Ultrastructure of protonephridia in Xenotrichula carolinensis syltensis and Chaetonotus maximus (Gastrotricha: Chaetonotida): comparative evaluation of the gastrotrich excretory organs

In an attempt to obtain detailed information on the entire protonephridial system in Gastrotricha, we have studied the protonephridial ultrastructure of two paucitubulatan species, Xenotrichula carolinensis syltensis and Chaetonotus maximus by means of complete sets of ultrathin sections. In spite of some differences in detail, the morphology of protonephridia in both examined species shows a common pattern: Both species have one pair of protonephridia that consist of a bicellular terminal organ, a voluminous, aciliar canal cell and an adjacent, aciliar nephridiopore cell. The terminal organ consists of two monociliar terminal cells each with a distal cytoplasmic lobe. These lobes interdigitate and surround cilia and microvilli of the terminal cells. Where both lobes interdigitate, a meandering cleft is formed that is covered by the filtration barrier. We here term the entire structure composite filter. The elongated, in some regions convoluted protonephridial lumen opens distally to the outside via a permanent nephridiopore. A comparison with the protonephridia of other species of the Gastrotricha allows hypothesising the following autapomorphies of the Paucitubulata: The bicellular terminal organ with a composite filter, the convoluted distal canal cell lumen and the absence of cilia, ciliary basal structures and microvilli within the canal cell. Moreover, this comparative survey could confirm important characteristics of the protonephridial system assumed for the ground pattern of Gastrotricha like, for example, the single terminal cell with one cilium surrounded by eight microvilli.     

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     

Development of the excretory system in the polyplacophoran mollusc,Lepidochitona corrugata: The protonephridium

A single pair of protonephridia is the typical larval excretory organ of molluscs. Their presence in postlarval developmental stages was discovered only recently. We found that the protonephridia of the polyplacophoran mollusc, Lepidochitona corrugata, achieve their most elaborate differentiation and become largest during the postlarval period. This study describes the protonephridia of L. corrugata using light and electron microscopy and interactive three‐dimensional visualization. We focus on the postlarval developmental period, in which the protonephridia consist of three parts: the terminal part with the ultrafiltration sites at the distal end, the voluminous protonephridial kidney, and the efferent nephroduct leading to the nephropore. The ultrafiltration sites show filtration slits between regularly arranged thin pedicles. The ciliary flame originates from both the terminal cell and the duct cells of the terminal portion. The efferent duct also shows ciliation. The most conspicuous structures, the protonephridial kidneys, are voluminous swellings composed of reabsorptive cells (“nephrocytes”). These cells exhibit strong vacuolization and an infolding system increasing the basal surface. The protonephridial kidneys, previously not reported at such a level of organization in molluscs, strikingly resemble (metanephridial) kidneys of adult molluscan excretory systems. J. Morphol., 2011. © 2011 Wiley‐Liss, Inc.     

Ultrastructure of the protonephridia of larval Rugiloricus cf. cauliculus, male Armorloricus elegans, and female Nanaloricus mysticus (Loricifera)

The protonephridial system of several Loricifera was studied by transmission electron microscopy. A larval specimen of Rugiloricus cf. cauliculus possesses two protonephridia, which are "capped" frontally by a compact mass of still undifferentiated gonadal cells. Each protonephridium consists of four monociliary terminal cells and four canal cells with a diplosome but no cilia. Because of incomplete series of sections and unsatisfactory fixation, the outleading cell(s) could not be detected. In a male specimen of Armorloricus elegans, each gonad contains two protonephridia that open into the gonadal lumen. Each protonephridium consists of two monociliary terminal cells, each forming a filter, two nonciliated canal cells, and two nephroporus cells. The protonephridial lumina of the latter cells fuse to one common lumen, which unites with the gonadal lumen. Preliminary observations on the protonephridia of a female Nanaloricus mysticus reveal a more complicated arrangement of interdigitating terminal and canal cells. One or two terminal cells form their own individual filter or four cells form a common compound filter. The cilium of the terminal cells of all species investigated are surrounded by a palisade of nine microvilli that support the filter barrier made of an extracellular matrix. An additional filter diaphragm could be traced between the pores in the cell wall of each terminal cell of A. elegans. The urogenital system of the Loricifera differs from that of the Priapulida in that the protonephridia of the former are completely integrated into the gonad, whereas the excretory organs of the latter open into the urogenital duct caudally of the gonads.     

Ultrastructure of the protonephridia of Seison annulatus (Rotifera)

Summary Each of the two protonephridial systems of Seison annulatus consists of three sections which are separated by cell borders with septate junctions: (a) a terminal syncytium with eight terminal organs and a capillary canal, (b) a canal syncytium which is divided into a multiciliary canal region and a main canal region, and (c) a nephroporus cell. The terminal syncytium is branched and linked twice to the canal syncytium. The supporting structure of each filtration barrier is a hollow cylinder which is perforated by pores and lacks microvilli (pillars). A protonephridial spine is situated in the multiciliary canal region and stabilizes the neck region. The ored, hollow cylinder and the protonephridial spine are new characteristics for the Rotifera.     

Ultrastructure of the nephridio-circulatory connections in Tubulanus annulatus (Nemertini,Anopla)

Summary The protonephridial terminal organs in the nemertean Tubulanus annulatus form an integral part of the blood vessel wall. Both endothelial and muscle-cell layers of the vessel's wall are discontinued at the site of each terminal organ. The terminal organs are usually composed of from one to three terminal cells enclosing a central lumen provided with many microvilli and separated from the blood vessel's lumen by a membranous filtration area. The latter is perforated by numerous winding clefts formed by interdigitation of minute cytoplasmic pedicels arising from processes issued by each of the involved terminal cells. Ultrafiltration of blood plasma takes place across a filtration membrane which spans the cleft system and the basal lamina of the terminal cells. Fluid is propelled into the lumen of the terminal organs through the activity of ciliary bundles, one for each terminal cell involved, perhaps supplemented by vascular turgor. All efferent conduits of the protonephridium have profuse infoldings of the luminal cell surfaces and/or numerous pinocytotic pits suggestive of reabsorption of substances from the primary urine.Abbreviations BL basal lamina - C cilium - CP coated pit - CT collecting tubule - CV inzcoated vesicle - D dictyosome - E endothelial cell - F fenestration of endothelial cell - FA filtration area - FM filtration membrane - G glycogen granule - LV lateral vessel - M mitochondrion - MC muscle cell - MV microvillus - N nucleus of terminal cell - NE nucleus of endothelial cell - NP nephridiopore - PC protonephridial capillary cell - PT protonephridial tubule - R rootlet - TC terminal cell     

Ultrastructural observations of the protonephridia of Polymerurus nodicaudus (Gastrotricha: Paucitubulatina)

Kieneke, A. and Hochberg, R. 2012. Ultrastructural observations of the protonephridia of Polymerurus nodicaudus (Gastrotricha: Paucitubulatina). —Acta Zoologica (Stockholm) 93 : 115–124. We studied different regions of the protonephridia of the limnic gastrotrich Polymerurus nodicaudus by means of light and electron microscopy to determine how freshwater species might differ from their marine relatives. Microscopic and ultrastructural characters are in accordance with another limnic species of Paucitubulatina, Chaetonotus maximus, whose protonephridial system has been previously reconstructed. Shared protonephridial characters of both species include the presence of highly elongate terminal organ cilia, microvilli, and the canal cell lumen as well as the presence of a conspicuous anterior loop of the protonephridial lumen. These features are not present in representatives of earlier, marine, paucitubulatan lineages (i.e., Xenotrichulidae) and so are assessed as evolutionary novelties that were likely important for the successful colonization of the freshwater environment.     

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     

Feinstruktur der Protonephridien von Paromalostomum proceracauda (Plathelminthes,Macrostomida)

Zusammenfassung Die Protonephridien von Paromalostomum proceracauda bestehen aus je einem Terminalkomplex und einem ausleitenden Kanal. Jeder Terminalkomplex setzt sich aus drei multiciliären Terminalzellen mit jeweils separatem Filtrationsapparat zusammen; die Zellen sind gestaffelt hintereinander angeordnet und bilden ein gemeinsames Reusenlumen. Der Nephridialkanal, der nicht an der Ultrafiltration, sondern nur an Resorptionsvorgängen beteiligt ist, besteht aus mindestens zwei röhrenförmigen, hintereinander liegenden ciliären Zellen. Die jeweils letzte Kanalzelle bildet auch den Nephridialporus. Proximal sind die Protonephridien bis zur Basis der Epidermis vollständig von einer interzellulären Matrix umhüllt.

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Fine structure of the protonephridia of Paromalostomum proceracauda (Plathelminthes, Macrostomida)

Summary The protonephridia of Paromalostomum proceracaud consist, respectively, of a terminal complex and a draining canal. Each terminal complex is composed of three multiciliary terminal cells with separate filtration apparatuses; the cells are staggered, forming a joint basket lumen. The nephridial canal consists of two or more tube-shaped ciliary cells, which are arranged in series; these cells do not participate in ultrafiltration but in resorption processes. The last canal cell also forms the nephroporus. Up to the epidermis, the protonephridia are proximally surrounded by an intercellular matrix.

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Abkürzungen cw Cilienwurzel - ep Epidermiszelle - i Interzellularsubstanz - k Kanalzelle - kl Kanallumen - l Leptotrichien - m Muskulatur des Hautmuskelschlauches - n Kern einer Terminaloder Kanalzelle - r Reusenapparat - rl Reusenlumen - rs Reusenstab - t1, 2, 3 Terminalzelle 1, 2, 3 - v Vakuole     

Ultrastructure and development of the nephridia in Anaitides mucosa (Annelida,Polychaeta)

Different developmental stages (trochophores, nectochaetae, non-mature and mature adults) of Anaitides mucosa were investigated ultrastructurally. A. mucosa has protonephridia throughout its life; during maturity a ciliated funnel is attached to these organs. The protonephridial duct cells are multiciliated, while the terminal cells are monociliated. The single cilium is surrounded by 14 microvilli which extend into the duct lumen without coming into any contact with the duct cells. Corresponding ultrastructure and development indicate that larval and adult protonephridia are identical in A. mucosa. Differences between various developmental stages can be observed only in the number of cells per protonephridium. A comparison between the funnel cells, the cells of the coelothel and the duct cells reveals that the ciliated funnel is a derivative of the duct. Due to the identical nature of the larval and postlarval protonephridia, such a funnel cannot be a secondary structure. In comparison with the mesodermally derived metanephridial funnel in phoronids it seems likely that the metanephridia of annelids and phoronids evolved convergently.     

Investigations on the protonephridial system of post-larval Gyrocotyle urna and Amphilina foliacea (Cestoda).

The gross morphology and ultrastructure of the different parts of the protonephridial system of the monozoic tapeworms Gyrocotyle urna and Amphilina foliacea are described. The terminal cell in both species has numerous cilia which are interconnected and extend into the lumen of the first canal cell. The filtration area is built up from projections of two cells, the terminal cell and the first canal cell. The first canal cell forms a solid hollow cylinder without a cell gap and a desmosome as found in Neodermata other than cestodes and Udonella. In Gyroctyle the nucleus of the first canal cell is located in the wall cytoplasma whereas more distally located ductules of both species have subepithelial cell bodies containing the nuclei. In both taxa the protonephridial canal system is reticulate. In Amphilina the distal canals lack non-terminal ciliary flames, such ciliary tufts can be found in the larger capillaries of Gyrocotyle. The capillary cilia have rootlets and the ultrastructure of the duct wall cytoplasm containing large numbers of vesicles indicates highly active transport processes. The morphology of the protonephridial systems is discussed with regard to the evolution of Neodermata (especially of the Cestoda) and the function of the protonephridial system in cestodes as a probable organ of nutrient distribution.     

Ultrastructure of the protonephridial system in Neodasys chaetonotoideus (Gastrotricha: Chaetonotida) and in the ground pattern of Gastrotricha

The taxon Neodasys has a basal position within Gastrotricha. This makes it very interesting for phylogenetic considerations in this group. To complete the reconstruction of the nephridial system in the stem species of Gastrotricha started earlier, we have studied the whole protonephridial system of Neodasys chaetonotoideus by means of complete sets of ultrathin sections and TEM. In many characters, protonephridia of N. chaetonotoideus resemble those of macrodasyidan gastrotrich species. For example, each of the six protonephridia, arranged in three pairs, consists of three distinct cells that constitute the continuous protonephridial lumen. Especially, the terminal cell of the protonephridia of N. chaetonotoideus shows a striking pattern: The perforation of the filter region is a meandering cleft that is continuous with the seam of the enfolded lumen of that cell. With the results presented here and that of former TEM studies, we give a comprehensive idea of the excretory organs in the ground pattern of Gastrotricha. Moreover, we can elaborate on the hypothesized protonephridial system in the stem species of Bilateria. We suggest that a meandering filtration cleft is a feature of the ground pattern of the Bilateria.     

The Protonephridia of the Arctic Kinorhynch Echinoderes aquilonius (Cyclorhagida,Echinoderidae)

The cyclorhagid kinorhynch Echinoderes aquilonius Higgins & Kristensen, 1988 possesses a single pair of protonephridia located in segments 10 and 11. The protonephridia consist of: (1) three terminal cells T-1, T-2. T-3, each with two cilia; (2) a single non-ciliated canal cell; (3) a nephridiopore cell with many microvilli and a cuticular sieve plate. The protonephridia of Echinoderes are presumed to develop from the ectoderm near the area of the sieve plate on the eleventh segment, and are suspended in the dorso-lateral pseudocoelomic cavity where they are surrounded by a basal lamina. One of the terminal cells (T-1) secondarily penetrates the basal lamina of the tenth segment and a part of the cell attaches to the cuticle. The kinorhynch protonephridia are compared with the excretory organs of other Bilateria. expecially the ‘aschelminths’, and apomorphic characters of the kinorhynch protonephridia are defined.     

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     

扩张莫尼茨绦虫(绦虫纲:圆叶目)的原肾管及原肾管概念的评述

本研究应用透射电子显微镜研究了扩张莫尼茨绦虫原肾管的细胞学特征 ,莫尼茨绦虫原肾管的焰茎球为一个过滤器结构 ,类似于“挡河坝”样构造 ,此构造由端细胞和近管细胞外突形成的肋条 (或称杆 )相互交错排列而成。肋条之间由细胞外物质构成的“膜”结构连接 ,过滤作用通过该“膜”发生。焰细胞与近管细胞交界处有裂缝或孔与细胞外的结缔组织 (实质组织 )相通 ;原肾管的毛细排泄管细胞质索之间没有隔状联结 ;毛细排泄管及排泄管的管腔内有大量珠状微绒毛突起以增加表面积。从扩张莫尼茨绦虫及其它一些无脊椎动物原肾管的研究结果表明 ,原原肾管概念将焰细胞作为封闭的盲端已不再合适 ,需要进行修订 ,建议修订为 :原肾管是一种焰细胞系统 ,通常由焰细胞、管细胞和肾孔细胞组成 ,焰茎球作为过滤装置与周围的结缔组织 (实质组织 )有或没有裂缝 (孔 )相通     

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.     

The excretory organs in Sphaerodorum flavum (Phyllodocida, Sphaerodoridae): a rare case of co-occurrence of protonephridia, coelom and blood vascular system in Annelida

The excretory organs of Sphaerodorum flavum (Sphaerodoridae) were investigated by TEM and reconstructed from serial ultrathin sections. These organs are segmentally arranged paired protonephridia, which are in close association with a well-developed blood vascular system. Each protonephridium consists of a terminal part made up of two monociliary terminal cells (solenocytes), and a nephridioduct, formed by two cells. The two solenocytes lie close together. Each cilium is surrounded by 12 microvillar rods projecting from the perikaryon of each solenocyte. These rods form a weir-like structure in the coelomic space. The distal part of the weir is embedded in the proximal nephridioduct. The largest part of the cell bodies of the solenocytes, containing the nucleus, is lateral or basal to the weir-like structures. The lumen of the nephridioduct is formed by two multiciliated cells, which enclose the extracellular nephridial canal one behind the other. The canal opens through the nephropore beneath the cuticle without penetrating the cuticle. Both nephridioduct cells are surrounded by a blood vessel, which is partially folded into several layers. The significance of a simultaneous occurrence of protonephridial excretory organs and a well-developed blood vascular system as well as coelomic cavities is discussed. The results of this investigation indicate a close relationship of Sphaerodoridae to Phyllodocidae instead of to Syllidae within the Phyllodocida. Accepted: 27 November 2000     

The protonephridial system of the tusk shell, Antalis entalis (Mollusca, Scaphopoda)

The development and microanatomy of the protonephridial system in larvae and postmetamorphic juveniles of Antalis entalis (Dentaliidae) have been examined by means of a semithin serial sectioning and reconstruction technique. One late larval stage has been additionally examined by transmission electron microscopy. The protonephridium appears during larval development and is reduced in the juvenile approximately 13 days after metamorphosis. This is the first unambiguous evidence of a protonephridium in a postlarval mollusc. When fully developed the protonephridium is unique in consisting of two cells only, a terminal cell (=cyrtocyte) and a duct-releasing cell with glandular appearance. The polyciliary terminal cell has several distinct ultrafiltration sites, resembling conditions in bivalve protonephridia. The large duct-releasing cell shows a very large nucleus probably reflecting polyploidy. Its basal infoldings and many mitochondria suggest metabolic activity, the cytoplasm is characterised by many distinct granules. The unique features of the scaphopod protonephridial system are compared with available data on the protonephridia of other molluscan classes. The finding gives additional evidence that protonephridia belong to the ground pattern of the Mollusca. Accepted: 22 January 2001     

The fine structure of the protonephridial system in the land nemertean Pantinonemertes californiensis (Rhynchocoela,Enopla, Hoplonemertini)

Summary The protonephridial terminal organ in the nemertean Pantinonemertes californiensis is composed of two cells that are similar in size and shape and are mirror images of each other. Basally in the organ the two cells combine to form a binucleate cytoplasmic mass. Apically they are intimately joined to form a subcylindrical thin-walled weir apparatus; this part is supported by two opposed cytoplasmic columns running the length of the weir region, one originating from each of the two cells, and by a number of regularly spaced circular bars that arise from the two columns. The ciliary flame consists of 94–114 cilia that originate in the bases of the two cells, and it is surrounded by a palisade of incomplete circlets of long, straight microvilli. The convoluted protonephridial tubule is rich in structures that indicate intensive reabsorption from the primary urine. It is argued that the terminal organs in Pantinonemertes and Geonemertes are fundamentally similar and differ only in the amount of microtubules present in the longitudinal supports.Abbreviations BL basal lamina - CF ciliary flame - CT connective tissue - CV coated vesicle - E endocytotic pit - FM filtration membrane - G Golgi complex - LC longitudinal cytoplasmic column - M mitochondrion - MT microtubules - MV microvilli - N nucleus - NPC nucleus of protonephridial capillary cell - PC protonephridial capillary cell - R rootlets - TB transverse bar - TC terminal cell - WE weir, exterior of fenestrated wall - WI weir, interior of same     

Ultrastructure and relationship between protonephridia and metanephridia in Phoronis muelleri (Phoronida)

Summary The actinotrocha of Phoronis muelleri has one pair of ectodermally derived, monociliated protonephridia. The duct runs mainly between the epidermis and the lining of the hyposphere coelom, pierces the septum and extends into the blastocoel. The proximal part is branched and closed up by terminal complexes consisting of two morphologically different cells which both serve filtration. During metamorphosis, the terminal complexes and the branches of the duct are cast off. The cells degenerate, pass into the remaining duct and are endocytosed by the duct cells. After metamorphosis the remaining part of the protonephridial duct is U-shaped, blindly closed and borders on the prospective lophophoral vessel. In a later stage the duct receives a ciliated funnel, which consists of monociliated epithelio-muscle cells and is a derivative of the lining of the metacoel. Thus, a part of the protonephridial duct of the larva and the whole metanephridial duct of the adult are identical. Aspects of a possible homology between phoronid nephridia and such organs in other bilaterians are discussed.