Ultrastructure of the metanephridia of Terebratulina retusa and Crania anomala (Brachiopoda)

Adult specimens of Terebratulina retusa and Crania anomala have one pair of metanephridia. Each metanephridium is composed of a ciliated nephridial funnel (nephrostome) and an outleading nephridial canal, thus, these organs are open ducts connecting the metacoel of the animal with the outer medium. In both species, the inner side of a nephrostome is lined by a columnar monociliated epithelium which contacts the coelothel within one of the two ileoparietal bands. The coelothel contains basal filaments (in C. anomala these are definite myofilaments). The canal epithelium also consists of monociliated columnar cells which differ from the nephrostome epithelial cells in size and some cell components. Within the nephropore, the canal epithelium makes contact with the so-called inner mantle epithelium which lines the mantle cavity. The nephrostome epithelial cells and the canal epithelial cells never contain any contractile filaments. There are always continuous transitions between these different epithelia and distinct borders cannot be observed. The present results, especially in comparison to Phoronida, do not contradict the hypothesis of a coelothelially derived nephridial funnel and an ectodermal nephridial duct in Brachiopoda. But with regard to the differences between Phoronida and Brachiopoda (larval protonephridia and podocytes in the adults are unknown in Brachiopoda), further investigations have to be done to test the hypothesis of such heterogeneously assembled metanephridia.     

Structure and development of the nephridia of Tomopteris helgolandica (Annelida)

 Nephridial diversity is high in Phyllodocida (Annelida) and ranges from protonephridia to metanephridia. The nephridia of Tomopteris helgolandica (Tomopteridae) can be characterized as metanephridia which bear a multiciliated solenocyte. This cell is medially apposed to the proximal part of the nephridial duct and bears several cilia, each of which is surrounded by a ring of 13 microvilli. An extracellular matrix connects the microvilli and thus leads to the impression of a tube surrounding the central cilium. Each tube separately enters a subjacent duct cell and the cilia extend into a cup-shaped compartment within the duct cell. This compartment is not connected to the duct. The funnel consists of eight multiciliated cells and is connected to the nephridial duct, which initially runs intercellularly and later percellularly. The last duct cell bears a neck-like process which pierces the subepidermal basal membrane and is connected to epidermal cells forming a small invagination, the nephropore. The nephridia of T. helgolandica develop from a band of cells and all structural components are differentiated at an early developmental stage. Further development is characterized by enlargment of the funnel, ciliogenesis in the solenocyte, merging of different sections of the duct and, finally, the formation of the nephropore. An evaluation of the nephridia of T. helgolandica leads to the hypothesis that the nephridial diversity in Phyllodocida can be explained by the retainment of different stages in the transition of protonephridia into metanephridia; this is caused by the formation of a ciliated funnel at different ontogenetic stages. Although the protonephridia in Phyllodocida are regarded as primary nephridial organs, protonephridia are also presumed to have evolved secondarily in progenetic interstitial species of the Annelida by an incomplete differentiation of the nephridial anlage. Accepted: 18 December 1996     

Ultrastructure of the preseptal part of metanephridia in Nais variabilis and Dero digitata (Annelida, Clitellata)

The composition and arrangement of cells in the preseptal region of metanephridia have been examined by ultrastructural methods in two naidid species, Nais variabilis and Dero digitata. Within this region special attention has been paid to the portion around the orifice and the region where the metanephridium penetrates the septum. In N. variabilis, the preseptal region is composed of four cells and, in D. digitata, three cells are present. In both species three cells correspond in position and ultrastructural details and, hence, are interpreted as homologous. These are the mantle cell, the flame cell, and the canal cell. The mantle cell covers the preseptal region and surrounds the opening. The margin around the orifice is endowed with cilia, which extend into the coelomic space and beat irregularly. They do not enter the orifice and, thus, are not part of the internal ciliary flame. Posteriorly, in D. digitata, the mantle cell originates from the septal wall, i.e., its extensions spread in the plane of the frontal coelothelium of the septum. In N. variabilis, the mantle cell is continued by a further cell, enwrapping the posterior region of the preseptal part. This cell, called the septal cell, is anchored in the septal wall like the mantle cell in D. digitata. Both cells are interpreted as mesodermal components of the metanephridium. The flame cell lies beneath the mantle cell. In front, on its dorsal wall, many cilia are inserted which extend posteriorly into the nephridial canal forming a flame. In D. digitata, the caudal extension of this cell was examined in more detail; it originates from an intraseptal position. The canal cell lines the anterior lumen of the nephridial duct. While the mantle cell and flame cell enclose the organ from a dorsal position, the canal cell lies opposite embracing the lumen from a ventromedial position. Behind, it extends into the postseptal region for a certain distance. It is concluded that metanephridia in the Clitellata have a coelothelial component and, probably, are not just descendants of a single cell, the nephridioblast. The results further indicate that a flame cell and a mantle cell or some corresponding coelothelial cells may be constitutive elements of the ground plan of the clitellate metanephridium. Phylogenetic consequences for non-clitellate Annelida are discussed. Accepted: 21 December 1999     

THE FUNCTIONAL ORGANIZATION OF FILTRATION NEPHRIDIA

(1) Based on the classical studies of Goodrich, protonephridia are believed to be phylogenetic antecedents of metanephridia. It is argued here that the primary factor determining the type of nephridium expressed is body size rather than phylogenetic status. (2) The proposed model defines a nephridium functionally and predicts two general configurations for filtration nephridia in animals. (3) Application of the model to metanephridial and protonephridial systems indicates differences in the sites of ultrafiltration and mechanisms of pressure generation. (4) Metanephridial systems function by muscle-mediated filtration of vascular fluid into a coelomic space before modification by an excretory duct. (5) Protonephridial systems function by cilia-mediated filtration of extracellular fluid into the lumen of a protonephridial terminal cell before modification in an adjoining duct. (6) The model predicts a correlation between animals with blood vessels and metanephridia, and animals without blood vessels and protonephridia. The correlation is shown to be nearly perfect. (7) Exceptions to the model are discussed. (8) Original experimental evidence is given for the permeability of the protonephridial terminal cell to iron dextran and its reabsorption by the protonephridial duct in the polychaete, Glycera dibranchiata. (9) Experimental data for proto- and metanephridial systems are summarized and shown to support the proposed model. (10) The ultrastructure of the exceptional amphioxus ‘protonephridium’ is reviewed and original data are presented. Its organization is structurally and perhaps functionally intermediate between proto- and metanephridial systems. (11) An original ultrastructural comparison is made of monociliated nitration cells in a size range of larval invertebrates from five phyla. Filtration cells that are structurally intermediate between protonephridial solenocytes and metanephridial podocytes are noted in larvae intermediate in body size between the two extremes. The comparative data suggest that (i) podocytes and solenocytes are homologous cells and (ii) that body size is correlated with which of the two designs is expressed. (12) The fates of larval podocytes are followed through metamorphosis in three species. The results confirm the equivalence of podocytes and solenocytes as suggested by the comparative analysis. They further indicate that which morph is expressed is a function of body design factors discussed in the model. (13) Protonephridia are believed to be primitive to metanephridia because they occur in presumably primitive animals and in ontogenetic stages of many animals with metanephridia as adults. It is suggested here that the distribution of protonephridia is related to small body size and the lack of blood vessels, regardless of phylogenetic status. The occurrence of protonephridia in the larvae of species with metanephridia as adults is explained similarly as a function of the small larval size and lack of blood vessels.     

Ultrastructure of the nephrostome in Enchytraeus albidus (Annelida, Clitellata)

 The nephrostome of Enchytraeus albidus exhibits a longitudinal slit-like opening on the dorsal side of a bulbous organ which is mainly composed of three cells, one flame cell situated centrally and endowed with a ciliary flame, and two cells lying superficially, called the mantle cell and the accessory mantle cell. The mantle cell occupies the ventral side of the organ extending on both sides up to the opening to constitute its immediate border on the frontal and lateral sides. Here it forms a kind of crest which is heavily subdivided into many protrusions and infoldings endowed with long cilia which exclusively spread into the coelomic cavity. The accessory mantle cell borders the narrow posterior slit of the opening, forming the roof of the initial canal which is devoid of cilia. From its anterior region a projection arises extending above the opening. The flame cell forms a groove from which the ciliary flame arises which extends into the canal. At its posterior region it replaces the accessory mantle cell displacing it onto the dorsal surface of the organ. It is argued that the mantle cell and the accessory mantle cell have presumbly originated from coelothelial cells. Thus the metanephridium may be a composite organ developing from a nephridioblastic and a coeloblastic source. A discussion of results in other annelid species indicates that metanephridia in the Annelida may have evolved more than once. Accepted: 13 October 1997     

Protonephridial organs in Myzostoma cirriferum (Myzostomida)

Myzostoma cirriferum Leuckart, 1836 possesses five paired, serially arranged, blindending nephridial organs which are described for the first time. Ultrastructural investigations reveal that each nephridium is composed of three terminal cells and one tubular cell that forms the emission tubule. The central lumen of the individual terminal cells contains six to nine flagella, each of which is surrounded regularly by cytoplasmic rods arranged in parallel. Weir-like fenestrations in the peripheral wall of the terminal cells make up the connection between the central lumina and the extracellular space around the nephridial organ. The canal of the emission tubule possesses cilia, microvilli and cytoplasmic structures, suggesting involvement of this cell with active transport and storage. It opens into the cuticle at the ventral surface of the animal.     

Permeability of the podocytes of Lumbricus terrestris (Annelida, Oligochaeta) to different markers

 The distribution of different injected markers between blood vessels and the coelomic cavity of Lumbricus terrestris was investigated by light and electron microscopy in order to show the direction of filtration and the permeability of the basement membrane of podocytes. The present results revealed that ultrafiltration takes place across the ventral vessel as well as through the peri-intestinal blood sinus of the typhlosolis. Furthermore, the filtration processes seem to be restricted to the front part of the body. Fluorescein isothiocyanate (FITC) [molecular weight (MW) 389.4 Da], Procion yellow (MW 873 Da), FITC-labelled dextrans (MW 39 kDa) and gold particles up to a diameter of 10–12 nm passed the podocytes. Evans blue (MW 960.8 Da) could not permeate through the podocytes. The injected gold particles were found inside the extracellular channels of the podocyte, between the microvilli-like processes of the podocyte and on the coelomic side of the peritoneal epithelium. The appearance of gold particles in the previously described structures indicated that filtration takes place from the lumen of the ventral vessel to the coelomic cavity. Accepted: 21 October 1996     

Early development of metanephridia in the caudal budding zone of a clitellate annelid, Dero digitata (Naidida): an electron-microscopical study

Ultrastructural analysis has revealed that metanephridia in Dero digitata arise from three nephroblast cells in the frontal epithelium of a septum suggesting its mesodermal origin. Each cell has a fixed developmental destination, one nephroblast cell produces the entire canal part and two cells give rise to the nephrostome. The nephroblast cell nearest to the body wall enlarges and proliferates a first set of canal cells, then one of the two proximally adjacent nephroblast cells differentiates into the envelope of the nephrostome generating the marginal cilia of the opening (mantle cell) and the second one transforms into the anteriormost cell of the funnel, producing a flame of cilia that beats into the canal lumen (flame cell). Thereafter, new canal cells appear, mainly by mitosis of the first cell, enlarging the body of the nephridium whose further differentiation was not analysed. Comparison with other clitellate species suggests a mantle cell (or some marginal cells) and a flame cell (or a central cell) to be special characters of the metanephridium in the stem species of the Clitellata and that, compared to many polychaete species, its early development assumes a special course by a precocious determination and arrangement of nephroblast cells, which, in both groups, probably originate from an identical mesodermal stem cell. Results further indicate that the nonclitellate Aeolosomatidae, by virtue of corresponding nephrostomata, are possibly closer related to the Clitellata.     

The Nephridium of the Bonellia viridis Male (Echiura)

The nephridium of the dwarf male of Bonellia viridis was investigated by means of transmission electron microscopy. The nephridium proved to be of a distinct protonephridial type and not a metanephridium as maintained in the older literature. The nephridium is composed of a ciliated duct that projects into the coelom. Five crown cells at the end of the duct function as terminal filtration cells. Each crown cell has a bundle of about 20 cilia, surrounded by a labyrinthic weir of cell processes that are presumably involved in filtration. The ciliary bundles enter the nephridial duct through perforations of the adjacent tubule cells. This finding of a protonephridium in a minute, coelomate animal that lacks a circulatory system corroborates a recently formulated functional theory on the distribution of nephridial types.     

Ultrastructure and significance of the transitory nephridia in Erpobdella octoculata (Hirudinea, Annelida)

In early developmental stages of Erpobdella octoculata two pairs of transitory nephridia occur which degenerate during the formation of the body segments. Because in the ground pattern of Annelida the first nephridia formed during ontogenesis are protonephridia, it can be assumed that the transitory nephridia of E. octoculata are homologous to the larval protonephridia (head kidneys) of Polychaeta. To test this hypothesis two cryptolarvae of E. octoculata were investigated ultrastructurally. Both pairs of transitory nephridia are serially arranged to either side of the midgut vestigium. Each organ consists of a coiled duct that opens separately to the exterior by an intraepidermal nephridiopore cell. The duct is percellular and formed by seventeen cells. Adluminal adherens and septate junctions connect all duct cells; the most proximal duct cell completely encloses the terminal end of the duct lumen. A filtration structure characteristic for protonephridia is lacking. Additionally, the entire organ lacks an inner ciliation. Morphologically and ultrastructurally the transitory nephridia of E. octoculata show far reaching congruencies with the segmental metanephridia in different species of the Hirudinea. These congruencies support the assumption that formation of transitory nephridia and definitive metanephridia in Hirudinea depends on the same genetic information. The same inherited information is assumed to cause the development of larval head kidneys and subsequently formed nephridia in different species of the Polychaeta. Thus, the presumed identical fate of a segmentally repeated nephridial anlage supports the hypothesis of a homology between the transitory nephridia in Hirudinea species and the protonephridial head kidneys in the ground pattern of the Polychaeta. We, therefore, assume that functional constraints lead to a modification of the protonephridial head kidneys in Hirudinea and explain ultrastructural differences between the transitory nephridia in Hirudinea and the protonephridia in Polychaeta. Accepted: 11 December 2000     

Structure of the nephridium in Ophryotrocha puerilis (Polychaeta,Dorvilleidae)

Summary The nephridia of Ophryotrocha puerilis are segmental organs. The nephrostome opens at the posterior margin of a setigerous segment into the coelomic cavity of this segment. The nephridial canal is made up of about 15 cells. These cells form an S-shaped tubule which extends into the following segment. The lumen of the nephridial canal ranges from 2 to 7 m in diameter. The nephropore opens laterally on the ventral surface of the body wall.In cross sections, one, two, or three cells are seen forming the canal. The inner surfaces of the canal cells are of different appearances along the canal. Since no regular pattern of cell distribution was found along the canals of different nephridia it is assumed that changes in cell structure along the canal are due to functional states or properties rather than to anatomically fixed regional differences. The canal cells either show smooth contours or they form brush borders of microvilli or sponge-like inner surfaces with a system of vacuolar canals running through the cytoplasm. Most of the canal cells are filled with various kinds of vesicles. Usually two or three cells contain larger vesicles up to 2.5 m in diameter with more or less electron-dense contents. Some of these vesicles resemble lysosomes. There are at least three bundles of cilia in each canal. In young specimens the number of cilia in one bundle is smaller (10–15) than in adult specimens (60–70). The nephridia do not show sex specific differences. The female nephridia do not function as genital ducts. As judged from the sizes of sperm and nephridia it appears to be possible that sperm are shed via male nephridia.     

Ultrastructure of the lycophora larva of Gyrocotyle urna (Cestoda,Gyrocotylidea)

Summary The ultrastructure of the protonephridial system of the lycophore larva of Gyrocotyle urna Grube and Wagener, 1852, is described. It consists of six terminal cells, at least two proximal canal cells, two distal canal cells and two nephridiopore cells. The terminal cells and the proximal canal cell build up the filtration weir with its two circles of weir rods. The proximal canal cell constitutes a solid, hollow cylinder without a cell gap and desmosome. The distal canal cell is characterized by a strong reduction of the canal lumen by irregularly shaped microvilli. The nephridiopore region is formed by a nephridiopore cell; its cell body is located at some distance proximally within the larva. The connection among different canal cells is brought about by septate desmosomes. Morphological, evolutionary and functional aspects of the protonephridial system within Platyhelminthes are discussed. The structure of the proximal canal cells without a desmosome is considered an autapomorphy of Cestoda.Abbreviations ci cilia of the terminal cell - Co distal canal cell - col lumen of the distal canal cell - Ep epidermis - er outer rods of the filtration weir - il inner leptotriches - ir inner rods of the filtration weir - ld lipid droplets - mt microtubule - mv microvilli - Nc nephridiopore cell - Ne neodermis anlage cells - nu nucleus - pC proximal canal cell - ro ciliary rootlets - sd septate desmosome - Tc terminal cell     

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.     

Ultrastructure du rein labial céphalique deCampodea chardardi Condé (Diplura,Insecta)

Résumé L'ultrastructure du rein labial céphalique deCampodea chardardi Condé a été étudiée. Cette néphridie comprend trois parties: le saccule terminal, le labyrinthe et le canal excréteur.Les cellules du saccule sont des podocytes typiques, contenant de nombreuses vacuoles de pinocytose et des inclusions diverses. La lumière est envahie par des micro-organismes bacilliformes.Le labyrinthe possède des cellules à indentations basales profondes avec de nombreuses mitochondries, et des microvilli distaux.Le canal excréteur débouchant sur la face ventrale du labium est caractérisé par la présence d'une intima cuticulaire.Le rein labial des Diploures a été comparé avec des organes segmentaires néphridiens d'autres Arthropodes, et avec le néphron des Vertébrés.

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The fine structure of the labial cephalic kidney ofCampodea chardardi Condé (diplura, insecta)

Summary The ultrastructure of the cephalic labial kidney ofCampodea chardardi Condé has been studied. Each nephridium is subdivided into three segments: end-sac, coiled tubule and efferent-duct.The cells of the sacculus are typical podocytes, and contain numerous pinocytotic vesicles and various vacuoles. The lumen contains micro-organisms.The cells of the coiled tubule bear basal infoldings with numerous mitochondria and distal microvilli.The efferent duct terminates close to the ventral face of labium, and possesses characteristic cuticular intima.The labial kidney of Diplura is compared with published data on the nephridial organs of other Arthropods and Vertebrate nephron.

Nous tenons à remercier Monsieur le Professeur Noirot pour ses encouragements et conseils.     

Ultrastructure and functional morphology of the protonephridia and segmental fenestrated lacunae inProtodrilus rubropharyngeus (Polychaeta,Protodrilidae)

The protonephridia ofProtodrilus rubropharyngeus are described. They consist of a terminal cell, one nephridiopore cell, and different types of duct cells (proximal, medial, distal) with the duct running intracellularly. Reabsorption takes place in the duct by means of very unique lamellar foldings. An interesting characteristic of the nephridial system inP. rubropharyngeus is the presumed double filtration of the primary urine that occurs in the walls of both the lateral blood lacunae and the terminal cell. The structure of excretory organs in relation to the particular coelomatic conditions found in different groups of polychaetes is discussed.     

Podocytes in bivalve molluscs: Morphological evidence for ultrafiltration

Summary In representatives from a survey of three major taxa of bivalves the pericardial glands were found in two distinct positions. In protobranches (Acila castrensis) and filibranch bivalves (Glycymeris subobsoleta, Chlamys hastata, Pecten caurinus, Placopecten magellanicus, Mytilus edulis andMytilus californianus) the pericardial glands are located on the auricular surface. In heterodonts (Mercenaria mercenaria, Clinocardium nuttallii andMya arenaria) the pericardial glands are found in an anterodorsal position to the pericardial cavity.The sites of ultrafiltration are described. They consist of podocytes with basally extending pedicels forming an interdigitating network apposed to a basal lamina. Other characteristics of this ultrafiltration barrier described are anionic sites on the basal lamina and presence of substructural components within the ultrafiltration slits between pedicels.The pathway for the ultrafiltrate in protobranchs and filibranchs is from the hemocoel through the basal lamina, through the ultrafiltration slits of the pedicel network, into the urinary spaces between the podocyte cell bodies and into the pericardial cavity. The pathway for the ultrafiltrate in heterodonts is from the hemocoel through the basal lamina, through the ultrafiltration slits of the pedicel network, into urinary spaces between the podocyte cell bodies, into the lumen of the pericardial gland tubules and into the pericardial cavity.All podocyte cells have electron dense granules, Golgi apparatus and vacuoles associated with their cytoplasm. Heterodont species have microvilli on the cell surfaces of the podocytes apposed to urinary spaces.In all cases the morphological sites of ultrafiltration were associated with the pericardial glands of the heart-pericardial complex.     

Nephridial development and body cavity formation in <Emphasis Type="Italic">Artemia salina</Emphasis> (Crustacea: Branchiopoda): no evidence for any transitory coelom

The Ecdysozoa-hypothesis on the origin of arthropods questions the homology of segmentation in arthropods, onychophorans, and annelids. The implication of convergent gain of metamery in these groups seems to conflict particularly with the correspondence in the development of serial coelomic cavities and metanephridia. Ultrastructural studies of the mesoderm development in Onychophora revealed that main correspondence with the state in annelids concerns the involvement of epithelial lining cells of the embryonic coelomic cavities in the formation of the visceral and somatic musculature. The significance of this correspondence, however, remained unclear as comparable data on the state in arthropods were still missing. Developmental studies on selected representatives covering all major arthropod subgroups aim to fill in this gap. Data were raised by a combination of transmission electron microscopy and fluorescent stainings of the muscular system and nuclei for the anostracan crustacean Artemia salina. In this species, putative transitory coelomic cavities proved to be absent in all trunk segments. In the second antennal and second maxillary segments small, compact nephridial anlagen develop into a sacculus and excretory duct. The sacculus originates from the terminal cells of the nephridial duct, which is formed in advance. The lumen of the sacculus is inconspicuous in its earliest functional stage and later enlarges to a bulb; it accordingly represents no remnant of any primarily large coelomic cavity. The muscular system is entirely formed prior to and independent of coelomic or nephridial anlagen. Visceral and somatic mesoderm already separate in the caudal body region. Transitory segmental clusters of mesodermal cells are composed of somatic cells only and accordingly represent no “somites”. Our observations overall do not provide any support for the homology of coelomic cavities in annelids and arthropods.     

Structure and development of nephridia in Annelida and related taxa

Two different kinds of filtration nephridia, protonephridia and metanephridia, are described in Polychaeta. During ontogenesis protonephridia generally precede metanephridia. While the latter are segmentally arranged, protonephridia are characteristic for the larva and are the first nephridial structure formed during ontogenesis. There is strong evidence that both organs depend on the same information and that their specific structure depends on the way in which the coelom is formed and which final expansion it gains. While metanephridia are regarded to be homologous throughout the polychaetes, protonephridia seem to have evolved in several lineages. Some of the protonephridia closely resemble less differentiated stages of metanephridial development, so that protonephridial evolution can be explained by truncation of the metanephridial development. Nevertheless, structural details are large enough to allow us to expect information on the polychaete evolution if the database on polychaete nephridia increases. A comparison of the polychaete metanephridia with those of the Clitellata and Sipuncula reveals some surprising details. In Clitellata the structure of the funnel is quite uniform in microdrilid oligochaetous Clitellata and resembles that of the aeolosomatids. Like the nephridia in the polychaete taxa Sabellida and Terebellida, those of the Sipunucla possess podocytes covering the coelomic side of the duct.     

Ultrastructure of the protonephridia in Pycnophyes kielensis (Kinorhyncha,Homalorhagida)

Summary Pycnophyes kielensis possesses one pair of protonephridia. The single excretory organ of a female consists of 25 cells: 22 terminal cells, 2 canal cells, and 1 nephroporus cell. Generally, all cells exhibit two cilia, the only exception being the nephroporus cell, which contains a diplosome instead. The slashed peripheral cytoplasmic walls of the 22 terminal cells altogether constitute one compound filter and a common filtration area. The protonephridia discharge via cuticularized cavities and six cuticularized tubes. Two accessory cells with modified cilia penetrate the nephroporus cell. These cells are considered to be receptor cells. The protonephridium of the first juvenile stage of P. kielensis is built up of only 5 cells: 3 terminal cells, 1 canal cell, and 1 nephroporus cell. It opens to the outside via 1 cuticularized tube. The protonephridia within both the Kinorhyncha and the Bilateria are discussed. Presumably excretory organs with compound filters developed independently within Bilateria.Abbreviations bb basal body - c canal cell - ca cuticularized cavity - ci cilium - cu cuticle - d dictyosome - de desmosome - di diplosome - dl dorsal longitudinal muscle - dv dorsoventral muscle - ecm extracellular matrix - ep epidermal cell - ex excretory organ - fc filter cleft - fi filter - fm fastening muscle cell - he hemidesmosome - i intestine - if intercellular fluid - m mitochondrium - mv microvilli - n nephroporus cell - nu nucleus - r ciliary rootlet - re accessory cell (presumable receptor cell) - sj septate junction - t terminal cell - tu cuticularized tube - v vesicle - w peripheral cytoplasmic wall