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The gastropod radula is highly diverse in structure in comparison with that of other mollusсs. The main radular types in the different phylogenetic groups of gastropods differ not only in the general morphology and configuration of the teeth but also in the mode of tooth synthesis and the ultrastructure of the formation zone. Previously, the formation and anlage of the radula in the ontogeny of radulae of all major phylogenetic gastropods groups have been studied, with the exception of the taenioglossan radula of Caenogastropoda. The data obtained in this study on the radular anlage and synthesis in one littorinimorph species Lacuna pallidula supplement the existing knowledge of diversity in gastropod radula formation. The radula is initially formed at the stage of the post-torsion veliger, with five teeth in each transverse row, and acquires the adult morphology before hatching from the egg mass. The larval radula is synthesized by a few morphologically uniform cells in the radular sac. Synthesis of the adult radula also occurs at the blind end of the radular sac, where groups of numerous odontoblasts each form one tooth, and membranoblasts, located on the ventral side, form the membrane. Characteristic features of the adult radular sac are an additional supporting cartilage-like structure at the radula curve in the middle of the radular sac, a well-defined proliferation zone in the zone of radula formation and the presence of an additional extracellular matrix around the teeth in the maturation zone.

Gastropoda is morphologically highly variable and broadly distributed group of mollusks. Due to the high morphological and functional diversity of the feeding apparatus gastropods follow a broad range of feeding strategies: from detritivory to highly specialized predation. The feeding apparatus includes the buccal armaments: jaw(s) and radula. The radula comprises a chitinous ribbon with teeth arranged in transverse and longitudinal rows. A unique characteristic of the radula is its continuous renewal during the entire life of a mollusk. The teeth and the membrane are continuously synthesized in the blind end of the radular sac and are shifted forward to the working zone, while the teeth harden and are mineralized on the way. Despite the similarity of the general mechanism of the radula formation in gastropods, some phylogenetically determined features can be identified in different phylogenetic lineages. These mainly concern shape, size and number of the odontoblasts forming a single tooth. The radular morphology depends on the shape of the formation zone and the morphology of the subradular epithelium. The radula first appears at the pre- and post-torsional veliger stages as an invagination of the buccal epithelium of the larval anterior gut. The larval radular sac is lined with uniform undifferentiated cells. Each major phylogenetic lineage is characterized by a specific larval radula type. Thus, the docoglossan radula of Patellogastropoda is characterized by initially three and then five teeth in a transverse row. The larval rhipidoglossan radula has seven teeth in a row with differentiation into central, lateral and marginal teeth and later is transformed into the adult radula morphology by addition of lateral and especially marginal teeth. The taenioglossan radula of Caenogastropoda is nearly immediately formed in adult configuration with seven teeth in a row. This article is protected by copyright. All rights reserved.

Development of the limpet Testudinalia testudinalis collected from the White Sea was studied using light and scanning electron microscopy. Cleavage and early development of T. testudinalis were con...

The radula is the basic feeding structure in gastropod molluscs and exhibits great morphological diversity that reflects the exceptional anatomical and ecological diversity occurring in these animals. This uniquely molluscan structure is formed in the blind end of the radular sac by specialized cells (membranoblasts and odontoblasts). Secretion type, and the number and shape of the odontoblasts that form each tooth characterize the mode of radula formation. These characteristics vary in different groups of gastropods. Elucidation of this diversity is key to identifying the main patterns of radula formation in Gastropoda. Of particular interest would be a phylogenetically closely related group that is characterized by high variability of the radula. One such group is the large monophyletic superfamily Conoidea, the radula of which is highly variable and may consist of the radular membrane with five teeth per row, or the radular membrane with only two or three teeth per row, or even just two harpoon-like teeth per row without a radular membrane. We studied the radulae of two species of Conoidea (Clavus maestratii Kilburn, Fedosov & Kantor, 2014 [Drilliidae] and, Lophiotoma acuta (Perry, 1811) [Turridae]) using light and electron microscopy. Based on these data and previous studies, we identify the general patterns of the radula formation for all Conoidea: the dorsolateral position of two groups of odontoblasts, uniform size, and shape of odontoblasts, folding of the radula in the radular sac regardless of the radula configuration. The morphology of the subradular epithelium is most likely adaptive to the radula type.

The rhipidoglossan radula, consisting of numerous teeth in each transverse row, is characteristic of phylogenetically distant groups of gastropods, including Vetigastropoda, Neritimorpha and 'lower' Heterobranchia. Previous studies have revealed the main patterns in the formation of the rhipidoglossan radula of vetigastropods, the main feature of which is the division of the formation zone into two horns, where marginal teeth are formed by a multilayered epithelium (odontoblasts). This work is devoted to the study of the formation of the rhipidoglossan radula of Nerita litterata using light and electron microscopy. The data obtained show that, despite the different external morphology of the radular sac of neritids and vetigastropods, the radular sac of N. litterata, like that of vetigastropods, is divided into two parts, in which the marginal teeth are similarly formed by odontoblasts located in more than one layer. It seems probable that this complex, three-dimensional structure of the formation zone is associated with a broad radula with numerous elongate marginal teeth and could be characteristic of other gastropods with this type of radula. Additional supporting rods located along the odontoblasts and consisting of vacuolated cells were first discovered in Nerita.

Aplacophora is a small enigmatic group of worm-like molluscs covered by sclerites. The general morphology of the posterior end, including the gonopericardial system, in many species is well studied using light microscopy. However, knowledge of the ultrastructure of this complex is fragmentary and many questions concerning the excretion of metabolites in this group still remain. The gonopericardial system of Chaetoderma nitidulum Loven, 1844 (Caudofoveata) from the White Sea was studied using light microscopy and 3D reconstruction from a series of semi-thin sections. The ultrastructure of the gonopericardial system was described in detail for the first time. The gonopericardial complex consists of gonads connected to the pericardium by ciliated paired gonopericardioducts. The pair of gonoducts begins at the lateral sides of the pericardium and opens into the mantle cavity. Gonoducts consist of upper and lower parts that are morphologically differentiated. The walls of the upper gonoducts consist of cells that likely participate in the excretion of metabolites and reabsorption of useful substances. Lower gonoducts are bulk bags with a small cell wall, in which there is a ciliary tract, likely for the excretion of metabolites and reproductive products. Spermatozoa were found at different stages of development in different parts of the pericardium and gonoducts. On the basis of previously published and newly obtained data, the process of excretion by C. nitidulum is proposed.

The radular morphology of the patellid species Testudinalia testudinalis (O. F. Müller, 1776) from the White Sea was studied using light, electron, and confocal microscopy. The radula is of the docoglossan type with four teeth per row and consisting of six zones. We characterize teeth formation in T. testidinalis as follows: one tooth is formed by numerous and extremely narrow odontoblasts through apocrine secretion; this initially formed tooth consists of numerous vesicles; the synthetic apparatus of the odontoblasts is localized in the apical and central parts of the cells throughout the cytoplasm and is penetrated by microtubules which are involved in the transport of the synthesized products to the apical part of the odontoblast; the newly formed teeth consist of unpolymerized chitin. Mitotic activity is located in the lateral parts of the formation zone. The first four rows contain an irregular arrangement of teeth, but the radular teeth are regularly arranged after the fifth row. The irregularly arranged teeth early on could be a consequence of the asynchronous formation of teeth and the distance between the odontoblasts and the membranoblasts. The morphological data obtained significantly expands our knowledge of the morphological diversity of the radula formation in Gastropoda.

The rhipidoglossan radula, which is characterized by presence of a central tooth, several lateral teeth, and numerous (more than 10) long marginal teeth in each transverse row, is found in three different subclasses, that is, Vetigastropoda, Neritomorpha and "lower" Heterobranchia. Details of radula formation and its ultrastructure have not been studied in any species with a rhipidoglossan radula. For the first time, we present such data for one vetigastropod species, Puncturella noachina. The radula itself and the radula formation zone were studied using light and electron microscopy (scanning and transmission), as well as confocal laser scanning microscopy. We identify the major features of Vetigastropoda rhipidoglossan radula formation, that is: the posterior bifurcation of the radula formation zone, creating paired horns into which the zones of formation of the marginal teeth extend; the supporting structure in the radula formation zone extends ventrally to strengthen this division; the odontoblasts of the marginal teeth form a multi-layered epithelium; membranoblasts do not differ from odontoblasts in ultrastructure; in some membranoblasts and cells of the sub- and supraradular epithelium basal bodies were found in the apical regions of the cells.

International audience; Spicules are mineral-based biocomposites skeletal structures that are widely distributed among phylogenetically distant groups of invertebrates (Porifera, Cnidaria, Mollusca, Echinodermata). Subepidermal spicules are formed under the ectodermal epithelium and are characterized for all groups except mollusks (Aplacophora, Polyplacophora, Bivalvia), their spicules are located on the surface of the body. However, one group of mollusks (Gastropoda: Heterobranchia) have unique subepidermal spicules that have never been detected above the ectodermal epithelium and similarly to those characterized for Porifera, Cnidaria and Echinodermata. Understanding subepidermal spicule formation in mollusks could help solve the question on the origin of spicules. Spicules in nudibranchs have been described for more than 150 years, yet ontogenetic dynamics of spicules have never been studied and the full mechanism of their formation remains unknown. Herein we investigate the spicule formation in different stages of postlarval development of the nudibranch Onchidoris muricata (O.F. Müller, 1776). For the first time, ontogenetic transformations of the spicule complex are described using experiments and different morphological methods. Our studies demonstrate that spicules of O. muricata form in the subepidermal space in early developmental stages immediately after veliger settlement. A single spicule forms inside a huge vacuole within a sclerocyte and remains there throughout the entire life of the specimen. Signs of spicule or sclerocyte migration under the epithelium in postlarval development was not found. Spicules only form during larval settlement, increasing only in size as development furthers. For the first time, spicule mineralization zones were detected at the tips of the spicules as well as the presence of collagen I in the overall composition of the spicules. Thus, our findings suggest that spicules form by an ectodermal cell that emerged under the ectodermal epithelium during the earliest stages of postlarval development.

Nudibranch molluscs represent an interesting model group to study the evolution of feeding apparatus and feeding modes, being characterized by specialized buccal complex in combination with extremely diverse dietary preferences and multiply prey shifts in evolutionary history. However, the plasticity of the buccal complex morphology in response to diet and specific feeding modes remains understudied. Here we study the general morphology and ontogenesis of the buccal complex in Eubranchus rupium (Nudibranchia: Fionidae). Specific goals are to provide a detailed description of buccal structures morphology in post-larval stages, suggest the feeding mechanism and discuss the phylogenetic value of the morphological characteristics of buccal armature within the genus Eubranchus. Methods included in vivo observations of the feeding process for E. rupium, light microscopic methods, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and confocal laser scanning microscopy (cLSM). According to our results, E. rupium is a mechanical driller, boring holes in hydrozoan perisarc and sucking internal content. The mechanical drilling is supplied by functionally uniserial radula with plate-like laterals teeth of exclusively supportive function and by massive buccal musculature. Comparative phylogeny-based analysis suggests that the drilling feeding mode is common for the genus Eubranchus and indicates radular characters may have a high phylogenetic signal. The buccal complex morphology and feeding mode were found to be similar in both adults and post-metamorphic specimens, its general structures occur even in settled veligers. Juveniles and adults compete for food source, but the competition is smoothed due to characteristics of prey species growth and life cycle.

The drilling mode of feeding is known from two clades of Gastropoda: Caenogastropoda and Heterobranchia. However, the level of convergence and parallelism or homology among these two lineages is unclear. The morphology of the buccal complex is well studied for drilling caenogastropods, but poorly known for drilling nudibranchs. It is also unclear whether the drilling feeding mechanism is similar between inside gastropods. Accordingly, a comparison between the feeding mechanisms of drilling nudibranchs and caenogastropods can help to understand the evolutional trends inside gastropods. In this study, we redescribe the morphology of the buccal complex of drilling dorid nudibranch Vayssierea cf. elegans, and compare it to that of previous investigations on this species and closely related dorid species. We describe the feeding mechanism of this species based on the obtained morphological and literature data and compare it to the feeding mechanisms described for drilling caenogastropods. The feeding apparatus of Vayssierea cf. elegans corresponds to the general morphology of the dorid buccal complex; that is, it has a similar arrangement of the buccal musculature and pattern of radular morphology. However, there are also adaptations to the drilling feeding mode similar to those found in Caenogastropoda: that is, specialized dissolving glands and lateral teeth with elongated pointed cusps; and even Sacoglossa: the specialized muscle for sucking. The feeding process of Vayssierea cf. elegans includes the same two stages as those described for drilling caenogastropods: (a) the boring stage, which is provided by mechanical and chemical activity, and (b) the swallowing stage.

We studied the innervation of unpaired branchial appendages using confocal laser scanning microscopy in two annelid species: Terebellides cf. stroemii (Trichobranchidae) and Cossura pygodactylata (Cossuridae). The branchial filament was found to be innervated by one unpaired segmental nerve coming from the ventral nerve cord at the level of chaetiger 2 in C. pygodactylata. This nerve lies transversally along the body wall and comes laterally to the base of the branchial filament. Such a structure indicates that (1) the branchial filament is unpaired and neither represents merged paired branchiae nor is related in its origin to prostomial appendages or peristomial ones, and (2) the branchial filament is the result of a shift of one of the paired segmental branchiae with parallel reduction of the second branchia in this pair. The branchial organ of T. cf. stroemii is innervated by paired segmental nerves from the first and second chaetigers. The segmental nerves are connected by longitudinal nerves going on the lateral sides of the body closer to the dorsum. One nerve on each side proceeds to the anterior part of the branchial stalk, and another nerve proceeds to the posterior part. All these nerves have transversal connections. The anterior stalk nerves come into the anterior branchial lobes, while the posterior nerves come to the posterior lobes. In the branchial lobes, the nerves give rise to thinner nerves, which go to each of the branchial lamellae. The present study confirms that the branchial organ of Terebellides has a paired nature and originates from segmental branchiae. Based on the number of innervating segmental nerves, we can conclude that the branchial organ of T. cf. stroemii is formed by two pairs of merged branchiae.

The anatomy of Cossura pygodactylata Jones, 1956 (Annelida; Cossuridae) is presented, based on the study of specimens collected during the BIOICE project in Icelandic waters and additional material from the White Sea. General gross external and internal anatomy were investigated by means of micro-computed tomography (micro-CT) and compared with results obtained with light microscopy histological sectioning (LMHS). Micro-CT revealed a highly regionalised gut, posterior to the pharyngeal cavity and oesophagus, divided in an anterior stomach and a posterior intestine with different morphologies. The stomach is in the thoracic region, dorsally positioned and divided in a short and narrow anterior part and a longer posterior one, with an increasing diameter towards its posterior end. Both stomach regions bear a dorsal infolding of tissue, along its inner wall, that resembles to the typhlosole of the Clitellata. The heart is dorsally located and contains a heart body above the hind stomach. The intestine is wider than the stomach, without dorsal infolding inside the lumen and filling most part of the internal body cavity. A transitional cup-shaped gut element connecting the stomach and the intestine could be observed at the level of the last thoracic segment. In general, the resolution of micro-CT images reflects reasonably well the main elements of gross external and internal anatomy of cossurids, and that of gut architecture in particular.

The general and fine morphology of the buccal armature and concomitant epithelia in Flabellina verrucosa were examined using light microscopy, cLSM, TEM, and SEM. A 3D-reconstruction of the radular sheath terminal end was constructed as well. Based on the obtained data, we suggest the mechanisms of jaw and radula syntheses for this species. The jaw plate’s growth is provided by the apocrine secretion of the gnathoblasts. There are two primary areas of jaw synthesis: the anterior area is responsible for growth formation of the jaw portion used in the masticatory process, and the posterior area provides the growth of the jaw plate. These types of synthesis (common for both areas) were described in detail for the first time for Gastropoda. The radula growth is provided by the microvillar activity of odontoblasts and membranoblasts. The rachidian tooth is synthesised by a single cell, and the lateral teeth are synthesised by group of 3–5 cells. The tooth formation includes four main stages: (1) tooth mould formation; (2) 90° turn of the tooth mould; (3) chitin accumulation, and (4) tooth maturation. The wide range of synthesis mechanisms is provided by a combination of two factors: the type of secretion and the amount of cells.

The morphology of the jaw apparatus of Mooreonuphis stigmatis was studied at different stages of development using light and electron microscopy as well as 3D reconstruction techniques. Three generations of maxillary apparatuses were identified. The first (larval) type of maxillae was found in juveniles that have from 4 up to 15 chaetigers. They are formed by short microvilli and have no capacity for continuous growth, as they are completely replaced by the subsequent generation of jaws. The second (juvenile) type of maxillae appears in individuals that have from 10 to 18 chaetigers. They are fully penetrated by long microvilli and have a layer of collagen fibrils in the basicuticle. The third (adult) type of maxillae is found in worms that have at least 92 chaetigers. These maxillae have long microvilli that penetrate the intermediate layer of the basicuticle but do not reach the thick collagen layer. The mandibles and two generations of maxillae (adult and juvenile) have the capacity for continuous growth. Previously, it was thought that different types of jaw structures are characteristic of different annelid taxa (for example, Dorvilleidae, Adercodon , etc.). Our data suggest that all three types of structures are found during developmental changes of the jaws in one species. The similarity between the larval maxillae of M. stigmatis and maxillary apparatus of Dorvilleidae supports the progenetic origin of the latter. The features of fine morphology suggest that the larval maxillae of Onuphidae have more chances to be fossilized than the adult maxillae. Consequently, the larval maxillae of labidognath Eunicida could be better represented in fossil material, while the calcified adult maxillae have a lower preservation capacity.

Buccal complex of Gastropoda is a complex structure consisting of the radula, odontophore and the buccal muscles. The general morphology and function of the buccal complex of Gastropoda was well-studied in several aspects. However, there are only a few integrated studies on both general and fine morphology, and the mechanism of feeding performed on opisthobranchs. Opisthobranchs' feeding mechanisms are very specific and diverse, because opisthobranch molluscs have highly-specified feeding preferences. Un- like the majority of opisthobranchs, Flabellina verrucosa (Gastropoda: Opisthobranchia) has a wide range of feeding objects. The feeding mechanism of this species can be an example of the non-specified feeding mode. General and fine morphology of the buccal complex of F. verrucosa is studied in the present work. Based on three-dimensional reconstruction of the buccal complex and data on the fine morphology of muscles, we suggest the mechanism of the functioning of the food-obtaining apparatus. Prey is pulled into the buccal cavity due to blowing negative pressure and triturated using the radula. This feeding mechanism is suggested for Gastropoda for the first time and could be compared only with that in Tochuina tetraquetra and Dendronotus iris (Nudibranchia: Dendronoti- da), although the morphology of radula in these three species differs considerably. How to cite this article: Mikhlina A.L., Vortsepneva E.V., Tzetlin A.B. 2015. Functional morphology of the buccal complex of Flabellina verrucosa (Gastropoda: Opisthobran- chia) // Invert. Zool. Vol.12. No.2. P.175-196. doi: 10.15298/invertzool.12.2.04

Juvenile and adult Cossura pygodactylata Jones 1956 from the White Sea were studied using confocal laser scanning microscopy, light microscopy, scanning and transmission electron microscopy. Transformations of the anterior musculature and digestive tract during ontogenesis were investigated. The early juveniles were shown to be lecithotrophic; their pharyngeal cavities were not connected to the intestines, which contained yolk granules. The juveniles bore prototrochs, which are used for movement, although juveniles had parapodial musculature similar to that of the adults. The juveniles presumably inhabit the upper semi-liquid layer of the silt. The muscles of the prostomium and circumbuccal complex change dramatically during ontogenesis. The ultrastructure of the buccal tentacles is redescribed. The tentacles consist of outer ciliated epithelial cells and an inner cylinder formed by epithelio-muscle cells. The blood sinus is situated between the central cylinder and the epithelium. Both juveniles and adults have developed circulatory systems. The whole dorsal vessel forms the heart with walls that consist of cells with circular cross-striated muscular fibres. The inner lumen is occluded by the heart body which is formed by a single row of cells that are tightly pressed together and connected by adherens junctions along their anterior and posterior surfaces. They contain granules and vesicles and bear numerous processes on the outer surface. The heart body most likely has a secretory haemopoetic function. A hypothetical mechanism of protraction and retraction of the buccal tentacles is suggested, and the participation of muscle contraction and relaxation in these movements is described. It is proposed that the protraction of the tentacles is provided by cell rigidity and increases in the blood volume in the tentacles blood sinuses. The development of the circulatory system is likely related to the need to keep the tentacles exposed during feeding while the anterior part of the body cavity is filled with muscle cell processes and there is no coelomic liquid flow. The proposed mechanism of feeding inside the sediment contrasts with that of surface feeding suggested by Tzetlin (Mem Mus Natl Hist Nat 162:137–143, 1994).

The topology of the heart and the pericardium of the chiton Stenosemus albus (Linnaeus, 1767) (Mollusca: Polyplacophora) is described as a result of the study of the general morphology and ultrastructure of the cardiac complex. Application of 3D-reconstruction technology to a series of histological sections detected three invaginations of the pericardium of S. albus. The dorsal invagination forms the ventricle, while two lateral invaginations form auricule walls. The auricular cavities are surrounded by the basement membrane of the pericardium from the inner side and by connective tissue from the outside. The investigation of the ultrastructure of the cardiac complex showed that the zones of ultrafiltration in S. albus are located both in the pericardium, as in all other chitons described earlier, and in the ventricle as well, as we describe here for the first time.

Jaws of various kinds occur in virtually all groups of Mollusca, except for Polyplacophora and Bivalvia. Molluscan jaws are formed by the buccal epi- thelium and either constitute a single plate, a paired formation or a serial structure. Buccal ectodermal structures in gastropods are rather different. They can be nonrenewable or having final growth, like the hooks in Clione (Gastropoda, Gymnosomata). In this case, they are formed by a single cell. Conversely, they can be renewable during the entire life span and in this case they are formed by a set of cells, like the forma- tion of the radula. The fine structure of the jaws was studied in the gastropod Puncturella noachina. The jaw is situated in the buccal cavity and consists of paired elongated cuticular plates. On the anterior edge of each cuticular plate there are numerous longitudinally ori- ented rodlets disposed over the entire jaw surface and immersed into a cuticular matrix. The jaw can be divided into four zones situated successively toward the anterior edge: 1) the posterior area: the zone of forma- tion of the thick cuticle covering the entire jaw and forming the electron-dense outer layer of the jaw plate; 2) the zone of rodlet formation; 3) the zone of rodlet arrangement; and 4) the anterior zone: the free scrap- ing edge of the plate, or the erosion zone. In the gen- eral pattern of jaw formation, Puncturella noachina resembles Testudinalia tessulata (Patellogastropoda) studied previously. The basis of the jaw is a cuticular plate formed by the activity of the strongly developed microvillar apparatus of the gnathoepithelium. How- ever, the mechanism of renewal of the jaw anterior part in P. noachina is much more complex as its scrap- ing edge consists not just of a thick cuticular matrix rather than of a system of denticles being the projec- ting endings of rodlets. J. Morphol. 000:000-000

The musculature of adult specimens of Cossura pygodactylata was studied by means of F-actin labelling and confocal laser scanning microscopy (CLSM). Their body wall is comprised of five longitudinal muscle bands: two dorsal, two ventral and one ventromedial. Complete circular fibres are found only in the abdominal region, and they are developed only on the border of the segments. Thoracic and posterior body regions contain only transverse fibres ending near the ventral longitudinal bands. Almost-complete rings of transverse muscles, with gaps on the dorsal and ventral sides, surround the terminal part of the pygidium. Four longitudinal bands go to the middle of the prostomium and 5–14 paired dorso-ventral muscle fibres arise in its distal part. Each buccal tentacle contains one thick and two thin longitudinal muscle filaments; thick muscle fibres from all tentacles merge, forming left and right tentacle protractors rooted in the dorsal longitudinal bands of the body wall. The circumbuccal complex includes well-developed upper and lower lips. These lips contain an outer layer of transverse fibres, and the lower lip also contains inner oblique muscles going to the dorsal longitudinal bands. The branchial filament contains two longitudinal muscle fibres that do not connect with the body musculature. The parapodial complex includes strong intersegmental and segmental oblique muscles in the thoracic region only; chaetal retractors, protractors and muscles of the body wall are present in all body regions. Muscle fibres are developed in the dorsal and ventral mesenteries. One semi-circular fibre is developed on the border of each segment and is most likely embedded in the dissepiment. The intestine has thin circular fibres along its full length. The dorsal blood vessel has strong muscle fibres that cover its anterior part, which is called the heart. It consists of short longitudinal elements forming regular rings and inner partitions. The musculature of C. pygodactylata includes some elements that are homologous with similar muscular components in other polychaetes (i.e., the body wall and most parapodial muscles) and several unique features, mostly at the anterior end.

The fine structure of the jaw apparatus was studied by scanning electron microscopy in eight species of Patellogastropoda. The jaw apparatus is an unpaired two-layered dorsolateral structure with anterior and posterior wings attached to the odontophore by muscles. The jaw of Testudinalia tesulata (O.F. Muller, 1776) is a derivative of the cuticle typical for the foregut. The tissue forming the jaw is a specialized foregut epithelium (gnathoepithelium), consisting of a special type of cells called gnathoblasts. The jaw grows in areas of the epithelium characterized by high concentration of electron-dense vesicles, ER and long microvilli that penetrate deep into the jaw plate. This indicates that the gnathoblasts take an active part in jaw growth. In most cases, these areas of the gnathoepithelium are highly folded. The main differences between the species studied are form and thickness of the frontal edge of the jaw. These differences do not correlate with the systematic position of the species studied but likely depend more on the feeding mode. The transmission electron microscopy studies yielded new morphological criteria for comparison between various gastropod species and other members of Trochozoa, in particular, Annelida. The jaws of Annelida are cuticular structures formed on the surface of specialized epithelial cells, often also called gnathoblasts. The jaw of Patellogastropoda can be attributed to the first type of annelid jaw formation characterized by an epithelium with long microvilli and continuous growth.

The nervous system of the ectoparasitic male of Scolelepis laonicola (Tzetlin, 1985), which attaches to the dorsal side of the female, was investigated by immunohistochemical methods in combination with confocal laser scanning microscopy. The male’s nervous system is reduced; no ganglia are found in the central nervous system. The circumesophageal connective is split into dorsal and ventral roots. Two median and one paramedial nerve run along the midventral axis of the male. The peripheral nervous system is well developed. Five commissures arise from each main cord per segment. There are two dorsolateral and two ventrolateral longitudinal nerves. The biggest segmental transverse nerve runs to the parapodia and diverges into a few fine nerves at the top of the parapodia. The nerve cord turns 90° in the male-female contact zone. Well-developed peripheral nerves, the presence of lateral nerves, and the absence of differentiated ganglia indicate the progenetic origin of the S. laonicola male. Well-developed parapodial nerves in immobilized mature males suggest an important role of the parapodia prior to settlement and possible presence of chaetae in the larva.

The muscular system of the dwarf ectoparasitic male of Scolelepis laonicola (Tzetlin, 1985) was reconstructed by Falloidin-TRITC technique and confocal scanning microscopy. The anterior part of the male penetrates the female. All four main longitudinal muscle strands of the male’s body enter the female; they are twisted about 90°. Oblique muscle fibers form a pear-shaped structure; they are very similar to the peristomial muscles in Prionospio cirrifera Wirèn. The well-developed musculature of the virtually immobile parasitic male of S. laonicola indicates good swimming abilities of juvenile males at the presettlement stage of the free-living larva.

The morphology of the obligately ectoparasitic polychaete Asetocalamyzas laonicola was studied by light and electron microscopy, and its taxonomic position was determined using molecular methods. The parasite has an extensive coelomic cavity, complete septae, and well-developed segmental nephridia, circulatory, and digestive systems. The nervous system is rudimentary and without ganglia. The parasite's anterior region penetrates the tissues of the host, and opens into the host's body cavity. The epidermal tissues of the parasite and the host are highly integrated in the area of contact, and the parasite's cuticle is continuous with that of the host. Blood vessels of the parasite and the host may interlace in the fusion zone. The dorsal side of the parasite faces the dorsal side of the host. All parasites were males, but all hosts were females. In order to elucidate the uncertain systematic position of the parasite, molecular systematic studies were conducted. Parasite and host 18S rDNA sequences were virtually identical and revealed that both belong to the spionid cluster. These sequences differed from those of Scolelepis squamata and Scolelepis bonnieri by 2.7% and 0.9%, respectively. In addition, of seven partial sequences of the mitochondrial COI gene obtained from three parasites and four hosts, six were identical, and in one host–parasite pair, COI sequences differed by one substitution. Partial ITS2 sequences from one host–parasite pair were analyzed and also found to be similar but not identical, with two indels in a 645-bp alignment. We conclude that the parasite is in fact a dwarf male of its conspecific spionid female host. Consequently, A. laonicola is transferred to Scolelepis (Spionidae), forming the new combination Scolelepis laonicola.

The sperm ultrastructure and spermatogenesis of the ectoparasitic polychaete Asetocalamyzas laonicola Tzetlin, 1985 (Calamyzidae) is investigated. The male cells are located freely in the coelom. The spermatocytes are large cells of irregular shape; their nuclei have condensed chromatin in the periphery. Spermatocyte cytoplasm is granular and electron-dense with several spherical mitochondria. During early developmental stages spermatids are aggregated into a rosette (four cells). The early spermatids have a tiny acrosomal vesicle at one side of the cell, a few round mitochondria at another, and electron dense nuclei. The late spermatids have elongated mitochondria, a well-developed acrosome and a flagellum. The mature sperm are threadlike with a round acrosomal vesicle, an electron-dense structure. The elongated nuclei have anterior and posterior depressions. The supporting root zone of the acrosome is located behind the acrosomal vesicle in the anterior invagination of the nuclei. Six elongated mitochondria surround the flagellum and form the midpiece of the sperm. A single centriole lies in the posterior depression of the nucleus. The middle part of the flagellum possesses a normal (9+2x2) pattern. Probably, the terminal part of flagellum is modified. The sperm structure suggests internal fertilization or another type of specialized sperm transfer in A. laonicola.