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.     

Development of excretory organs in <Emphasis Type="Italic">Phoronopsis harmeri</Emphasis> (Phoronida): From protonephridium to nephromixium

Larval protonephridia appear as paired ectodermal invaginations on the posterior body end of the larva (actinotrocha), at early stages of its development. The protonephridium of the early actinotrocha has a straight canal and one group of solenocytes distally. The protonephridium of the late actinotrocha has a U-shaped canal and two (upper and lower) groups of solenocytes. After metamorphosis, solenocytes degenerate and the canal is connected with metacoel. The metanephridial funnel is formed from the upper metacoelomic wall epithelium and the lateral mesentery. The definitive nephridium consists of two parts: the ectodermal canal (derived from the protonephridial canal) and the mesodermal funnel, a derivative of the coelomic epithelium. Thus, the phoronid excretory organ is a nephromixium. Consecutive stages of the evolution of nephridia in phoronids are discussed.     

The nephridia of the interstitial polychaete Hesionides arenaria and their phylogenetic significance (Polychaeta,Hesionidae)

Summary The small hesionid polychaete Hesionides arenaria possesses paired segmental excretory organs that closely resemble solenocytic protonephridia. The nephridium consists of one terminal cell and four tubule cells which form the emission channel. From the terminal cell, up to six flagella arise each surrounded by a weir of ten regularly arranged cytoplasmic rods. The structure of the cytoplasm of three of the following cells suggests that they function in active transport and storage. Because all of the larger, more primitive species of this family are equipped with metanephridia, the possibility is discussed that these organs have been developed out of metanephridia. The Hesionides arenaria nephridium may be a morphological stage in the evolutionary pathway from metanephridia to solenocytes. This would mean that solenocytes can no longer be considered to be homologous in every case with other protonephridial organs in polychaetes and may well be derived several times independently out of metanephridia or true protonephridia.     

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.     

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     

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.     

Development of the excretory system in a polyplacophoran mollusc: stages in metanephridial system development

ABSTRACT: BACKGROUND: Two types of excretory systems, protonephridia and metanephridial systems are common among bilaterians. The homology of protonephridia of lophotrochozoan taxa has been widely accepted. In contrast, the homology of metanephridial systems -- including coelomic cavities as functional units -- among taxa as well as the homology between the two excretory systems is a matter of ongoing discussion. This particularly concerns the molluscan kidneys, which are mostly regarded as being derived convergently to the metanephridia of e.g. annelids because of different ontogenetic origin. A reinvestigation of nephrogenesis in polyplacophorans, which carry many primitive traits within molluscs, could shed light on these questions. RESULTS: The metanephridial system of Lepidochitona corrugata develops rapidly in the early juvenile phase. It is formed from a coelomic anlage that soon achieves endothelial organization. The pericardium and heart are formed from the central portion of the anlage. The nephridial components are formed by outgrowth from lateral differentiations of the anlage. Simultaneously with formation of the heart, podocytes appear in the atrial wall of the pericardium. In addition, renopericardial ducts, kidneys and efferent nephroducts, all showing downstream ciliation towards the internal lumen, become differentiated (specimen length: 0.62 mm). Further development consists of elongation of the kidney and reinforcement of filtration and reabsorptive structures. CONCLUSIONS: During development and in fully formed condition the metanephridial system of Lepidochitona corrugata shares many detailed traits (cellular and overall organization) with the protonephridia of the same species. Accordingly, we suggest a serial homology of various cell types and between the two excretory systems and the organs as a whole. The formation of the metanephridial system varies significantly within Mollusca, thus the mode of formation cannot be used as a homology criterion. Because of similarities in overall organization, we conclude that the molluscan metanephridial system is homologous with that of the annelids not only at the cellular but also at the organ level.     

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.     

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.     

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.     

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.     

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     

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.     

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 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     

Nachtrag zur “Wurmtheorie” der Vertebraten-Evolution1

This paper presents some new arguments for the metameric-wonn-theory for the evolution of the Vertebrates (Gutmann 1966a). Metameric coelomoducts in Enteropneust larvae (Goodrich 1947) which should be interpreted as metanephridia show that the Enteropneusts can be derived from metameric Chordate-like predecessors. The myomeres of Branchiostoma are no solid organs as there exist sclerocoels. These must be interpreted as vestigial coelomic cavities. They can be cited as a proof for the metameric worm-theory. They function as a canal system, which gathers excretory stuff in the myomeres which these organs could otherwise not get rid of. The coelom-cavities are cleaned by the protonephridia in the gill region. Some additional details of the phylogenetic transformation of metameric coelom cavities into myomeres are reconstructed. It is shown that the problem of coelomic and myomeric metamerism cannot be solved in the way proposed in the literature concerned with this question. The metameric-worm-theory for the evolution of the Vertebrates pretends that metameric metanephridia were fused on the lower level of Vertebrate phylogeny and formed the archinephric ducts. A paper of Goodrich (1947) shows that there are similar cases of fused metanephridia in some Annelids. These are parallels to the postulated formation of the metanephridia in the lowest Vertebrates. The archinephric duct acquired its muscular coat when it was formed by fusion of metanephridia in the bodywall. Muscles of the body wall took over a new function by making peristaltic movements of the newly formed archinephric ducts possible. When the archinephric duct was moved back into the coelom it did not lose the still functioning muscular coat.     

Comparative ultrastructural study of the protonephridium of Eteone longa and the metanephridium of Lumbricus terrestris]

In the initial part of the protonephridium of E. longa specialized cells, cyrtocytes, are observed in addition to cells of the protonephridial duct among which dark and light cells are distinguished. The metanephridial duct of L. terrestirs is characterised by a comples differentiation and subdivision into several parts whose cells deffer in their morphology. The functional role of the two excretory organs is discussed.     

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 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.