Ultrastructural investigations on the cephalic and metameric nuchal organs of Spio cf. filicornis (Polychaeta, Spionidae)

The nuchal organs of Spio cf. filicornis from northern Europe have been studied by scanning and transmission electron microscopy. Spio cf. filicornis is the first species in which metameric nuchal organs are described. The nuchal organs consist of a distinct cephalic nuchal complex followed by metameric structures for a variable number of chaetigers. Their microanatomy corresponds to the general structural plan of nuchal organs: these are ciliated supporting cells and bipolar sensory cells with sensory cilia traversing an olfactory chamber. The organs are overlaid by a secondary paving-stone-like cover and innervated by longitudinally elongated paired nuchal nerves. The findings clearly favour the hypothesis that the paired metameric ciliated structures found in some Spionidae are in fact homologous with the prostomial nuchal organs characteristic of Polychaeta.     

Ultrastructure of the extensively developed nuchal organs of Laonice bahusiensis (Annelida: Canalipalpata: Spionidae)

The nuchal organs of annelid Laonice bahusiensis (Spionidae) from northern Europe have been studied using scanning and transmission electron microscopy. L. bahusiensis is the first spionid species in which extensively developed, continuous nuchal organs are described. The nuchal organs of this genus are the longest known among polychaete annelids. They consist of paired double bands extending from the prostomium on a mid‐dorsal caruncle for about 24–30 setigers. Their microanatomy corresponds to the general structural plan of nuchal organs: there are ciliated supporting cells and bipolar sensory cells with sensory cilia traversing an olfactory chamber. The organs are overlaid by a secondary paving‐stone‐like cover and innervated by one pair of longitudinally elongated nuchal nerves. These findings clearly favor the hypothesis that the paired, extensively developed ciliated structures found in some Spionidae are homologous with the prostomial nuchal organs characteristic of polychaete annelids. J. Morphol. 2010. © 2009 Wiley‐Liss, Inc.     

Ultrastructure of the Genital Organs in the Interstitial Syllid Petitia amphophthalma (Annelida,Polychaeta)

Abstract The gonochoristic syllid Petitia amphophthalma is one of the truly interstitial polychaetes. P. amphophthalma does not show any epitokous modifications at maturity such as those that usually occur in syllids. The reproductive structures are unique: the male genital organs consist of a seminal vesicle in chaetigers 6–10, subdivided into a dorsal part tightly filled with spermatozoa and a ventral part with contents in different stages of spermatogenesis, one pair of sperm ducts and conspicuous gland cells situated in chaetigers 10 and 11. Their glandular secretions are discharged into the sperm duct together with those of other types of gland cells that form the duct. The oocytes develop freely within the body cavity of the females. Each of the fertile segments possesses a paired oviduct ending in a large ciliated funnel. Sperm ducts and oviducts are probably modifications of excretory organs; nephridia are absent in segments where gonoducts occur. A direct sperm transfer by lytic opening of the integument of the female and internal fertilization are inferred. Copyright © 1996 Published by Elsevier Science Ltd on behalf of the Royal Swedish Academy of Sciences     

Morphology and ultrastructure of the nephridial system of Hypania invalida (Grube, 1860) (Annelida,Polychaeta, Ampharetidae)

The excretory organs of the freshwater polychaete Hypania invalida have been examined using scanning and transmission electron microscopy. Three pairs of macroscopically and ultrastructurally different nephridia are present in the thorax. Intersegmental septa in the thorax are absent, with the exception of a single diaphragm between second and third chaetiger. The first pair of nephridia is anterior to this septum, the second pair crosses the septum, with the nephrostomes anterior and the ducts and the nephridiopori posterior to it, and the third pair of nephridia is entirely posterior to the diaphragm. The first two pairs of nephridia have ciliated nephrostomes of moderate size and long nephridial ducts that extend the length of the thorax. In contrast, the third pair is characterized by short ducts and very prominent nephrostomes. Macroscopically, seven different sections of nephridial duct cells can be distinguished along the length of the first two pairs of nephridia, whereas, on an ultrastructural basis, only six different regions can be identified. Only two regions of different duct cells can be recognized in the third pair of nephridia. Cells of the two anterior pairs of nephridia show typical characteristics of transport epithelia and most likely function as excretory organs. In contrast, the duct cells of the third pair are not that much differentiated and might primarily be responsible for the release of sexual products, as sperm was observed passing through these ducts. Podocyte‐like cells were observed to accompany nephridial ducts. J. Morphol., 2011. © 2010 Wiley‐Liss, Inc.     

The functional morphology and possible taxonomic significance of the parapodia of the maldanid polychaetes Clymenella torquata and Euclymene oerstedi

The morphology of the chaetae of the maldanid polychaetes Clymenella torquata (Leidy) and Euclymene oerstedi (Claparède) (= Caesicirrus neglectus Arwidsson, '11) are described and related to movements observed in the laboratory. Graphs are constructed of the number and length of the neuropodial chaetae of each chaetiger throughout the body of Clymenella and of species of the genera Euclymene, Macroclymene and Axiothella, and show a characteristic and relatively constant pattern for each species. This work suggests that in making taxonomic decisions, more attention should be paid to the overall pattern of the neuropodial chaetae and less to the confusing distinction between aciculae and rostral uncini.     

The functional anatomy and histology of the alimentary canal of the maldanid polychaetes Clymenella torquata and Euclymene oerstedi

The alimentary canal of the maldanid polychaetes Clymenella torquata (Leidy), and Euclymene oerstedi (Claparède (= Caesicirrus neglectus Arwidsson, 1911) resembles, in many ways, that of the arenicolids. It is divided into buccal mass, pharynx, oesophagus, stomach and intestine, the three latter regions showing further subdivision. The buccal mass and anterior pharynx together form an eversible proboscis. The pharynx, oesophagus, and greater part of the intestine are ciliated. Simple feeding experiments, and histochemical tests, suggest that the stomach is concerned with the digestion and absorption of proteins, fats and carbohydrates, that the anterior intestine is a digestive and major absorptive region, and that the posterior intestine is a storage region. Waste materials are stored mainly in the wall of the oesophagus. A certain amount of intracellular digestion is carried out in the intestine of Euclymene but not in Clymenella. The difference is attributed to the richer, diatomaceous diet of Clymenella. British individuals of this species, being apparently selective feeders, differ not only from Euclymene but also from American ones, both of which ingest the substratum non-selectively.
The pharynx, oesophagus and rectum are surrounded by plexuses of blood capillaries, while the remaining regions are associated with a blood sinus system which varies in position and form in the different regions, lying deepest in the absorptive intestine. The gut muscle seems to be more concerned with moving the blood forward through the sinus system and into the anterior plexus than with moving the food backward. One region of the stomach musculature is especially concerned with this circulation. Rectal respiration probably occurs.     

Structure of the prostomial appendages and the central nervous system in the Protodrilida (Polychaeta)

Summary The prostomial appendages and the central nervous system have been investigated by electron microscopy in Protodriloides chaetifer, P. symbioticus, Protodrilus haurakiensis, P. oculifer, P. ciliatus, P. helgolandicus, P. adhaerens, Saccocirrus krusadensis and S. papillocereus. The tentacles are highly developed, mobile sensory structures and consist of cuticle, epidermis, a different number of intraepithelial nerves, a small blind-ending blood vessel and a bundle of longitudinal muscle fibres. An internal canal is only present in Protodrilus and Saccocirrus species. On the tentacles seven types of sensory cells have been found including different multiciliated and uniciliated sensory cells with cilia penetrating the cuticle, sensory cells with non-penetrative cilia, phaosomes and basal ciliated sensory cells. The latter are described for the first time in polychaetes. From the specific pattern of innervation by up to five nerves originating close to the brain from the dorsal and ventral roots of the circumoesophageal connectives it is evident that the prostomial appendages represent palps. In the palps the nerve fibres form neuroneuronal, myoneuronal and epithelioneuronal synapses. The brain also gives rise to the stomatogastric nerves and various dorsal nerves. The palp canals are separated from the surrounding tissue by a prominent extracellular matrix. The wall is formed by muscle cells. The centre is usually completely filled with the cell bodies of these muscle fibres and large coelenchyme-like cells. These cells move freely in the canals and they are very likely the structural basis for the hydroskeletal function of the canals. The canals are completely separated from other body cavities and fluid is probably driven into the canals from the blood vascular system via podocytes located in a specific zone in the prostomium. In particular, the structure of the central nervous system with its nerves, the pattern of innervation of the palps and the palp canal system are compared with those of other polychaetes with special emphasis to the Spionida, the taxon presumed to include the sister group of the Protodrilida.     

Two new species of polychaetes from the sublittoral bottoms off Antofagasta, Northern Chile: Clymenella fauchaldi n. sp. (Maldanidae) and Mooreonuphis colosensis n. sp. (Onuphidae)

Individuals belonging to two new species, from two genera Clymenella Verril (Maldanidae) and Mooreonuphis Fauchald (Onuphidae), have been collected and described from a sublittoral bottom area close to Antofagasta, northern Chile. The environment consists of medium to fine sands and the samples have been collected at ca. 60 m depth. Clymenella fauchaldi sp. n. is closely related to Clymenella minorArwidsson, 1911, but clearly differs from other species of the genus in the type of notochaetae, winged capillaries, and neurochaetae, thick acicular spines, that are borne on the first chaetigers. Mooreonuphis colosensissp. n.is related to Mooreonuphis nebulosa (Moore, 1911) and Mooreonuphis peruana (Hartman, 1944), but differs mainly in the form of the mandibles, in the maxillary formula, location of the subacicular hooks on chaetigers, number of the ceratophore rings and the length of the medium and lateral antennae styles.     

Organogenesis in the leech: development of nephridia,bladders and their innervation

The formation of the definitive excretory system (nephridium and bladder complex) in Hirudo medicinalis during the last two thirds of embryonic development was observed with light- and electron microscopy, immunocytochemistry, and nuclear labeling. In jawed leeches, two excretory systems develop and function successively. The nephridia of the cryptolarva are associated with the larval sac and persist until the definitive nephridia are sufficiently developed to be functional. Development of the definitive excretory system begins with the differentiation of the (ectodermal) bladder and urethra. The cells from which they arise incorporate bacteria and are thereby recognizable at day 8. The (mesodermal) urine-forming tissues of the nephridium (canalicular cells and central canal cells) appear a day later. By day 17, the nephridia are in contact with the outlet region and structurally able to function. Each nephridium is individually innervated by a peripheral neuron, the nephridial nerve cell, which expresses FMR Famide-like immunoreactivity and begins growing into the nephridium on day 11. Organogenesis of the leech nephridium is compared with the formation of excretory organs in other species. The temporal correlation of innervation and the development of the transporting cells is discussed. Correspondence to: A. Wenning     

Ultrastructural investigation of the nuchal organs ofPygospio elegans (Polychaeta). I. Larval nuchal organs

The structural differentiation of the nuchal organs during the post-embryonic development ofPygospio elegans is described. The sensory organs are composed of two cell types: ciliated cells and bipolar primary sensory cells, constituting the nuchal ganglion, which is associated with both the sensory epithelium and the brain. Since the sensory neurons are largely integrated into posterolateral parts of the cerebral ganglion, the nuchal organs are primary presegmental structures. The microvilli of the ciliated cells form a cover over the cuticle with a presumed protective function. An extracellular space extends between cuticle and sensory epithelium. The distal dendrites of the sensory cells terminate in sensory bulbs, bearing one modified sensory cilium each that projects into the olfactory chamber, embedded within the secretion of the ciliated cells. During development, the nuchal organs increase in size. This is accompanied by a shift in position, an expansion of the sensory area, and secretory activity of the ciliated cells. The nuchal ganglion differentiates into three nuchal centres forming three distinct sensory areas around the ciliated region. Each nuchal complex reveals two short nuchal nerves comprising the sensory axons, which enter the posterior circumesophageal connective. The sensory cells lying in the brain exhibit neurosecretory activity; the sensory cilia enlarge their surface area by dilating and branching. Nuchal organs accomplish the basic structural adaptions of chemoreceptors and show structural analogies to arthropod olfactory sensilla; thus, there is every reason to suppose chemoreceptor function.     

Origin and differentiation of nephridia in the Onychophora provide no support for the Articulata

Comparative morphology currently permits no unambiguous decision on the primary homology of the nephridia of Annelida and Arthropoda. In order to obtain additional information on this subject, ultrastructure of morphogenesis and further differentiation of nephridia was studied in the onychophoran Epiperipatus biolleyi (Peripatidae). In this species, the nephridial anlage develops by reorganization of the lateral portion of the embryonic coelomic wall that initially gives rise to a ciliated canal. All other structural components, including the sacculus, merge after the nephridial anlage has been separated from the remaining mesodermal tissue. The nephridial sacculus does not represent a ‘persisting coelomic cavity’, since it arises de novo during embryogenesis. There is no evidence for ‘nephridioblast‘ cells participating in the nephridiogenesis of Onychophora, which is in contrast to the general mode of nephridial formation in Annelida. Available data on nephridiogenesis in euarthropods (Chelicerata, Myriapoda, Crustacea, and Hexapoda) also provide no evidence for nephridia of Annelida and Arthropoda being a synapomorphy of these taxa. These findings accordingly weaken the traditional Articulata hypothesis.     

Evidence for a cerebral cholinergic system and suggested pharmacological patterns of neural organization in the prostomium of the polychaete Nereis virens (SARS)

The histological visualization of choline acetyltransferase (CAT) and acetylcholinesterase (AChE) on frozen sections of prostomia of Nereis virens indicate a concentration of cholinergic activity in the anterior brain. Components are probably sensory epithelial cells with cholinergic axons entering the brain in cephalic nerves and efferent cholinergic axons to prostomial muscle leaving the brain in the same nerves. There are also subepidermal cholinergic cells that may be second order motor neurons serving epidermal mucous cells. The smaller, second lobe of the corpora pedunculata and its associated vertical fibre tract are CAT⁴ and appear continuous, on each side of the cerebral ganglion, with a dorsal and a ventral longitudinal bundle of AChE⁺ fibers. This system tapers to nothing at the level of the posterior eyes. There is a small AChE⁺ component to each optic nerve and AChE is present in the nuchal epithelium. These observations are discussed in relation to earlier studies on aminergic and neurosecretory activity in the same ganglion.     

Ultrastructure of the nuchal organs in the polychaete Platynereis dumerilii (Annelida,Nereididae)

Abstract. We examined the nuchal organs of adults of the nereidid polychaete Platynereis dumerilii by means of scanning and transmission electron microscopy. The most prominent features of the nuchal organs are paired ciliary bands located dorsolaterally at the posterior margin of the prostomium. They are composed of primary sensory cells and multiciliated supporting cells, both covered by a thin cuticle. The supporting cells have motile cilia that penetrate the cuticle and are responsible for the movement of water. Subapically, they have a narrowed neck region; the spaces between the neck regions of these supporting cells comprise the olfactory chamber. The dendrites of the sensory cells give rise to a single modified cilium that crosses the olfactory chamber; numerous thin microvillus-like processes, presumably extending from the sensory cells, also traverse the olfactory chamber. At the periphery of the ciliated epithelium runs a large nervous process between the ciliated supporting cells. It consists of smaller bundles of sensory dendrites that unite to form the nuchal nerve, which leaves the ciliated epithelium basally and runs toward the posterior part of the brain, where the perikarya of the sensory cells are located in clusters. The ciliated epithelium of the nuchal organs is surrounded by non-ciliated, peripheral epidermal cells. Those immediately adjacent to the ciliated supporting cells have a granular cuticle; those further away have a smooth cuticle. The nuchal organs of epitokous individuals of P. dumerilii are similar to those described previously in other species of polychaetes and are a useful model for understanding the development of nuchal organs in polychaetes.     

Morphology and ultrastructure of the anterior end of Diplocirrus longisetosus Marenzeller, 1890 (Flabelligeridae, Polychaeta, Annelida)

The morphology and ultrastructure of the sedentary polychaete Diplocirrus longisetosus Marenzeller, 1890, collected from the White Sea, were studied using dissection, histological methods, light microscopy, and both scanning and transmission electron microscopy. The prostomium and peristomium carry a pair of palps, eight branchiae, a pair of nuchal organs and two nephridiopores, ciliated folds and the mouth. The prostomium, peristomium and the first chaetigerous segment with all appendages comprise the so-called siphon complex. The mouth leads to a pharyngeal organ that is closed ventrally and composed of a ventral muscle bulb adjoined dorsally by two folds projecting into the pharyngeal lumen. These parts are connected and enveloped by the longitudinal investing muscle. No tongue-like organ is present. The nervous system of the siphonal part comprises the brain, the circum-oesophageal connectives and the ganglia of the peristomium and first chaetigerous segment.     

Anterior regeneration in the polychaete Marenzelleria viridis (Annelida: Spionidae)

The objective of this study was to examine the regeneration capacity of the spionid polychaete Marenzelleria viridis from Long Island, New York. In the field, ~7% of the worms exhibited regeneration of the anterior end. In the laboratory, worms were ablated at the 10th–50th chaetiger and their regeneration documented. Anterior morphogenesis was similar to that previously reported for spionids, with wound healing, blastema formation, differentiation of segments, and formation of feeding and sensory structures (mouth, palps, nuchal organs) occurring within 14 d. Unlike in some spionids, the segments do not appear to all form simultaneously from the blastema; rather, external differentiation of segments was observed from posterior to anterior on the regenerate. The number of segments replaced was equal to the number ablated for up to 10 segments. A maximum of 17 segments were replaced when 20–30 chaetigers were ablated, and the number replaced decreased to 14 when 40–50 chaetigers were ablated. Survival and normal growth of the worms decreased with more chaetigers ablated; a significantly higher number of worms died or grew abnormally with ≥30 chaetigers ablated, compared to worms with ≤20 chaetigers ablated. Members of M. viridis could be valuable model organisms in the study of the cellular mechanisms involved in regeneration, and further research on regeneration in the field should be completed.     

Ultrastructural investigation of the nuchal organs of Pygospio elegans (Polychaeta)

Summary The differentiation of the dorsal organs as well as the structure of the nuchal organs and their relation to the central nervous system in adult Pygospio elegans were studied by electron microscopy and compared to the nuchal organs of the larvae. The nuchal organs are represented by paired ciliary bands on the dorsal side of the first setiger, delimiting a median caruncle that is completely filled with epidermal and nervous tissue. They are composed of ciliated supporting cells and bipolar primary sensory cells constituting the nuchal ganglia, which are integrated into the brain. Microvillus-like processes of the ciliated cells give rise to a secondary covering layer over the sensory epithelium. The size of the nuchal organs is a sexually dimorphic feature.Dorsal organ formation is concomitant with the onset of sexual maturation in the male sex only. They appear as metameric ciliary bands on the dorsal side of the anterior body region and consist of ciliated cells accompanied by lateral accumulations of tubular gland cells. In the gametogenic segments they are structurally associated with the male genital pores and may be involved in reproduction. The results refute previous theories that dorsal organs are sensory and have a common origin to nuchal organs.Abbreviations ac anterior commissure of the brain - ace anterior circumesophageal connective - bb basal body - bl basal lamina - c cuticle - ca caruncle - cc ciliated cell - ci sensory cilium - co microvillar cover - d septate desmosome - db dorsal blood vessel - dn dorsal nerve cord - ea efferent axons - ec epidermal cell - eg elementary granules - g Golgi complex - i filamentous inclusion - lm longitudinal muscles - ly lysosome - mc motile cilia - mv microvillus - n neuron - ng nuchal ganglion - nn nuchal nerve - nu nucleus - oc olfactory chamber - pa palp - pc posterior commissure of the brain - pce posterior circumesophageal connective - rer rough endoplasmic reticulum - sI setiger I - sb sensory bulb - sc sensory cell - sd sensory dendrite - ser smooth endoplasmic reticulum - tf tonofilament bundle - v clear vesicles - za zonula adherens     

Structure of the kidney in the coelacanth Latimeria chalumnae with reference to osmoregulation

The morphology of the nephrons of the coelacanth Latimeria chalumnae was investigated by light microscopy. Each nephron is composed of a large renal corpuscle with well‐vascularized glomerulus, non‐ciliated neck segment, proximal convoluted tubule divided into distinct first and second segments, non‐ciliated intermediate segment, distal tubule, collecting tubule and collecting duct. The parietal layer of the Bowman's capsule of the renal corpuscle is composed of low cuboidal cells. The short non‐ciliated neck segment is lined by cuboidal epithelium. The first and second proximal segments display a prominent brush border and contain amorphous material in their lumen. The second proximal segment differs from the first segment in having taller columnar epithelium and a relatively narrow lumen. The intermediate segment is lined by non‐ciliated columnar epithelium and its lumen appears empty. The distal tubule is narrow in diameter and its cuboidal epithelium is devoid of intercalated cells. A unique feature of L. chalumnae is having binucleate cells in the tubule and collecting duct epithelium. The renal arteries have poorly developed tunica media and its cells contain granular material. The structure of L. chalumnae nephrons correlates well with their osmoregulatory function and resembles those of euryhaline teleosts.     

Comparative ultrastructure of juvenile and adult nuchal organs of an annelid (Polychaeta: Opheliidae)

Opheliid nuchal organs are composed of ciliated cells, retractor muscles, and sensory cells. The perikarya of sensory cells are located in the posterior portion of the brain, and their distal processes extend along the body wall, as the nuchal nerve, and terminate just anterior to the ciliated region. The nuchal nerve of the juvenile is composed of 30–35 dendrites; the adult nuchal nerve has 35–40 dendrites. The ends of the sensory dendrites form sensory bulbs which are clustered around the olfactory chamber, and each bulb bears a modified cilium. Sensory cilia lose their axonemes and extend as microvillous-like structures into the olfactory chamber. Supportive cells delineate approximately the posterior and dorsal portions of the chamber with sensory bulbs forming the remaining ventral and anterior portions. On the lateral aspect of the chamber, cuticular matrix extends into it, and in this area supportive cells bear microvilli which extend into the matrix. The adult nuchal organ is larger than that of the juvenile, and the sensory portion of the olfactory chamber wall is expanded. Expansion of the sensory area is apparently the result of size increase in sensory bulbs and by intrusion of supportive cells between sensory bulbs.     

Ultrastructure of nuchal organs in some marine polychaetes

Polychaetes normally possess one pair of nuchal organs at the posterior edge of the prostomium or peristomium. They have been regarded as chemosensory organs. The nuchal organs of four marine polychaete species with different habits were investigated by electron microscopy. Although the shapes of nuchal organs can vary greatly from simple ciliary bands (Scolelepis squamata, Spionidae) to retractile tongue-like, piston- or finger-shaped forms (Eteone longa, Anaitides mucosa, Phyllodocidae; Heteromastus filiformis, Capitellidae), the structural components, including the ciliated supporting cells, sensory cells, and nuchal epidermal cells, are essentially similar. The differences basically concern 1) the position of the sensory cells with relation to the ciliated supporting cells, 2) the location and structure of the nuchal nerve, and 3) the structure of the nuchal cuticle. The diverging nature of this modified cuticle is described and discussed in detail. Comparisons are made with the fine structure of nuchal organs of other polychaete species. Similarities of cellular components of nuchal organs are found not only in the four species studied here but also in all nuchal organs investigated so far. This is hypothesized to be due to the fact that the polychaete stem species already possessed nuchal organs with the respective cell types. Differences in the number and distribution of cellular components and in the overall shape of nuchal organs are thought to have evolved in correlation with the equipment of other cephalic appendages and with different habits and modes of nutrition.