The Ultrastructure of the Protonephridium of the Actinotroch Larva (Phoronida)

The actinotroch larva of Phoronis muelleri has a pair of protonephridia located beneath the tentacle ring and draining the blastocoel; each protonephridium is composed of about 25 solenocytes and a nephroduct which opens in a nephropore on the ventral side of the metasome. The neck of the solenocytes consists of bars, mutually interconnected by a fenestration lamina. Inside the neck microvilli originate proximally in the proximal intrachoanal field and extend through the neck into the nephroduct. There is no canal cell. In cross section the nephroduct is composed of 5–7 monociliary cells, with the cilium protruding through a border of microvilli and extending into the nephroduct. The whole protonephridium is surrounded by a basal lamina. Comparisons of the actinotroch protonephridium with those of other groups have not revealed any convincing homologies. The protonephridia of the protostomians are all considered to be of ectodermal origin, while the cyrtopodocytes of Branchiostoma are mesodermal. The protonephridium of the actinotroch is ectodermal.     

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     

Vestigial prototroch in a basal nemertean, Carinoma tremaphoros (Nemertea; Palaeonemertea)

Nemerteans have been alleged to belong to a protostome clade called the Trochozoa that includes mollusks, annelids, sipunculids, echiurids, and kamptozoans and is characterized by, among other things, the trochophore larva. The trochophore possesses a prototroch, a preoral belt of specialized ciliary cells, derived from the trochoblast cells. Nemertea is the only trochozoan phylum for which presence of the trochophore larva possessing a prototroch had never been shown. However, so little is known about nemertean larval development that comparing it with development of other trochozoans is difficult. Development in the nemertean clade Pilidiophora is via a highly specialized planktonic larva, the pilidium, and most of the larval body is lost during a drastic metamorphosis. Other nemerteans (hoplonemerteans and palaeonemerteans) lack a pilidium, and their development is direct, forming either an encapsulated or planktonic "planuliform" larva, producing a juvenile without a dramatic change in body plan. We show that early in the development of a member of a basal nemertean assemblage, the palaeonemertean Carinoma tremaphoros, large squamous cells cover the entire larval surface except for the apical and posterior regions. Although apical and posterior cells continue to divide, the large surface cells cleavage arrest and form a contorted preoral belt. Based on its position, cell lineage, and fate, we suggest that this belt corresponds to the prototroch of other trochozoans. Lack of differential ciliation obscures the presence of the prototroch in Carinoma, but differentiation of the trochoblasts is clearly manifested in their permanent cleavage arrest and ultimate degenerative fate. Our results allow a meaningful comparison between the development of nemerteans and other trochozoans. We review previous hypotheses of the evolution of nemertean development and suggest that a trochophore-like larva is plesiomorphic for nemerteans while a pilidium type of development with drastic metamorphosis is derived.     

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 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     

The larval nephridia of the brackish-water polychaete,Nereis diversicolor

The larval nephridia of the brackish-water polychaete Nereis diversicolor are described for the first time, and have been studied to determine if their times of development and structural characteristics are consistent with a role in the osmotic regulation of the larva. As shown in serial paraffin sections and by interference-contrast optics, the nephridia of the three-setiger larva consist of a single pair of very large metanephridia, arising in the 3rd larval setiger, but with their elongated terminal ducts and coiled ciliated tubules pushed forward into the 2nd setiger; their open metanephrostomes and anterior anchoring filaments lie dorsal to the 2nd set of setae. In contrast, the definitive or juvenile metanephridia, arising in the 4th and subsequently formed setigerous segments, have short terminal ducts and coiled ciliated tubules confined to the segments on which their external nephropores open; their nephrostomes are ventrally located and open into the rear of the next anterior segment. These findings are in contrast to the claims of Edouard Meyer (1887), who described two pairs of closed protonephridia in the 2nd and 3rd larval setigers of Perinereis cultrifera. Although it is not excluded that the single larval pair of metanephridia of N. diversicolor may arise as protonephridia, Meyer's claim of two pairs of larval protonephridia was an observational error. The larval nephridia of the marine Platynereis dumerilii resemble in form, but are considerably smaller than, those of N. diversicolor. It is concluded that the hypertrophied pair of larval metanephridia of N. diversicolor is an evolutionary adaptation to existence in habitats of low and unpredictably varying salinity. Their development occurs irrespective of the prevailing salinity; hence, it must be genetically determined.     

The morphology of the apical organ and adjacent epithelium of pilidium prorecurvatum, a pelagic larva of an unknown heteronemertean (Nemertea)

The morphology of pilidia ex gr. recurvatum from Peter the Great Bay (Sea of Japan) was studied by confocal laser scanning and transmission-electron microscopy. The studied pilidium larvae differ from pilidium recurvatum in lacking a posterior ciliary ring and by the presence of a caudal tuft. On this basis, pilidium prorecurvatum is proposed as a new name for the lavae. The apical organ of pilidium prorecurvatum is represented by a thickened epithelium, which consists of uniform columnar monociliary collar cells and is innervated by a pair of serotonergic intraepithelial neurons. The bodies of the serotonergic neurons are located outside of the apical organ, but occasional axons were found at the organ base. The rest of the pilidial epithelium is represented by flattened polygonal multiciliated cells with sparse microvilli; the bodies of two neurons lie in the helmet epithelium immediately adjacent to the apical organ. Morphologically, the apical organ of the pilidium corresponds well to that of other lophotrochozoan larvae, but their homology remains unclear.     

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.     

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.     

The fate of the larval epidermis in the Desor-larva of <Emphasis Type="Italic">Lineus viridis</Emphasis> (Pilidiophora,Nemertea) displays a historically constrained functional shift from planktotrophy to lecithotrophy

Pilidiophora constitutes a clade of nemerteans characterized by a peculiar larval type, the pilidium. A characteristic of this larva is the transitory epidermis in which the juvenile develops from imaginal discs. The primary function of this larval envelope is assumed to be feeding and dispersal. When juvenile development is complete, the larval epidermis is ruptured and swallowed by the juvenile. According to recent cladistic and molecular analyses of the Nemertea, the intracapsular Desor-larva of the sibling species Lineus viridis and L. ruber is thought to have evolved from a pelagic pilidium. The general course of development has been demonstrated to be similar to that of the pilidium, in which the juvenile forms from imaginal discs under the larval epidermis. The two Lineus species, however, differ in their mode of larval feeding: L. ruber being ootrophic and L. viridis being lecithotrophic. In order to elucidate the transition from the planktotrophic pilidum to lecithotrophic development, I studied the early cleavage and metamorphosis from intracapsular Desor-larva to juvenile stages in L. viridis from the island of Sylt, using light microscopical, electron microscopical, and fluorescent staining methods. Due to the specific cleavage pattern with equally sized 1st quartet animal blastomeres and vegetal blastomeres in L. viridis, the larval epidermis later contains a considerable amount of the yolk reserve. During metamorphosis, the larval epidermis is ingested by the juvenile thus displaying behavior similar to that of the pilidium larva. In contrast to the pilidium, the function of the larval epidermis of the Desor-larva has shifted from feeding and dispersal to direct food supply. Thus, the development of L. viridis is a perfect example for strong historical constraints that prevent ancestral larval structures from being lost.     

Protonephridia and Metanephridia - their relation within the Bilateria

Two different kinds of nephridia occur within the Bilateria, protonephridia closed up by a terminal cell and metanephridia opening into the coelomic cavity. Both initially filter and subsequently modify intercellular fluids. Whereas metanephridia are strictly correlated to a coelom, proto-nephria occur in acoelomate as well as in coelomate organisms. Protonephridia of different bilaterian taxa correspond to each other in several structural features. Therefore, it is hypothesized that protonephridia are homologous organs throughout the Bilateria. They must have evolved once as one pair of monociliated organs orinatinng from the ectoderm and consistin of one terminal, one duct and one nephropore cell In the ground pattern of the Bilateria the cilium of the terminal cell has only one rootlet and is surrounded by resumably eight strengthened and elongated microvilli. Cilium and microvilli extend into the hollow cyinder of the terminal cell, which is oriented distally and is attached to the adjacent duct cell by desmosomes. This cylinder is perforated by clefts and represents the supporting structure of the filtration barrier consisting of extracellular matrix. In the Annelida and Phoronida, the metanehridia at the postlarval stages are ontogenetically preceded by protonephridia in the larva, but far reaching structural and developmental differ ences exist between the metanephridia of both. In horonids the rotonephrdial duct of the larva is retained in the postlarva and acquires a coelothelially derived funnel, whereas in annelids the metanephridia are uniform organs orihating from a solid anlage, which is a repetition of the protonehridial anlage of the larva. The differences contradict a homology of the metanephridia in Annegda and Phoronida. We therefore have to conclude that metanephridia must have evolved indeendently, at least two times. The comparative analysis of nephridia in the Bilateria allows the following hyothesis: Pro tonephridia were evolved in a monohasic acoelomate organism in the stem fineage of the Bilateria. During the evolution of biphasic life cycles consisting of an acoelomate larva and a coelomate adult, the information about the differentiation of protonephridia has been preserved in the early acoelomate developmental (larval) stages. During postlarval development and the formation of a coelom the protonephridia have either been retained or modified into meta nephridia. Accordin to the differences between the metanehridia of phoronids and annelids, we emphasize that. tiere is no possibility to trace back all bilaterian taxa with a coelom to a common stem species.     

No direct contact between the excretory system and the circulatory system in Prostomatella arenicola Friedrich (Hoplonemertini)

Excretory and circulatory systems in Prostomatella arenicola are examined at the ultrastructural level. Interdigitating cells, which rest on a thin fibrillar basal lamina, line the lumina of the lateral vessels. A layer of muscle cells and an underlying sheath of fibrillar extracellular material surround each vessel.The excretory system consists of one pair of laterally situated branched protonephridia. Each protonephridium is composed of several terminal cells, an efferent duct and a nephridiopore. The terminal parts of the protonephridia are not restricted to the vicinity of the circulatory system; they can also be found dorsally or laterally to the nerve cords between muscle cells. The presumed filtration area arises as a hollow cylinder from the terminal cell. This cylinder is perforated by numerous clefts which are never bridged by a filter diaphragm. Instead, each terminal cell cylinder is surrounded by an extracellular matrix. The terminal cells neither extend into the lumen of the lateral vessel nor contact the vessel lining cells.Phylogenetic implications of the results are discussed.     

Evolutionary Origin of the Vertebrate Nephron

SYNOPSIS. The primitive form of the vertebrate nephron consistsof a vascular nitration surface overlain with podocytes, a specializedcoelomic cavity to receive the ultrafiltrate, and a tubule formodification to final urine. Although previously thought tobe unique to the vertebrates, this design is now known to bewidespread among invertebrates, including most of the protochordates,and especially their larvae. Goodrich' rejection of the homologyof invertebrate nephridia and the vertebrate nephron, basedon a lack of germ-layer correspondence, is shown to be eitherunsupported by facts or logically dubious. Comparative morphologyof adult and larval invertebrates suggests that filtration excretoryorgans, as protonephridia and metanephridial systems, evolvedin the lineage to the bilaterally symmetrical animals and eachconsisted minimally of a filtration cell, a urinary compartment,and tubule joined to the exterior. Invertebrate metanephridialsystems and protonephridia are discussed as homologous structurescomposed of homologous cells (podocytes, terminal cells; alsonephrocytes). The ontogenetic and phylogenetic distributionof nephridia is correlated with body design, especially bodysize.     

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     

Rhogocytes in gastropod larvae: developmental transformation from protonephridial terminal cells

Rhogocytes, terminal cells of protonephridia, and podocytes of metanephridial systems share an architectural feature that creates an apparent sieving device. The sieve serves to ultrafilter body fluid during the excretion and osmoregulation process carried out by nephridial systems, but its function in rhogocytes is unclear. Rhogocytes are molluscan hemocoelic cells that appear to have various functions related to metabolism of metal ions, including synthesis of hemocyanin in some gastropods and metal detoxification in pteriomorph bivalves. A hypothesis that proposed developmental and possibly evolutionary conversion between protonephridial terminal cells and rhogocytes has never been further explored; indeed, information on the occurrence of rhogocytes in molluscan developmental stages is meager. We used transmission electron microscopy to show that rhogocytes are present within larvae of eight species of gastropods sampled from the three major gastropod clades with a feeding larval stage in the life history. In larvae of a heterobranch gastropod, a rhogocyte was located next to each terminal cell of a pair of protonephridia that flanked the foregut, whereas all six species of caenogastropod larvae and a neritimorph larva that we examined had rhogocytes, but no protonephridia, in this location. We did not find ring‐shaped profiles of hemocyanin decamers within rhogocytes of larvae or pre‐hatch embryos. Rhogocytes in newly released larvae of Nerita melanotragus contained orderly bundles of cylinders, but the diameter of the cylinders was only 70% of the diameter typical of hemocyanin multidecamers. By examining embryos of the caenogastropod Nassarius mendicus at four successive developmental time points that bracketed the occurrence of larval hatching, we found that terminal cells from non‐functional protonephridia in pre‐hatch embryos transformed into rhogocytes around the time of hatching. This empirical evidence of ontogenetic transformation of protonephridial terminal cells into rhogocytes might be interpreted as developmental recapitulation of an evolutionary transition that occurred early in molluscan history.     

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     

Trochophore concepts: ciliary bands and the evolution of larvae in spiralian Metazoa

‘Trochophore’ is a term used in a strict sense for larvae having an opposed-band method of feeding, involving a prototroch and metatroch. Other ciliary bands such as a telotroch and neurotroch may be present. The trochophore has been proposed to represent the ancestral larval form for a group of metazoan phyla (including all members of the Spiralia). The name trochophore is also often applied to larvae that do not conform to the above definition. A cladistic analysis of spiralian taxa (with special reference to polychaete annelids), based on a suite of adult and larval characters, is used to assess several hypotheses: (1) that the trochophore (in a strict sense) is a plesiomorphic form for the Spiralia; (2) that die stricdy defined trochophore is plesiomorphic for members of the Spiralia such as the Polychaeta. The homology of each of the various separate ciliary bands of spiralian larvae, and features such as the apical tuft and protonephridia is also assessed. The results favour the conclusion that the trochophore, if defined as a feeding larval form using opposed bands, should not be regarded as an ancestral (= plesiomorphic) type for the Spiralia, or any other large taxon such as the Polychaeta or Mollusca. The evidence suggests that the various ciliary bands have differing evolutionary histories, and only the Echiura (possibly an annelid group) has members with the classical trochophore. The trochophore is re-defined as a larval form with a prototroch. This broad definition covers a wide variety of larvae, and matches the current usage more accurately than the restricted term. Features such as the neurotroch, telotroch and opposed-band feeding show convergence and reversals. The nature of the metatroch requires further investigation. The presence of a prototroch (and hence trochophore larvae) is used to identify an apomorphy-based taxon, Trochozoa, that includes the first ancestor to have evolved a prototroch and all its descendants. This minimally includes the Annelida [sensu lato), Echiura, Entoprocta, Mollusca and Sipuncula and is a less inclusive taxon than the Spiralia.     

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