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Only seven frenulate species are currently known along the Eurasian coast of the Arctic Ocean. We describe a new genus and a new species of frenulates Crispabrachia yenisey, gen. nov. et sp. nov. The morphological analysis involved standard anatomical techniques, semithin sections and scanning electron microscopy (SEM). The molecular study included four markers (partial COI, 16S, 18S and 28S) and implemented Bayesian and Maximum likelihood phylogenetic approaches. The description of Crispabrachia gen. nov. is the first documented finding of frenulates in the Kara Sea at the estuary of the Yenisey River in rather shallow water (28 m). The establishment of a new genus is warranted based on the composition of morphological characters and several specific features including free, comparatively short curly tentacles, a triangular cephalic lobe with amplate base, the valvate extension of the posterior part of the forepart and prominent papillae on the nonmetameric region. The tube structure with prominent frills and the worm’s numerous tentacles, metameric papillae with cuticular plaques and segmental furrow on the forepart indicate that the new genus belongs to the polybrachiid group. Although the type locality in the Yenisey River estuary is unusual for siboglinids in general, the physical conditions here are common for other frenulates habitats, i.e. salinity ~30–33, bottom water temperature –1.5°C. This finding was made in the Yenisey Gulf in the region with the highest methane concentrations in the southern part of the Kara Sea that reflects permafrost degradation under the influence of river flow. Further study of the region would help to understand the factors influencing frenulate distributions and improve our knowledge of their biodiversity.

Deep-sea hemichordates Torquaratoridae gen. sp. reach high abundance up to 12 spec. m–2 at the depths of 1830–2130 m on the slope of the Volcanologists Massif in the south-western part of the Bering Sea, dominating in the benthic community at these depths. Their abundance exceeds by two orders the values recorded earlier. In order to clarify this phenomenon, we examined the gut contents of Torquaratoridae gen. sp. The detritus particles and frustules of planktonic diatoms Thalassiosira, Coscinodiscus, Actinocyclus, Chaetoceros, Neodenticula, and Grammatophora were the most common in the gut, as well as the remains of skeletons of benthic invertebrates with little admixture of mineral particles. According to obtained data, Torquaratoridae gen. sp. are mobile deposit feeders with high selectivity to fresh phytodetritus, able to compete with holothurians occupying similar trophic niche. Unusually high abundance of acorn worms is apparently related to high organic matter flux to the seafloor as a result of spring phytoplankton bloom in the surface water layer of the Bering Sea.

The excretory system ultrastructure and immunocytochemistry have been investigated in the plerocercoid Pyramicocephalus phocarum. It has been shown that P. phocarum has independent terminal cells, cyrtocytes. The entire canal system is a single undivided syncytium, which includes nephridial funnels of the terminal tubules, and peripheral and central canals. The nephridial funnel and cyrtocyte form a filtration complex of the protonephridial type. In the caudal region, several peripheral canals open into a deep fold of the tegument, the urinary bladder. The excretory pores are separated from the tegument by annular septate desmosomes. There are no cell junctions inside the excretory system. The presence of the F-actin ring and the expression of non-synaptic serotonin in the collar area have been detected in cyrtocytes by immunocytochemistry methods.

The tubeworm Riftia pachyptila is a key primarily producer in hydrothermal vent communities due to the symbiosis with sulphur-oxidizing bacteria, which provide nourishment to the worm from sulphides, oxygen and carbon dioxide. These substances diffuse from the vent water into the bloodstream of the worm through their tentacular crowns, and then to the bacteria, hosted in a specialized organ of the worm, called a trophosome. The uptake rates of these substances depend on the surface/volume relationship of the tentacles. We here describe two morphotypes, ‘fat’ and ‘slim’, respectively, from the basalt sulphide-rich vents at 9 °N and 21 °N at the East Pacific Rise, and the highly sedimented, sulphide-poor vents at 27 °N in the Guaymas Basin. The ‘fat’ morphotype has a thicker body and tube, longer trunk and smaller tentacular crowns, whereas the ‘slim’ morphotype has shorter trunk, thinner body and tube, and presents longer tentacular crowns and has a higher number of tentacular lamellae. Given the dependence on sulphides for the growth of R. pachyptila, as well as high genetic connectivity of the worm’s populations along the studied localities, we suggest that such morphological differences are adaptive and selected to keep the sulphide uptake near to the optimum values for the symbionts. ‘Fat’ and ‘slim’ morphotypes are also found in the vestimentiferan Ridgeia piscesae in similar sulphide-rich and poor environments in the northern Pacific.

The gill skeleton of the enteropneust Saccoglossus mereschkowskii consists of a series of tridents. The central prong of each trident bifurcates in its ventral end. The most anterior gill skeletal element has a simple horseshoe shape. Homologues of the elements of the enteropneust gill apparatus were found in the structure of the gill apparatus of Cephalochordata. The organization of the gill skeleton of Enteropneusta and Cephalochordata can be derived from the metameric horseshoe-shaped elements. The similarity of the structure of the gill skeleton of Enteropneusta and Cephalochordata contradicts a common "upside-down theory" of the origin of Chordata.

The problem of the origin of the metazoan life cycle is analyzed on the base of various hypotheses on the origin of multicellular animals. According to the Gastraea theory and the Phagocytella theory, the ancestral metazoan life cycle was holopelagic. In the framework of the hypotheses of the primary sedentarity, the metazoan ancestor had a pelago-benthic life cycle with floating larvae, the synzoospores. In accordance with this hypothesis, Eumetazoa originated from the progenetic larvae of the sedentary ancestor. The primary life cycle of Eumetazoa (i.e., metazoans with a nervous system, musculature, mouth, and gut) was holopelagic. Particularly this life cycle is typical for recent Ctenophora, which is the earliest branch of the eumetazoans. Cnidaria and Bilateria are sister groups. Their last common ancestor acquired the pelago-benthic life cycle de novo. The pelagic part of the life cycle of cnidarians is comprised of the blastula and gastrula stages only. Some anthozoans still maintain the planktotrophic gastrula larvae in their life cycles. Planulae of Medusozoa are simplified lecithotrophic larvae that had lost the function of spread because of the appearance of the medusa stage in the life cycle. In triploblastic Bilateria, the prolongation of the pelagic stage of the life cycle occurred due to the appearance of the ciliated bilaterally symmetrical larvae, which are actually the juveniles raised into the water column. This phylogenetic modus can be designated by the special term “larvalization.” Thus, the ciliated pelagic larvae of all bilaterians have a common origin from the juvenile stages of the last common bilaterian ancestor. Their ciliated bands came from the modified ciliated tentacular apparatus of juvenile stages of the last common bilaterian ancestor. Homologous elements in the ciliated bands of trochozoan and deuterostomian larvae are traced.

Frenulate siboglinids are a characteristic component of communities living in various reducing environments, including sites with hydrocarbon seeps. High concentrations of hydrocarbons in the sediments of the Arctic basin seas, including the Barents Sea, suggest the presence of a rich siboglinid fauna there. This reflects the fact that microbiological oxidation of methane occurs under reducing conditions, generating high concentrations of hydrogen sulfide in the sediment. This hydrogen sulfide acts as an energy source for the sulfide-oxidizing symbionts of siboglinids. Here we report on the findings of the frenulate siboglinid species Nereilinum murmanicum made between 1993 and 2020 in the Barents Sea. These data significantly expand the range of this species and yield new information on its habitat distribution. The depth range of N. murmanicum was 75–375 m. The species was most abundant from 200 to 350 m and was associated with temperatures below 3 °C and salinities from 34.42 to 35.07. Most of the findings (43 locations or 74%) fall on areas highly promising for oil and gas production. Twenty-eight locations (48%) are associated with areas of known oil deposits, 22 locations (37%) with explored areas of gas hydrate deposits. N. murmanicum was also found near the largest gas fields in the Barents Sea, namely Shtokman, Ludlovskoye and Ledovoye.

Allometric growth in post-embryonic development was studied in the hydrothermal vestimentiferan Oasisia alvinae Jones, 1985. In the process of growth, the length of the trunk region increases relative to the total body length from 51 to 83.4%, whereas the relative sizes of the obturacular region, vestimental region, and the opisthosoma decrease. This is connected with a strong development of the trophosome and gonads in the trunk region. It is suggested that the predominant growth of the trunk region during ontogeny is a common pattern for all vestimentiferans. We discuss the differences in body proportions of vestimentiferans that live in hydrothermal vents and vestimentiferans of cold hydrocarbon seeps.

The structure of early juvenile specimens of vestimentifera Ridgeia piscesae (Annelida, Siboglinidae) was studied. As adults, vestimentiphers are deprived of the intestine and have a trophosome, in which cells chemoautotrophic bacteria are living. In early juvenile individuals of 280-300 microns in size, it was found that trophosome develops from cells of the coelomic lining on the surface of the intestine and on the side walls of the body. This structure of the rudiment of the trofosome suggests that the bacteria are first captured by the cell wall of the body and then transferred to the cell on the surface of the intestine.

Siboglinids (Annelida, Siboglinidae) as possible indicators of carbohydrates on the case of the Sea of Okhotsk Geographical and bathymetric distribution of siboglinides in the sea of Okhotsk was studied. More than 75% of all siboglinides were found at a depth of 400 m. These findings were mainly concentrated in the North-Western part of the shelf. A comparison of the distribution of siboglinides in the Sea of Okhotsk and generalized geological data on the distribution of hydrocarbons showed that siboglinides in the sea of Okhotsk were mostly found in the areas of hydrocarbon manifestations and were absent in the central regions, where in the bottom sediments and in the near-surface layer of water we have registered minimal methane and hydrocarbon concentrations.

Lineus species are the most abundant nemerteans that inhabit boreal regions of the Atlantic and Arctic Oceans and are the focus of many zoological studies. However, its taxonomy and species composition in Arctic areas remain poorly known. In this study, we performed an updated genetic analysis of nemerteans from the “Lineus ruber–viridis” complex, clarified its species composition in the White Sea, and identified their diagnostic features. Molecular data based on mtDNA analysis indicate the presence of L. ruber, L. viridis, and L. clandestinus in the studied area, which was consistent with morphological data. Short descriptions of external morphological features are provided. External morphologies of European and White Sea Lineus species have some peculiarities: L. clandestinus from the White Sea lacks the iridescent ventral fold, and L. ruber has an unusual “green” form. The recovered genetic structure differed among the three species, which may have resulted from different dispersal scenarios during the last glacial maximum. The low level of nucleotide diversity combined with the typical star-like structure of the network indicated that an ancestral northern L. ruber population underwent a bottleneck event in the recent historical past.

Nudibranch mollusks (Gastropoda: Heterobranchia) are widely known for their ability to incorporate some active biochemical compounds of their prey, or even organelles and symbionts of the prey, which assured biological success of this group. At the same time, the process of nematocysts obtaining and incorporation into specific structures called cnidosacs by cladobranch mollusks remain poorly studied. This highlights a necessity of additional ultrastructural studies of cnidosac and adjacent organs in various aeolid mollusks using modern microscopic methods as they may provide new insight into the cnidosac diversity and fine-scale dynamics of nematocysts sequestration process. The present study is focused on the general and fine structure of the cnidosac area in cladobranch Aeolidia papillosa (Aeolidiidae). Specific goals of our study were to provide a detailed description of histological and ultrafine structure of epidermis, upper parts of the digestive glands and the cnidosac, its innervation and proliferation using standard histological techniques, confocal laser scanning microscopy (CLSM) and transmission electron microscopy. Our results clearly demonstrated that A. papillosa cnidosac is a much more complex structure, than it was thought, especially compared with simple cnidosacs found in flabellinids and facelinids. Using CLSM for functional morphological analysis provides a better resolution in visualization of structural elements within a cnidosac compared with traditional histological techniques. We revealed the presence of two cell types in the cnidophage zone: cnidophages and interstitial cells, which differ in ultrastructure and function. Our results also document the presence of a specific cnidopore zone, lined with differentiated cuboid epithelium bearing long microvilli, which likely provides a unidirectional flow of nematocysts during kleptocnides extrusion. For the first time, occurrence of vacuoles containing protective chitinous spindles in the cnidosac epithelium was shown.

Siboglinids are a characteristic feature of reducing environments. More than 75% of all siboglinids were found in the Sea of Okhotsk at a depth of less than 400 m, while some species are known to inhabit the abyssal depth in other regions. Among the six species of siboglinids encountered in the Sea of Okhotsk, only two are widespread: Siboglinum caulleryi and Oligobrachia dogieli. A significant number of all findings belong to the area where, according to geological data, the methane concentration varies between 0.22 and 4.46*109 nmol/kg. There is a vast territory in the central part of the Sea of Okhotsk that is not inhabited by siboglinids and is characterized by minimum methane concentration values. Thus, data on the Sea of Okhotsk indicate that siboglinids are related to sites of methane seepage.

Here we report the first finding of a frenulate pogonophoran (Annelida, Siboglinidae) in the southern part of the Kara Sea. This finding was made in the Yenisei Gulf in the region of the highest methane concentrations, resulting from the degradation of permafrost under the influence of river flow. It has been suggested that pogonophorans are indicators of hydrocarbon manifestations of various genesis.

We have conducted comparative analysis of the structure of the dorsal lips of the polychaete Eudistylia polymorpha from the family Sabellidae and the obturacula of Oasisia alvinae (Vestimentifera). It has been concluded that the obturacula of Vestimentifera are homologs of the dorsal lips of Polychaete from the family Sabellidae. It has been suggested that the head lobe of siboglinids of the subfamily Frenulata is homologous to the fused obturacula of Vestimentifera.

Studies of echinoid microscopic anatomy over the last two centuries have created a number of inaccuracies and mistakes that have accumulated in the descriptions of the intricate organization of the coelomic system of Echinoidea. To clarify the situation, we reconstructed the axial complex and radial complex of the echinoid Strongylocentrotus pallidus. The water ring is located between the perivisceral coelom and the perioral coelom. The oral haemal ring lies between the coelothelia of the water-vascular, perivisceral, and perioral rings. The axial part of the axial organ communicates with the oral haemal ring in interradius CD, but the axial coelom does not form the axocoelomic perihaemal ring. The ventral intestinal haemal vessel originates from the oral haemal ring in radius A, and then branches into a network of capillaries, from which the dorsal intestinal vessel starts. The pericardial coelom envelopes the pericardial part of the axial organ, the lacunae of which communicate with the haemocoel of the body wall and with the axial part of the axial organ. The genital haemal ring and the dorsal intestinal vessel communicate with the axial organ. The genital coelom passes in the CD interradius on the side opposite to the hindgut. There is a somatocoelomic perihaemal ring, which sends a pair of coelomic outgrowths into each radius, accompanied by a radial haemal vessel in the oral part. The mistakes and inaccuracies of early descriptions of the echinoid axial complex are listed. The axial complex and associated structures of sea urchins are compared with other eleutherozoans.

The ultrastructure of the lophophoral coelomic lining in the articulate brachiopod Hemithiris psittacea was studied using electron microscopy and confocal laser scanning microscopy. The coelomic system of the lophophore consists of large and small canals; both extend along each brachium. A small coelomic canal gives rise to a blind coelomic canal into each tentacle. The lophophoral coelothelium consists of two types of cells, epithelio-muscle and peritoneal cells, and exhibits different types of organization that is known in the Bilateria: from an epithelium consisting only of epithelio-muscle cells to a pseudostratified myoepithelium. The lophophoral coelothelium forms the musculature and blood vessel wall of the brachia and tentacles. The lophophoral blood vessel runs in the extracellular matrix of the septum that separates the large and small canals. In the lophophoral vessel, the upper wall consists of epithelio-muscle cells, whereas the lower wall is formed by peritoneal cells. The lophophoral vessel gives rise to blind branches into each tentacle. The lophophoral wall is highly muscular and contains both longitudinal and transverse muscles. Because the spirolophe lophophore of H. psittacea is supported only by short crura, the coelomic system of the lophophore is assumed to stabilize the lophophore by regulating the hydrostatic pressure of the coelomic fluid.

Ultrastructural study of the excretory tree of vestimentifera Ridgeia piscesae has shown that it consists of tubules that are blind at their distal ends. The tubules are lined with ciliated cells and have one or two multiciliated terminal cell(s) at the distal ends. In the tubule walls, there are putative ultrafiltration sites. The excretory tree tubules are interpreted as the secondary protonephridia.

The structure of cnidosacs in nudibranch mollusc Aeolidida papillosa (Linnaeus, 1761) before and after the discharging of kleptocnidae has been studied. In the apical zone of the cnidosac, the basal laminae of epidermis and gastrodermis are interrupted, and the muscle layers of the cnidosac and the epidermis are absent. We suggest the formation of a temporary channel during the discharging of the cnidosac. Through this channel, nematocysts move from the cnidosac to the cnidopore, which forms on the top of the ceras.

The giant dimensions of vestimentiferan Riftia pachyptila (Jones, ) are achieved thanks to the well-developed vascular system. In the vestimentum, there is a complicated net of lacunae, including the brain blood supply and the ventral lacuna underlying the ciliary field. The trunk region has an extensive network of blood vessels feeding the gonads («rete mirabile»). The thick muscular lining of the mesenterial vessels in the trunk and the dorsal vessel in the opisthosome serves as an additional pump, pushing blood into numerous vessels in the segments. It was hypothesized that the blood envelope of the ventral blood vessel in the trunk provides the blood supply to the trophosome. The 3D reconstruction has revealed that there are two vascular systems of the tentacular crown of R. pachyptila. Blood runs into the tentacles via axial afferent vessels, as described earlier only for Riftia, and also via basal ones, as described for other vestimentiferans except Riftia. The basal ones are poorly developed, and the number of lamellar blood vessels is small, indicating a lack of demand for these within huge R. pachyptila. It appears that the presence of these vessels is the preserved ancestral state of Vestimentifera. In different portions of the dorsal vessel, the morphology of the intravasal body varies, depending on function.

The absence of axial and pericardial coeloms and the axial organ, presence of a mesocoelomic tentacular apparatus, correspondence of the larval sagittal plane with the madreporic plane of the adult, and some other peculiarities set holothurians apart from all other echinoderms. We studied the holothuroid Chiridota laevis with histological methods and constructed a 3D-model. The bilateral symmetry of the water-vascular system is clear in spite of a peculiar coelomic connection with one of the tentacles in radius D. The somatocoelomic and axocoelomic perihaemal coeloms are absent. There are perioral and water-vascular circumoral coelomic rings. The hyponeural ring is absent. Each radial complex includes a water-vascular canal, a hyponeural canal, two nerve bands (ectoneural and hyponeural), and the non-coelomic epineural canal. The stone canal starts from the water ring on the aboral side in the CD interradius and ends with a madreporic ampulla, from which a few pores open into the perivisceral coelom, and one pore canal opens into the environment. This organization seems to be the intermediate state between most echinoderms, in which the madreporic pores open into the environment, and the typical organization of holothurians, in which the madreporic pores open into the perivisceral coelom. The histological structure of the oral haemal ring resembles the axial organ of other Eleutherozoa. The haemocoel lacune of the stone canal communicates with the haemal system of the gonad, the oral haemal ring, and the gut haemocoel lacunae. The genital coelom is represented by narrow spaces surrounding the “genital rachis” of the gonad.

A review of the taxonomy and distribution of vestimentiferan tubeworms has been performed. The subfamily Vestimentifera comprises three infrafamilies: Lamellibrachiinae, Escarpiinae, and Tevniinae. The complete descriptions and illustrations of 19 vestimentiferan species known to date are given. Comparative tables of the morphological characteristics of all vestimentiferan genera have been compiled. Lamellibrachiinae appear to be the most eurybathic group, representatives of which inhabit depths of 82–3200 m. Most representatives of Tevniinae reside deeper than 1500 m (except A. spiralis, which can be found at a depth of 750 m). Tevniinae live exclusively on the rocky substrate of hydrothermal vents. Lamellibrachiinae and Escarpiinae inhabit both soft and rocky substrates of cold seeps and the periphery of hydrothermal vents. Tevniinae occur only in the hydrothermal vents of the Pacific Ocean. Escarpiinae are found in the Pacific and Atlantic oceans. Lamellibrachiinae is the most widespread group that inhabits the Pacific and Atlantic oceans, including the Mediterranean Sea. The hypothesis on the Pacific origin on vestimentiferan tubeworms has been discussed. The eurybathic Lamellibrachiinae and Escarpiinae penetrated into the Atlantic Ocean through shallow-water basins of the Tethys Ocean and channels in the place of Mesoamerica, while the deep-sea Tevniinae were not able to do that.

In comparison with Asteroidea, the axial complex of ophiuroids has some important features, which are the result of shifting of the madreporite from the aboral side to the oral side. In contrast to Asteroidea, the stone canal of ophiuroids connects with the water ring from the outside, not from the inside. In Ophiuroidea, the somatocoelomic perihaemal coelom is closer to the mouth than the axocoelomic ring. The water ring of ophiuroids is shifted to the oral side relative to the perihaemal coelomic rings. The genital coelom and gastric haemal ring are located on the outer side of the axial complex, whereas in Asteroidea, they are located on the inner side. The pericardial part of the axial organ is situated on the oral side. The interradial sections of the genital coelom and genital haemal ring are descended to the oral side. Our hypothesis considers that the ancestors of ophiuroids turned the aboral side of the animal to the substratum. It caused shifting of the madreporite to the oral side and closing of the anus.

The anatomy of Crinoidea differs from that of the other modern echinoderms. In order to see, whether such differences extend to the axial complex as well, we studied the axial complex of Himerometra robustipinna (Himerometridae, Comatulida) and compared it with modern Eleutherozoa. The axial coelom is represented by narrow spaces lined with squamous coelothelium, and surrounds the extracellular haemocoelic lacunae of the axial organ. The latter is located, for the most part, along the central oral-aboral axis of the body. The axial organ can be divided into the lacunar and tubular region. The tubular coelomic canals penetrating the thickness of the axial organ have cuboidal epithelial lining, and end blindly both on the oral and aboral sides. The axial coelom, perihaemal coelom, and genital coelom are clearly visible, but they connect with the general perivisceral coelom and with each other via numerous openings. The haemocoelic spaces of the oral haemal ring pass between the clefts of the perihaemal coelom, and connect with the axial organ. In addition, the axial organ connects with intestinal haemal vessels and with the genital haemal lacuna. Numerous thin stone canaliculi pierce the spongy tissue of the oral haemal ring. They do not connect with the environment. On the oral side, each stone canaliculus opens into the water ring. The numerous slender tegmenal pores penetrate the oral epidermis of the calyx and open to the environment. Tegmenal canaliculi lead into bubbles of the perivisceral coelom. Some structures of the crinoid axial complex (stone canaliculi, communication between different coeloms) are numerous whereas in other echinoderms these structures are fewer or only one. The arrangement of the circumoral complex of Crinoidea is most similar to Holothuroidea. The anatomical structure and histology of the axial complex of Crinoidea resembles the "heart-kidney" of Hemichordata in some aspects.

Background Entoprocta affinities within Lophotrochozoa remain unclear. In different studies, entoprocts are considered to be related to different groups, including Cycliophora, Bryozoa, Annelida, and Mollusca. The use of modern methods to study the neuroanatomy of Entoprocta should provide new information that may be useful for phylogenetic analysis. Results The anatomy of the nervous system in the colonial Barentsia discreta was studied using immunocytochemistry and transmission electron microscopy. The ganglion gives rise to several main nerves: paired lateral, aboral, and arcuate nerves, and three pairs of tentacular cords that branch out into tentacular nerves. The serotonergic nervous system includes paired esophageal perikarya and two large peripheral perikarya, each with a complex net of neurites. Each tentacle is innervated by one abfrontal and two laterofrontal neurite bundles. Sensory cells occur regularly along the abfrontal side of each tentacle. Star-like nerve cells are scattered in the epidermis of the calyx. The stalk is innervated by paired stalk nerves. Conclusions The neuroanatomy of the colonial Barentsia discreta is generally similar to that of solitary entoprocts but differs in the anatomy and ultrastructure of the ganglion, the number of neurite bundles in the calyx, and the distribution of serotonin in the nerve elements. A comparison of the organization of the nervous system in the Entoprocta and Bryozoa reveals many differences in tentacle innervations, which may indicate that these groups may not be closely related. Our results can not support with any certainty the homology of nervous system elements in adult entoprocts and adult “basal mollusks”.

The geographic and bathymetric distribution of siboglinids in the Sea of Okhotsk was studied. At least 75% of all siboglinid findings were at a depth up to 400 m. Most of them were concentrated in the northwestern part of the shelf. Comparison of the data on siboglinid distribution in the Sea of Okhotsk and the aggregate geological data on hydrocarbon distribution showed that, in the Sea of Okhotsk, siboglinids were mostly in the regions of hydrocarbon manifestations, but they were absent in the central areas with the minimum methane and hydrocarbon concentrations in both benthic sediments and the uppermost water layers.

The paper reports the study of the anatomy of early juvenile individuals of the vestimentiferan tubeworm Ridgeia piscesae (Annelida, Siboglinidae). Adult vestimentiferans lack the digestive tract but have the trophosome, whose cells are inhabited by chemoautotrophic bacteria. It has been shown, in 280- to 300-µm early juvenile individuals, that the trophosome develops from cells of the coelomic lining on the gut surface and on the lateral body walls. The observed proto-trophosome structure suggests that the bacteria are first captured by the coelomic cells of the body wall and then transferred to the coelomic cells located on the gut surface.

Comparative analysis provides evidence that bilateral symmetry is a primary character of Cnidaria. All anthozoan taxa are characterized by bilateral symmetry. The anthozoan pharyngeal plane is a plane of bilateral symmetry of mesenteries and, at the same time, it is a plane of bilateral symmetry of regulatory gene expression in anthozoan morphogenesis. In Medusozoa, the bilateral symmetry is replaced by radial symmetry, but some hydrozoans (for example, Corymorphidae) demonstrate bilateral symmetry. The bilateral symmetry of Cnidaria is thought to be inherited from the common ancestors of both cnidarians and triploblastic bilaterians. The secondary radial symmetry of Cnidaria evidently is a result of the adaptation to the sessile mode of life. The presence of both the marginal and labial rings of tentacles is supposed to be a plesiomorphic character of Cnidaria. In some groups of cnidarians, one of the tentacle rings may be reduced.

The madreporite of brittle stars is poorly studied, and the features of its structure are rarely used in the taxonomy. However, it is known that there is diversity in the madreporite structure. But are the ophiuroid madreporites phylogenetically informative? To check this hypothesis, we investigated the structure of the madreporite of 33 species of brittle stars from 4 families of Euryalida and 12 families of Ophiurida. The fixed specimens were processed with sodium hypochlorite using the standard procedure and then studied using SEM. If we combine our results with the modern phylogenetic data about brittle stars (O’Hara et al. in Curr Biol 24(16):1874–1879, 2014), we will find wide morphological diversity of the madreporites present in each of the three clades of Ophiuroidea. The madreporites with numerous pores, the well-developed oral shields in other interradii instead of irregularly arranged plates and the multiple madreporites occur in the representatives of all three clades. Only in Euryalida, which belongs to the clade A as well as the sister clade Ophiuridae + Ophiomusium, the definite oral shields are absent in all interradii except CD. Whereas in the family Ophiuridae (as in the clades B and C), the oral interradial shields are regularly formed. Contrary to this, the multiple madreporites and numerous madreporic pores appear to have evolved several times in different clades. Hence, the hypothesis that madreporite morphology is phylogenetically informative must be rejected since madreporites are highly homoplasious.

The ultrastructure of the axial organ of Asterias amurensis has been studied The organ is a network of canals of the axial coelom separated by haemocoelic spaces. The axial coelom is lined with two types of monociliary cells: podocytes and musculo-epithelial cells. Podocytes form numerous basal processes adjacent to the basal lamina on the coelomic side. Musculo-epithelial cells form processes running along the basal lamina. Some bundles of these processes wrapped in the basal lamina pass through haemocoelic spaces between neighboring coelomic canals. It is hypothesized that the axial organ serves for filtration of fluid from haemocoelic spaces into the axial coelom cavity, from which urine is excreted through the madreporite to the exterior.

Brachiopods have a complex blood system that includes a main central heart and several accessory hearts. Recent ultrastructural studies suggested that the accessory hearts have dual functions: blood propulsion and ultrafiltration. In the current study, the main heart of the brachiopod Hemithiris psittacea was studied by electron microscopy and confocal laser scanning microscopy. In addition, the structure and function of the main heart were compared with those of the accessory hearts. The main heart is located on the dorsal side of the stomach posterior of the gastroparietal mesentery. It is a diverticulum of the dorsal vessel that passes along the stomach surface. The main heart consists of a short stalk that attaches to the dorsal vessel and a wide sac-like part that projects into the perivisceral coelom. The heart wall is composed of an outer coelomic epithelium, an extracellular matrix (ECM), and amoebocytes that adhere to the inner surface of the ECM. The coelomic epithelium consists of two types of cells: myoepithelial and peritoneal. The myoepithelial cells resemble podocytes, strongly stain with phalloidin, but show a weak immunoreactivity reaction against α-tubulin. These cells form long basal muscle processes, which run in different directions, branch, interweave, and form a strong muscle coating of the heart. The muscle processes give rise to the secondary thin pedicels interdigitating with each other. The peritoneal cells exhibit a strong α-tubulin-like immunoreactivity, do not stain for phalloidin, and form basal protrusions that contain abundant microtubules and that extend between the muscle processes of the myoepithelial cells. The peritoneal cells contain well-developed rough endoplasmic reticulum and numerous inclusions, and apparently have storage and secretory functions. Amoebocytes form the inner layer of the main heart but do not form a true endothelium because they lack desmosomes and a basal lamina. In summary, the main and the accessory hearts are sites in the blood system with dual functions: propulsion of blood in the ramified vessels and sinuses, and ultrafiltration of liquid that moves from the blood system to the perivisceral coelom.

The hypothesis on non-random correlation between abnormalities in the structure of hinge plate and infection of mollusks Macoma balthica with trematodes of the family Gymnophallidae has been tested on the basis of material from the Barents Sea. Significant correlation between the presence of warts and infection was established upon intraand interpopulation comparison. The hypothesis states that parasitizing of trematodes in the extrapallial cavity of mollusks influences the mantle functioning and provokes abnormalities in the hinge plate structure.

The larval development of the Brachiopod Coptothyris grayi (Davidson, 1852) from the Sea of Japan is described for the first time. Ciliated blastula proved to represent the first free-swimming stage. The blastopore is initially formed as a rounded hole stretching later along the anteroposterior axis. The larva is first divided into two lobes (the apical lobe and the trunk); the mantle lobe is formed later as two lateral folds. Two pairs of seta bundles appear in the late stage larvae. The apical larval lobe in brachiopods is supposed to match the pre-oral lobe and anterior part of the trunk with tentacles in phoronids.

Introverts of Priapulus caudatus and P. tuberculatospinosus bear 25 rows of scalids, as well as 8 spines and scattered papillae in the region the circumoral lip. These, as well as the first ring of pharyngeal teeth in P. tuberculatospinosus, are sensory organs. Although superficially they differ between species, they are all characterized by apical and/or subapical openings which are located on tiny cuticular tubules. All sensory organs contain cilia bearing bipolar receptor cells. The 8 sensory spines situated between the circumoral area and the beginning of the scalids are ultrastructurally similar to the scalids. The introvert and pharyngeal body walls, and associated muscles are described. © 1994 Wiley-Liss, Inc.

Vestimentifera is a peculiar group of marine gutless siboglinids which has uncertain position in annelid tree. The detailed study of the fragmentary explored central nervous system of vestimentiferans and other siboglinids is requested to trace the evolution of the siboglinid group. Among all siboglinids the vestimentiferans preserve the gut rudiment what makes them a key group to homologize main cerebral structures with the ones of typical annelids, such as supra- and subesophageal commissures, cirsumesophageal connectives etc. Histologically we revealed main annelid brain structures in the compact large brain ofRiftia pachyptila: circumesophageal connectives (longitudinal nerve tracts) and commissures (dorsal, supra- and subenteral commissures). Innervation of tentacles makes them homologous to peristomial palps of the rest annelids. The single nerve cord is represented by paired intraepidermal longitudinal strands associated with the ventral ciliary field in vestimentum and bearing giant axons originating from at least four pairs of perikarya. The absence of regularly positioned ganglia and lateral nerves in the nerve cord in vestimentum and trunk and presence of them in the opisthosome segments. Among siboglinids, the vestimentiferans distinguished by a large and significatly differentiated brain which is reflection of the high development of the palp apparatus.Osedax, frenulates andSclerolinumhave less developped brain. Frenulates andSclerolinumhave good ganglionization in the opisthosome, which probably indicates its high mobility. Comparative neuroanatomical analysis of the siboglinids and annelid sister clades allows us to hypothesize that the last common ancestor of siboglinids might had brain with a dorsal commissure giving rise neurite bundles to palps and paired ventral nerve cord.

Histological examination of a specimen of a deep-sea enteropneusts that belongs to a yet undescribed species (Torquaratoridae gen. sp.) revealed numerous trunk coelomoducts. They open into the genital wing coelom as a typical funnels; short ducts communicate with environment through pores located on the outer side of the genital wings. Total number of coelomoducts in a specimen is estimated at several thousand. Trunk coelomoducts have not been found earlier in any member of the phylum. We believe that the release of the male gonad products occurs through coelomoducts of Torquaratoridae gen. sp.

Background. The method of ecological niche modeling (ENM) was applied to reconstruct the nesting conditions of one of the most widely-known pest species, Locusta migratoria asiatica, with a focus of nesting in Balkhash-Alakol basin. The ENM uses a set of input environmental variables to analyze and select the key factors from the entire input set. The key factors are the climatic variables which define the wellbeing of an organism; and the range of these variables may be calculated with statistical and GIS approaches. Materials and methods. The method of ENM used in current paper is referred to as “presence-only” since it utilizes the known localities of the animal (in our study, egg-clutches) to develop a model. The model outlines the area where the successful development of locust egg-boxes is most probable, rather than the actual nesting area. Further analysis of the identified key variables allows definition of the most vulnerable stages of the locust life-cycle. Results. The most important factors, influencing the development of the locust over its life-cycle, are: the ambient air temperature; the temperature of the soil during the cold season of the year; and soil moisture. The locust is an ectotherm organism, which has a restricted ability to regulate its body temperature; and the ambient temperature thus serves as a major factor affecting the animal’s behavior. Wintering egg-boxes are immobile and face even more environmental challenges than nymphs or adults do. The soil temperature may not depend upon a single variable, like the air temperature, but is a function of the complex relationship between the thermal properties of the air and soil. The process of the energy flux between soil and atmosphere incudes many factors, particularly related to soil-moisture content and the physical properties of the soil. The analysis of key variables should not be performed without an understanding of the complex relationships between the abiotic components of the environment. Conclusions. Comparative analysis of published data on locust adult and embryo physiology and key-variables, revealed by the model, confirmed the usefulness of the ENM approach for the study of the ecological peculiarities of a living species. Further development of this model with additional variables, gathered with remote sensing, should result in a probabilistic forecasting model aimed to withstand the locust outbreaks.

Tracing the evolution of the siboglinid group, peculiar group of marine gutless annelids, requires the detailed study of the fragmentarily explored central nervous system of vestimentiferans and other siboglinids. 3D reconstructions of the neuroanatomy of Riftia revealed that the "brain" of adult vestimentiferans is a fusion product of the supraesophageal and subesophageal ganglia. The supraesophageal ganglion-like area contains the following neural structures that are homologous to the annelid elements: the peripheral perikarya of the brain lobes, two main transverse commissures, mushroom-like structures, commissural cell cluster, and the circumesophageal connectives with two roots which give rise to the palp neurites. Three pairs of giant perikarya are located in the supraesophageal ganglion, giving rise to the paired giant axons. The circumesophageal connectives run to the VNC. The subesophageal ganglion-like area contains a tripartite ventral aggregation of perikarya (= the postoral ganglion of the VNC) interconnected by the subenteral commissure. The paired VNC is intraepidermal, not ganglionated over most of its length, associated with the ciliary field, and comprises the giant axons. The pairs of VNC and the giant axons fuse posteriorly. Within siboglinids, the vestimentiferans are distinguished by a large and considerably differentiated brain. This reflects the derived development of the tentacle crown. The tentacles of vestimentiferans are homologous to the annelid palps based on their innervation from the dorsal and ventral roots of the circumesophageal connectives. Neuroanatomy of the vestimentiferan brains is close to the brains of Cirratuliiformia and Spionida/Sabellida, which have several transverse commissures, specific position of the giant somata (if any), and palp nerve roots (if any). The palps and palp neurite roots originally developed in all main annelid clades (basally branching, errantian and sedentarian annelids), show the greatest diversity in their number in sedentarian species. Over the course of evolution of Sedentaria, the number of palps and their nerve roots either dramatically increased (as in vestimentiferan siboglinids) or were lost.

The article was published with incorrect scale bars in Figure 2. This correction stands to correct the scale bars for Fig. 2 as follows: a–d, g, j—10 mm; e, f, i—1 mm; h—5 mm; k—3 mm as provided here. The original article has been corrected.

Scanning electron microscopy was used to investigate the fine morphology of the frontal filaments (FFs) in nauplial stages of the barnacles Verruca stroemia (Muller, 1776) and Hesperibalanus hesperius (Pilsbry, 1916). Changes in the FF structure in the course of development were examined. The FFs that were revealed in nauplial stage II were retained at all the subsequent nauplial stages. Small openings, which were located either apically or subapically, were found at the top of each FF in both species. In addition, the surface of an FF bears longitudinal grooves. The FFs are not separated from the body; however, each of them has a constriction at the base. In the nauplii of both species, the FFs have a wide proximal and a thin distal part. The area between the proximal and distal parts in V. stroemia has the form of a single crease; in H. hesperius, it resembles an accordion-shaped series of creases. The length ratio of the proximal/distal parts remains constant during all nauplial stages, but it varies between species. The proximal part constitutes 20–25% of the FF length in V. stroemia, while in H. hesperius the proximal and distal parts are approximately equal. The frontal filament length increases proportionally to the length of the larval body in the course of development. The possible functions of FFs as sense organs, their occurrence among the crustaceans, and possible homology with the preantennal limbs are discussed.

Metameric gill slits are mysterious structures, unique for Chordata and Hemichordata, and also, perhaps, for the extinct Cambrian Calcichordata. There is a discussed hypothesis of the gill slits origin from the metameric nephridia. According to the hypothesis, the hypothetical metameric deuterostome ancestor had in each segment a pair of coelomoducts and a pair of intestinal pockets. In the anterior segments, the coelomoducts have fused with the intestinal pockets. As a result, each nephridium opened both into the gut and into the environment. Then the dissepiments and funnels reduced in all segments except the collar one. Thus, in recent enteropneusts, only the first pair of gill slits keeps the ancestral arrangement communicating at the same time with the gut, with the environment, and with the coelom of the preceding (collar) segment. In the anterior part of the branchio-genital trunk region of enteropneusts, the metameric intestinal pockets remained, as well as the metameric coelomoducts functioning as the ducts of the metameric gonads, i.e., as the gonoducts. The consequence of the hypothesis is that the metameric gill pores originate from the metameric excreting pores, and the metameric branchial sacs originate from the metameric endodermal pockets of the gut fused with the coelomoducts. The metameric gill slits by themselves correspond with metameric openings connecting the gut with metameric intestinal pockets. J. Exp. Zool. (Mol. Dev. Evol.) 324B: 647-652, 2015. © 2015 Wiley Periodicals, Inc.

The origin of the Bilateria and reconstruction of the last common bilaterian ancestor (LCA) are fundamental problems in zoology, including the question whether the LCA was a coelomic creature or not? Insight into the nature of the LCA might be obtained by investigating the coelomic system of poorly studied bilaterians. The Brachiopoda is a relict group of marine invertebrates whose anatomy has been seldom studied with modern methods. For most brachiopods, the coelomic system has been described as bipartite, i.e., as consisting of two parts: the lophophore coelom and trunk coelom. In the present report, a tripartite coelomic system is described for the first time in adult brachiopods, the linguliform Lingula anatina . In addition to a lophophoral and trunk coelom, L. anatina has a preoral coelom. The protocoel is located at the base of the epistome and has its own lining, which consists of non-muscular monociliated epithelial cells connected by desmosomes and tight junctions. Among brachiopods there are two types of the coelom: bipartite and tripartite. The same is known in phoronids and bryozoans (=ectoprocts). These three phyla – brachiopoda, phoronida, and bryozoa – are traditionally united into clade called lophophorata. An analysis of coelomic system organization revealed that the trimeric coelom is plesiomorphic for the lophophorates. The trimeric coelom is typical for the most of deuterostomes. The presence of the trimeric coelom in two main lineages of the bilateria allows to suggest that the LCA may have had a tripartite coelom and strongly specialized two the most anterior parts of the body – the prosome and the mesosome. This suggestion is consistent with published gene expression studies, in which Hox genes are never expressed in the two first segments of the body, whereas Otx genes are expressed in the most anterior segments of some bilaterians.

Several details of the anatomy of the axial complex in brittle stars remain unknown, and there are many mismatching descriptions of its structure in different sources. The comparison of the ophiuroid axial complex with other classes of echinoderms is important for understanding of the phylogenetic relations in Ophiuroidea. We describe the organization of the axial complex of Ophiura robusta, compare it with other Ophiuroidea and analyse how the specific structure of the brittle star axial complex could appear in evolution. The standard technique of dehydration of material in alcohols of increasing concentration was used, followed by embedding material in paraplast and dissection. In the main components, the axial complex of Ophiuroidea fits other Asterozoa. But there are some important differences. The stone canal connects with the ambulacral ring from the outside, not from the inside. The somatocoelomic perihaemal ring is closer to the mouth than the axocoelomic ring. The axial complex lies between the genital coelom and the digestive tract. The gastric haemal ring is located on the outer side of the axial complex. The “pericardial” part of the axial organ is shifted to the oral side, but all its anatomical connections are retained: with the genital haemal ring, with the haemocoel of the body wall, with the gastric haemal ring and the vessels of the axial part of the axial organ. All these features could be explained as a result of shifting of madreporic plate along interradius CD from the aboral side to the oral side.

The distal tip cell (DTC) is a large single cell located at the apex of nematode gonadal arms. It is well investigated in Caenorhabditis elegans Maupas, 1900 and described in many other species of class Chromadorea. It has been obtained convincing evidence that in C. elegans the DTC plays an essential role in the development of the gonads and regulation of gametogenesis. We have studied the fine structure of the distal part of the testis in free living marine nematodes Enoplus. TEM observations revealed the absence of a large specialized DTC at the apex of testes in E. michaelseni (Linstow, 1896) and E. brevis Bastian, 1865. The testis of both species is lined with numerous uniform testis epithelial cells (TEC) forming an uninterrupted epithelium adjacent to the basal lamina. Besides that, TEC form long outgrowths protruding deeply into the testis between spermatogonia. Unlike a single DTC of C. elegans, the multiple TEC in Enoplus spp. show hardly any signs of synthetic activity. They possess neither nucleoli nor a rough endoplasmic reticulum in their cytoplasm, but contain thick bundles of filaments giving no evidence for a glandular or a hormonal function. The multicellular epithelium of the distal part of the testis in Enoplus spp. complements other plesiomorphic characters of enoplid structure and development (e.g. indeterminate early development, absence of cell constancy, regeneration ability, preservation of the nuclear envelope in spermatozoa) and may be considered as additional evidence for the basal position of the order Enoplida within the phylum Nematoda. How to cite this article: Yushin V.V., Afanasiev-Grigoriev A.G., Malakhov V.V. 2014. The male gonad of the marine nematode Enoplus: No single distal tip cell but multiple uniform epithelial cells // Invert. Zool. Vol.11. No.2. P.361-372.