophore. In contrast to all other brachiopods, the brachial arms of P. atlanticus are extended backward. This type of organization may be regarded as modification of zygolophe lophophore, whose morphology is similar because of bending of distal ends of brachial axis, but whose brachial arms are extended forward. Apparently, P. atlanticus demonstrates a particular way of the evolution of the lophophore in brachiopods. Investigation of the lophophore in recent brachiopods will help to understand evolution of the brachiopods as whole including abundance of extinct brachiopods. At the same time, the study of the lophophore, which is the most prominent peculiarity of all lophophorates, may help to clarify the status of clade Lophophorata. According to all recent molecular results, brachiopods and phoronids form a united clade, which is called Brachiozoa. Although phoronids and bryozoans () have very similar morphology of the lophophore and body plan in general, Bryozoa is regarded as separated clade, which is not related to Brachiozoa. In organization of the lophophore of phoronids and brachiopods there are several main differences. First, in most of brachiopods, the tentacles are arranged in two rows – inner and outer; tentacles of inner and outer rows are alternated. In phoronids, there is a single row of tentacles. Interestingly, in trocholophe lophophore of brachiopods, bears a single row of tentacles. Moreover, in all brachiopods, there is a single row of tentacles (outer tentacles) near the mouth. Probably the presence of one row of tentacles is an ancestral state of the brachiopod lophophore organization. Second, in brachiopods, the brachial fold always conducts tentacles, whereas in phoronids, the epistome closely contacts the only tentacles of anal side. It looks like the space between tentacles and epistome is much wider in phoronids than in brachiopods. Thus, the each arm of phoronid lophophore bears two rows of tentacles (internal and external), epistome, and the food groove between inner tentacles and epistome. Third, in most of brachiopods, there are two lophophoral coeloms: large and small canals, whereas in phoronids, there is a single lophophoral coelom, which gives rise to canals into the tentacles. Lophophoral coelom of phoronids is similar to small canal of brachiopods, which also gives rise to coelom into the tentacles. The large canal is probably novelty of brachiopods with large lophophore: large canal works as skeleton of long and large lophophoral arms. Small trocholophe lophophore lacks the large canal. In case of zigolophe lophophore, the large canal undergoes changes: there is a fusion of large canals of two halves of each arm (Fig. 1). P. atlanticus exhibits unusual organization of the lophophore, which looks like modified zygolophe, but contains two large canals in each arm. This type may be regarded as transitional state between schizolophe and true zygolophe lophophore. This study is supported by Russian Foundation for Basic Research (#18-04-00586); the microtomography was done with support from Russian Science Foundation (#18-14-00082).