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.     

THE FUNCTIONAL ORGANIZATION OF FILTRATION NEPHRIDIA

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

Ultrastructure 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.     

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.     

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.     

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 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.     

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 phylogenetic significance of the head kidneys in <Emphasis Type="Italic">Thalassema</Emphasis><Emphasis Type="Italic">thalassemum</Emphasis> (Thalassematinae,Echiura)

Recent molecular analyses consistently resolve the “spoon worms” (Echiura) as a subgroup of the Annelida, but their closest relatives among annelids still remain unclear. Since the adult morphology of echiurans yields limited insight into their ancestry, we focused on characters of their larval anatomy to contribute to this discussion. Electron microscopical studies of the larval protonephridia (so-called head kidneys) of the echiuran species Thalassema thalassemum revealed distinct correspondences to character states in serpulid polychaetes, although a close relationship between Echiura and Serpulidae is not supported by any phylogenetic analysis. The larval head kidneys of T. thalassemum consist of only two cells, a terminal cell and a duct cell. The terminal cell forms a tuft of six cilia projecting into the lumen of the terminal cell. The cilia are devoid of circumciliary microvilli. A filter structure is formed by two to three layers of elongate microvilli that surround the lumen of the terminal cell in a tubular manner. A thin layer of extracellular matrix (ECM) encloses the outer microvilli of the tubular structure. The tips of the microvilli project into the lumen of the adjacent duct cell but are not directly connected to it. However, mechanic coupling is facilitated by the surrounding ECM and abundant hemidesmosomes. The distal end of the multiciliary duct cell forms the external opening of the nephridium; a specialized nephropore cell is absent. Apart from the multiciliarity of the duct cell, details of the head kidneys in T. thalassemum reveal no support for the current assumption that Echiura is closely related to Capitellida and/or Terebelliformia. Available data for other echiuran species, however, suggest that the head kidneys of T. thalassemum show a derived state within Echiura.     

Protonephridia in the larvae of the paleonemertean species Carinoma mutabilis (Carinomidae,Nemertea) and Cephalothrix (Procephalothrix) filiformis (Cephalothricidae,Nemertea)

During spiralian development, the first pair of nephridia forms anterior to the mouth. Each organ consists of a few cells, which is characteristic for spiralian larvae. In nemerteans, one of the unambiguously spiralian taxa, so far protonephridia, has been reported only in advanced pilidium larvae, where they likely persist as juvenile and adult nephridia. These organs have not been recorded in larvae of the basally branching nemertean taxa. In search for these organs, we examined the ultrastructure of pelagic planuliform larvae of the palaeonemerteans Carinoma mutabilis and Cephalothrix (Procephalothrix) filiformis. In both species, a pair of protonephridia is located at the level of the stomodaeum. Each protonephridium of C. mutabilis consists of two terminal cells, two duct cells and one nephropore cell, while that of C. filiformis consists of three terminal cells, three duct cells and one nephropore cell. In C. mutabilis and in C. filiformis, all terminal cells contribute to forming a compound filtration structure. In both species, the protonephridia seem to develop subepidermally, since in C. filiformis, the nephropore cells pierce the larval epidermis and in C. mutabilis, the nephropores are initially covered by the binucleated multiciliated trophoblast cells. On the fifth day, these cells degenerate, so that the protonephridium becomes functional. The occurrence of protonephridia in the larvae of both paleonemertean species is in accordance with the hypothesis that a common ancestor of Nemertea and Trochozoa had a larval stage with a pair of protonephridia. This does not contradict previous hypotheses on placing the Nemertea as an ingroup of the Trochozoa or Spiralia (= Lophotrochozoa). Whether these protonephridia are restricted to the larval phase or whether they are transformed into the adult protonephridia, like those of the pilidium larva, remains to be answered.     

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     

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.     

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.     

Larval morphology of the brooding clam Lasaea adansonii (Gmelin, 1791) (Bivalvia, Heterodonta, Galeommatoidea)

The strongly modified mode of development of the small and brooding galeommatoid bivalve Lasaea adansonii (Gmelin, 1791) [syn. Lasaea rubra (Montagu, 1803)] has been studied by means of transmission and scanning electron microscopy and by fluorescent staining of the muscular system and of two neurotracers, FMRFamide and serotonin. In addition, two developmental stages were visualized using computer-aided 3D-reconstruction. All larval stages of L. adansonii lack ciliary rings. The apical organ appears invaginated: the base of the duct contacts the cerebral ganglia and opens on the preoral region. Larval protonephridia are lacking. The adult kidneys develop independently of the pericardial cavity and contain a protonephridial part that enables excretory function until the pericardium is formed. The larval muscular system is composed of smooth muscle fibers; striated fibers are lacking. Posteriorly and immediately below the ligament, a paired cell of unknown function is present that contains serotonin and FMRFamide. In summary, L. adansonii exhibits the direct mode of development. Only few truly larval structures (e.g., the modified apical organ) are elaborated.     

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.     

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.     

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.     

Germ-line cysts are formed during oogenesis in Erpobdella octoculata (Annelida,Clitellata, Erpobdellidae)

Abstract

Erpobdella octoculata (Clitellata, Hirudinea, Erpobdellidae) has paired ovarian sacs, each containing several rod-shaped structures termed ovarian bodies. Oogenesis takes place within the ovarian bodies. We show that in the apical part of the bodies the germ-line cells form syncytial cysts of cells interconnected by stable intercellular bridges. Germ-line cyst architecture is broadly similar to that of other clitellate annelids; that is, each germ cell has only one intercellular bridge connecting it to the anuclear cytoplasmic mass, the cytophore. Unlike germ-line cysts described in other leech species, the cytophore in cysts of E. octoculata is poorly developed, taking the form of thin cytoplasmic strands. Oogenesis in E. octoculata is meroistic because the germ cells forming the cysts (cystocytes) have diverse fates, i.e., nurse cells and oocytes appear. One large ramified cell (apical cell) occurs within the apical part of the ovarian body. We compare the ultrastructure of the apical cell found in E. octoculata with that of apical cells described recently in some hirudiniform leeches. The germ-line cysts as well as the oocytes are enveloped by somatic follicular cells. As in other leeches, the follicular cells surrounding the growing oocytes have cytoplasm perforated by intracellular canals. In view of the many similarities between E. octoculata ovarian bodies and the ovary cords described in glossiphoniids and especially in hirudiniform leeches, we suggest that the ovarian bodies found in E. octoculata are in fact modified ovary cords.     

The fine structure of protonephridia in Gnathostomulida and their comparison within Bilateria

Summary The fine structure of the protonephridia of Haplognathia rosea (Filospermoidea) and Gnathostomula paradoxa (Bursovaginoidea) is described. Each protonephridium consists of three different cells: (1) a monociliated terminal cell which constitutes the filtration area, (2) a nonciliated canal cell showing a special protonephridial outlet system, and (3) an intraepidermal cell — the nephroporus cell — constituting the nephroporus. The protonephridia are arranged serially. There is no canal system connecting the protonephridial units.Protonephridial characters in other Bilateria are considered. The pattern of characters in the protonephridia in the last common gnathostomulid stem species and presumed apomorphies in the protonephridia of the Gnathostomulida investigated are discussed.Abbreviations used in figures ac acessory centriole - AC additional epidermal cell - bb basal body - bl basal lamina - bm bundle of microvilli - c cilium - cc cilium duct cell - cd cilium duct - cr ciliary rootlet - crs structures resembling ciliary rootlets - di diplosome - ds desmosome - dy dictyosome - f filtration area - g granules - m mitochondrium - mv microvillus - n nucleus - NC nephroporus cell - np nephroporus - oc outlet canal - TC terminal cell - tl tubules of lacunar system