Organization of the coelomic system is traditionally used as important feature for evolutionary reconstructions. Data on chaetognath coelom organization are limited to general descriptions of coelomic morphology and ultrastructure. We investigated the coelomic and circulatory systems of Parasagitta elegans using histology, transmission electron microscopy, and 3D reconstructions. The head contains two unpaired coelomic compartments of complex morphology: dorsal and mediolateral. Their fluids differ in basophilic staining and probably in containing substances: densely stained liquid of dorsal compartment supplies the nutrition of muscles and cerebral ganglion, whereas no stained liquid of mediolateral coelom lacks the nutrients. The main circulatory organ is the peri-intestinal sinus, an expansion of extracellular matrix (ECM) between the intestinal epithelium and the gut musculature. Dorsally, it expands inside the mesentery, forming the triangular in cross section dorsal sinus. The ventral blood vessel is represented by narrow space between the integument basal lamina and coelomic epithelium. Coelothelial cells of the ventral sinus contain basal muscle fibers, which contractions may regulate hemal fluid flow. The head circulatory system includes the dorsal vessel, which extends in the longitudinal mesentery along the obliquus superficialis muscle, and two large lateral sinuses, which are located in the posterior part of the head. We propose that in P. elegans the ventral mesentery is reduced to a muscular sheath around the ventral blood vessel, facilitating blood flow from the tail toward the head. In the tail, two blood vessels extend in the dorsoventral mesentery. The possible scheme of function of circulatory system and its connection with the head coeloms is suggests.

Unlike most other brachiopods, craniiforms lack a prominent pedicle, cement to a hard substrate by their ventral valve, and thereby can be easily distinguished in paleontological materials. In this study, we describe a new genus and species from the Porkuni Regional Stage of Northeastern Estonia (Baltica), corresponding to the lower part of the Hirnantian Stage of the Upper Ordovician. Pocillocrania rubeli gen. et sp. nov. was a small brachiopod, about 3-4 mm in size, that lived on dorsal (abfrontal) surfaces of reticulate colonies of stenolaemate bryozoans and, like other craniids, was cemented by its ventral valve. The ventral valve of P. rubeli n. gen. n. sp. has a cup-shaped form with a steep, straight posterior slope. The anterior part of the dorsal valve is concave and was probably deeply inserted into the ventral valve. The external surface of the dorsal valve bears peripheral hollow spines, which sometimes bifurcate. Almost all ventral valves, which have been studied, are attached to the reticulate vertical colonies of bryozoans Parachasmatopora porkuniensis Lavrentjeva, 1985. The cup-shaped ventral valve of P. rubeli n. gen. n. sp. was likely an adaptation to inhabiting living organisms. Due to silicification, both valves of P. rubeli preserve well-defined muscle scars, allowing reconstruction of the musculature of this new craniid species as consisting of five pairs of muscles, which participate in the movement of the valves. A morpho-functional analysis based on data from extant craniids suggests that the lophophore of P. rubeli n. gen. n. sp. was of the simple spirolophous type and consisted of two arms raised into the mantle cavity, each forming one spiral coil. Thus, despite its unusually shaped ventral valve, the musculature and lophophore of P. rubeli n. gen. n. sp. exhibit a structure typical of craniids.

Phoronida is a small group of marine animals, most of which are characterized by a long larval period and complex metamorphosis. As a result of metamorphosis, their body changes so much that their true anterior and posterior ends are very close to each other, and the intestine becomes long and U-shaped. Using histology and electron microscopy, we have shown that the elongation and change in shape of the digestive tract that occurs during metamorphosis in Phoronopsis harmeri larvae is accompanied by the formation of new parts and changes in ultrastructure. At the same time, our in situ hybridization data suggest that the posterior markers Cdx and Post2 are expressed in posterior tissues at larval stages, during metamorphosis, and in juveniles, and that changes in their expression correlate with remodeling of the posterior parts of the digestive tract. Our data may shed light on the evolution of body patterning in animals undergoing complex metamorphosis.

Although Brachiopoda represent a relatively small phylum, they exhibit significant morphological diversity. One of the central issues in understanding the evolution of the ancestral brachiopod body plan pertains to the homologies among body compartments across the different brachiopod subphyla: Craniiformea, Linguliformea, and Rhynchonelliformea. In this context, the pedicle stands out as one of the most contentious organs, characterized by differing origin and organisational aspects among various brachiopod taxa. In this study, we present original observations of Novocrania anomala and Lingula anatina to explore the prevailing hypotheses regarding pedicle evolution in brachiopods, notably the hypothesis of convergent evolution and the 'anal papilla hypothesis'. Our findings indicate that N. anomala possesses a prominent posterior outgrowth of the soft body that contains the coelomic cavity, which is lined by myoepithelium. This organizational structure shares many similarities with the well-developed pedicle found in L. anatina, suggesting that the posterior outgrowth of N. anomala may be homologous to the pedicle of other brachiopods. Therefore, it is likely that representatives from all three subphyla of brachiopods possess pedicles that were inherited from their last common ancestor, subsequently undergoing remodelling in distinct lineages in various ways.

Craniiformea is a clade of brachiopods, insufficiently studied in terms of functional morphology. While the valve-opening mechanism in Linguliformea and Rhynchonelliformea has been reconstructed, the data on the same mechanism in Craniiformea are incomplete, although several hypotheses concerning this issue have been provided since the end of the 19th century. To review these hypotheses, we have studied the ultrastructure of the main visceral coelomic compartments and the muscles involved in shell movements in Novocrania anomala. Our data document that the lateral oblique muscles, together with a well-developed longitudinal musculature of the body wall (described in craniiforms for the first time) compress the perivisceral coelom along the antero-posterior axis. As a result, the perivisceral coelom expands dorso-ventrally, pushing the dorsal valve up. Thus, that the valve-opening mechanism in Craniiformea is, in principle, similar to that in Linguliformea. We also describe the smooth and cross-striated parts of the anterior adductors, and demonstrate that all muscles of N. anomala are formed by myoepithelial cells.

Microsporidia (Opisthokonta: Rozellomycota: Microsporidia) are ubiquitous intracellular parasites infecting representatives of all major taxonomic groups of Animalia, from protozoans to mammals, and infecting marine, freshwater, and terrestrial hosts. A representative of the phylum Phoronida was recently added to the list of microsporidian hosts. Only one species Microsporidium phoronidi, a parasite of Phoronis embryolabi, has been recently described. The paper presents further study of this host-parasite system, specifically, the observation of an efficient anti-microsporidial defense reaction in a phoronid host, and a unique mechanism of clearing the host of infection. This defense reaction results in encapsulation of infected cells and subsequent releasing of the capsules through excretory ducts of metanephridia, together with larvae, which regularly leave the mother organism this way. We hypothesize that by encapsulation, phoronids destroy most of parasites, block spread of the infection throughout the body, and prevent horizontal transmission. At the same time, microsporidia that develop in vasoperitoneal tissue that nourish maturing oocytes and embryos, likely overcome the host defense by sporadic or regular infection of embryos. As a result, the parasite secures its persistence in host populations by vertical transmission, which, in turn, benefits evolving less pathogenic forms. Overall, such elaborated and well-balanced phoronid host-microsporidia parasite interactions may suggest long history of co-existence and deserve further studies. New data extend our knowledge about parasite-host interactions and immune response in Lophophorata.

Cell proliferation is a key driver of morphogenesis and body plan transformation in multicellular animals, yet its spatial organization remains poorly understood in many non-segmented spiralians. In this study, we examine the dynamics of cell division during larval growth and metamorphosis in the larvae and early juveniles of the phoronid Phoronopsis harmeri, using EdU incorporation, anti-phospho-histone H3 immunostaining, confocal laser scanning microscopy, and electron microscopy. Early larval proliferation is partly regionalized from the outset and becomes progressively more localized toward metamorphosis. We identify a tripartite organization of proliferative activity: (1) posterior ring-shaped domains in the telotroch that persist through metamorphosis and support elongation and anal chamber formation; (2) regional proliferative zones at tentacle bases, preoral and postoral regions; and (3) scattered proliferation driving the expansion of the trunk epidermis. This coexistence of posterior, regional, and scattered patterns underscores the developmental plasticity of phoronids and the diversity of growth strategies within Spiralia. Posterior proliferative domains in phoronids contribute important context to homology-convergence debates on posterior growth across spiralians, but are not decisive by themselves; viewed with the distributed epithelial proliferation, they underscore the coexistence of multiple proliferative programs within a single life cycle. In addition, we identify atypical mitotic characteristics in this species, including unconventional metaphase organization and signs of interkinetic nuclear migration in larval epithelia. Our results suggest that phoronids provide a valuable model for exploring how diverse architectures of cell proliferation contribute to larval growth, body elongation, and morphogenetic compartmentalization in Lophotrochozoa.

Cell proliferation is a key driver of morphogenesis and body plan transformation in multicellular animals, yet its spatial organization remains poorly understood in many non-segmented spiralians. In this study, we examine the dynamics of cell division during larval growth and metamorphosis in the larvae and early juveniles of the phoronid Phoronopsis harmeri , using EdU incorporation, anti-phospho-histone H3 immunostaining, confocal laser scanning microscopy, and electron microscopy. Early larval development is characterized by widespread proliferative activity across ectodermal and mesodermal tissues, which becomes progressively compartmentalized as the larva matures. Two structured ring-shaped posterior proliferative zones, pre- and post-telotrochal, emerge within the telotroch and persist through metamorphosis, supporting both larval elongation and the juvenile development of ascending gut branch. In contrast, the metasomal sac and future adult trunk epidermis expand via broadly distributed epithelial proliferation, without forming a localized growth zone. This suggests that P. harmeri combines conserved features, such as a posterior growth zone, with lineage-specific innovations in regional growth. In addition, we identify atypical mitotic characteristics in this species, including unconventional metaphase organization and signs of interkinetic nuclear migration in larval epithelia. Our findings highlight the coexistence of ancestral and derived proliferative mechanisms in phoronids and provide new insights into the evolution of axial elongation and morphogenetic compartmentalization in Lophotrochozoa. ### Competing Interest Statement The authors have declared no competing interest. Russian Science Foundation, 23-14-00020

Phoronida is a small phylum of benthic marine invertebrates that can occur in large numbers globally. The study of phoronid morphology and anatomy is important for understanding phoronid biology and the function of benthic communities dominated by phoronids. Because all phoronids are tube-living animals, the study of the morphology and ultrastructure of the body wall is an important step toward understanding the processes of the tube formation, growth, and renovation. This study used epoxy histology, scanning and transmission electron microscopy to describe the body regionalization and ultrastructure of the body wall epithelium of the unusual Phoronis embryolabi, which lives as a commensal in burrows of digging shrimps. The trunk of P. embryolabi consists of 8 zones, which are clearly distinguishable in living individuals. These zones are as follows: long head region, median sphincter with its three different parts (waist, upper and lower), muscular region, reproductive region, zone 7, and ampulla. Such body division can correlate with specificity of life style of P. embryolabi. The ultrastructure of the epithelium of all zones differ from each other in thickness, set and abundance of gland cells, structure of the extracellular matrix that underlies the epithelium, and abundance of neurites. The capacity and distribution of glandular cells correlate with tube formation and remodelling. Bacteria of two different types are described along body wall of all parts of the trunk; reciprocally advantageous phoronid-bacteria interaction is suggested. Our data suggest that P. embryolabi is able to build the tube at the anterior end rather than at the posterior end, as previously suggested for other phoronid species. At the same time, the certain mechanism of phoronid tube growth and remodelling is still unknown for phoronids as well as for many other tube-living invertebrates.

Brachiopods have the most complex lophophore in comparison with other lophophorates, i.e., phoronids and bryozoans. However, at early ontogenetic stages, brachiopods have a lophophore of simple morphology, which consists of the oral tentacles. Data on the ultrastructure of the oral tentacles is mostly missing. Nonetheless, it has recently been suggested that the structure of oral tentacles is ancestral for all lophophorates in general, and for brachiopods in particular. The fine structure of the oral tentacles in the brachiopod Hemithiris psittacea is studied using light microscopy, transmission and scanning electron microscopy, cytochemistry and confocal laser scanning microscopy. The oral tentacles have a round shape in transverse section, and four ciliary zones, i.e., one frontal, two lateral, and one abfrontal. Latero-frontal sensory cells occur among the frontal epithelium. Four basiepithelial nerves in the ciliary epithelium are colocalized with ciliary zones. Lophophores of simple morphology in phoronids and brachiopods are characterized by non-specified round forms of tentacles. In phoronids and bryozoans, tentacles have additional latero-frontal ciliary zones that function as a sieve during filtration. In most brachiopods, lateral cilia are involved in the capture of food particles, whereas latero-frontal cells are retained in the frontal zone as sensory elements. The oral tentacles of H. psittacea contain a coelomic canal and have distinct frontal and abfrontal longitudinal muscles, which are separated from each other by peritoneal cells. A similar structure of tentacle muscles occurs in all bryozoans, whereas in phoronids, the frontal and abfrontal tentacle muscles are not separated by peritoneal cells. We suggest that the lophophorates' ancestor had tentacles, which were similar to the tentacles of some phoronids with lophophore of simple morphology. We also assume that the structure of the oral tentacles is ancestral for all brachiopods and the specialization of brachiopod tentacles correlates with the appearance of the double row of tentacles.

Knowledge of gametogenesis and structure of mature gametes is important for understanding the reproductive biology of a species under study. Among phoronids, Phoronis embryolabi is distinguished by a most unusual type of development: larval viviparity. In the present work, we characterize spermatogenesis in this species using transmission and scanning electron microscopy. The results indicate that the development of male gametes occurs within the vasoperitoneal tissue. Early spermatocytes have a well-developed secretory apparatus comprising a Golgi body and endoplasmic reticulum. The Golgi body, which is retained until the late spermatid stage, produces acrosome material, thick glycocalyx, and membrane vesicles that are presumably necessary for the elongation of initially small cells into filiform mature spermatozoa. The germ cells retain their connections with each other until the late spermatid stage. The mature sperm of P. embryolabi can be regarded as an intro- sperm, which is characteristic of species with internal fertilization. The structure of mature sperm differs from that of other phoronid species, whose sperm can be described in terms of the form and location of organelles. P. embryolabi sperm differs in the location of the acrosome, in the presence of a collar around the base of the flagellum, and due to the close attachment of the flagellum to the non-flagellar part of the cell. Due to this attachment, a mature P. embryolabi spermatozoon is probably able become an undulating cell capable of movement in the densely packed inner space of an animal. The filiform shape of the mature sperm is likely correlated with the specificity of the P. embryolabi reproductive strategy, i.e. viviparity of larvae, when thousands of embryos, larvae, germ cells, and cells of vasoperitoneal tissue occupy the body cavity; thus, the spermatozoon has to squeeze in narrow spaces.

The musculature is one of the best studied organ systems in brachiopods, being approachable not only by dissecting recent species of brachiopods, but also by exploring muscle scars in fossil material. In the present study, the muscular anatomy of Novocrania anomala is studied using 3D reconstructions based on microcomputed tomography. Muscles of N. anomala may be subdivided into two groups: those related to movements of the lophophore, and those connected to movements of shell valves. Muscles, their morphology and possible functions, such as brachial protractors, elevators, and retractors, as well as anterior adductors, are described and discussed. We also provide the discussion of craniid muscle terminology, consider the valve-opening mechanism. The investigation of muscle scars on dorsal valves supports the conclusion that the shape of muscle scars should be used for description and distinction of recent and extinct species only when visible distinctness cannot be explained by substrate differences. This study, which is aimed at improving our understanding the anatomy and functioning of muscles in craniids, will be useful not only for zoologists, but also for paleontologists.

The fine structure of echiurid blood vessels in the proboscis is known in detail, but the circulatory system of the trunk is still understood mainly at the level of general anatomy. The trunk circulatory system was studied in Bonellia viridis females, and specialized podocytes were found to form the walls of the ring vessel and the anterior part of the ventral vessel. Podocytes were for the first time described in the echiurid circulatory system. Podocytes of B. viridis displayed a typical cell architecture, which is known for other bilaterians. A podocyte consists of a cell body; primary processes; and pedicels, which extend from the primary processes and are interconnected via specialized slit diaphragms. The presence of podocytes indicates that the ventral and ring vessels act as ultrafiltration sites, where the plasma is filtered through the basal lamina into the body cavity.

Since ctenostomes are traditionally regarded as an ancestral clade to some other bryozoan groups, the study of additional species may help to clarify questions on bryozoan evolution and phylogeny. One of these questions is the bryozoan lophophore evolution: whether it occurred through simplification or complication. The morphology and innervation of the ctenostome Flustrellidra hispida (Fabricius, 1780) lophophore have been studied with electron microscopy and immunocytochemistry with confocal laser scanning microscopy. Lophophore nervous system of F. hispida consists of several main nerve elements: cerebral ganglion, circumoral nerve ring, and the outer nerve ring. Serotonin-like immunoreactive perikarya, which connect with the circumoral nerve ring, bear the cilium that directs to the abfrontal side of the lophophore and extends between tentacle bases. The circumoral nerve ring gives rise to the intertentacular and frontal tentacle nerves. The outer nerve ring gives rise to the abfrontal neurites, which connect to the outer groups of perikarya and contribute to the formation of the abfrontal tentacle nerve. The outer nerve ring has been described before in other bryozoans, but it never contributes to the innervation of tentacles. The presence of the outer nerve ring participating in the innervation of tentacles makes the F. hispida lophophore nervous system particularly similar to the lophophore nervous system of phoronids. This similarity allows to suggest that organization of the F. hispida lophophore nervous system may reflect the ancestral state for all bryozoans. The possible scenario of evolutionary transformation of the lophophore nervous system within bryozoans is suggested.

In Bilateria, the formation of the coelomic mesoderm occurs in various ways and is of great significance for comparative embryology and phylogeny. Several early ontogenetic stages were studied in the brachiopod Coptothyris grayi by scanning electron microscopy and cytochemistry combined with confocal laser microscopy. Two sources of the mesoderm were observed to form simultaneously from the anterior and posterior walls of the archenteron at the gastrula stage. Both anterior and posterior rudiments form enterocoely as unpaired protrusions of the wall of the archenteron and are subsequently separated from it. The findings confirmed the previous data on enterocoely in brachiopods. Moreover, a dual origin of the coelomic mesoderm from an anterior and a posterior precursor was for the first time demonstrated for all brachiopods. Analysis of the literature showed that two sources of the coelomic mesoderm in ontogeny are characteristic of representatives of various groups of protostomes and deuterostomes. This fact may provide evidence for the earlier hypothesis of plesiomorphy of two sources of the mesoderm in Bilateria.

Nuchal organs are epidermal sensory structures present in most annelids. Based on one of the interpretations, they serve in larval settlement. Siboglinids lack nuchal organs in adult and larval stages, however, larvae of some siboglinids inhabiting seeps and hydrothermal vents are capable of swimming up to 100 km away from their home hydrothermal field to colonize a new one. One question that remains is, what organ are siboglinid larvae using to search and locate suitable substrates? To determine if any nuchal organs are present in siboglinid larvae, we studied the head and sensory apparatus in successive larval stages in a frenulate, Siboglinum fiordicum (Webb, 1963), using transmission electron microscopy and immunocytochemistry. In the early trochophore stage, we found an unpaired dorsal organ lying proximal to the posterior prototroch. This organ consists of trochoblast- and "covering" cells. Trochoblasts exhibited serotonin-like immunoreactivity and likely correspond to ciliated supporting cells, where cilia and microvilli project into the olfactory chamber. The "covering" cells are characterized by the presence of large nuclei with numerous pores and thick processes that project into the olfactory chamber, forming the contacts with the trochoblast projections. We have shown for the first time the presence of a nuchal-like organ in annelids as early as the trochophore stage. The presence of this organ in siboglinid trochophores while they are still in the inside the female tube suggests that this structure might be associated with functions other than settlement, such as communication or initiation of the departure from her tube.

List of spiders of different regions of Russian Empire, from Poland to the Caucasus and republics of Middle Asia, containing 118 species from 10 families is restored by pencil remarks by Alexander I. Kroneberg made in 1869-1871 in the book on spiders of Sweden by Westring (1862).This is the earliest correct data on the spider fauna of Moscow and its environs, as well as North-East of European Russia, Chernozyom zone and Lower Povolzhie.