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     

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.     

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 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     

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.     

Franciscideres gen. nov. – a new,highly aberrant kinorhynch genus from Brazil,with an analysis of its phylogenetic position

A new genus and species of Kinorhyncha, Franciscideres kalenesos gen. et sp. nov., is described from tidal and subtidal sandy habitats in Brazil. The new genus and species is characterized by an extremely flexible trunk without pachycycli that appears perfectly circular in cross-section, segments 1, 2 and 11 consisting of closed rings and 3 to 10 of single, bent plates with midventral articulations, a neck without placids that resembles an additional segment, densely packed scale-like, cuticular hairs, and a terminal segment with a middorsal spine and two sets of lateral terminal spines, but no midterminal spine. Phylogenetic analyses of 18S rRNA of Franciscideres kalenesos gen. et sp. nov. and 47 other kinorhynch ingroup taxa suggest that the new genus is a basal homalorhagid, whereas comparison of morphological characters indicates affinities between the new genus and the peculiar cyclorhagid Cateria.

http://zoobank.org/urn:lsid:zoobank.org:pub:64E29D97-DE1D-4511-8683-C969DD2EED43     

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     

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 Flame Bulbs and Protonephridial Capillaries of Microstomum sp. (Platyhelminthes,Macrostomida)

The terminal part of the protonephridia of Microstomum is formed by a branching proximal canal cell and (at least?) two terminal cells. Each weir consists of longitudinal (sometimes convoluted) ribs continuous with the cytoplasm of the terminal cell. Internal leptotriches arise from the terminal and proximal canal cells. Near the tip of the flame, the proximal canal cell tube is surrounded by the more external terminal cell and connected to it by a septate junction. Large cristate mitochondria are densely packed in the terminal and canal cells. The flame bulb of Microstomum differs markedly from that of other macrostomids (Macrostomum, Paramalostomum) examined. Phylogenetic implications are discussed.     

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.     

<Emphasis Type="Italic">Tubulideres seminoli</Emphasis> gen. et sp. nov. and <Emphasis Type="Italic">Zelinkaderes brightae</Emphasis> sp. nov. (Kinorhyncha,Cyclorhagida) from Florida

One new kinorhynch genus and species and one new species from the genus Zelinkaderes are described from sandy sediment off Fort Pierce, Florida. The new genus and species, Tubulideres seminoli gen. et sp. nov. is characterized by the presence of the first trunk segment consisting of a closed ring, the second segment of a bent tergal plate with a midventral articulation and the following nine segments consisting of a tergal and two sternal plates. Cuspidate spines are not present, but flexible tubules are located on several segments, and in particular concentrated on the ventral side of the second segment. Middorsal spines are present on all trunk segments and are alternatingly offset to a position slightly lateral to the middorsal line. Zelinkaderes brightae nov. sp. is characterized by its spine formula in having middorsal spines on trunk segments 4, 6 and 8–11, lateroventral acicular spines on segment 2, lateral accessory cuspidate spines on segments 2 and 8, ventrolateral cuspidate spines on segments 4–6 and 9, lateroventral acicular spines present on segments 8 and 9, and midterminal, lateral terminal and lateral terminal accessory spines on segment 11. The spine formula of Z. brightae nov. sp. places it in a position in between Z. submersus and a clade consisting of Z. klepali and Z. floridensis. The new findings on Z. brightae nov. sp. have led us to propose an emended diagnosis for the genus.     

The lamina propria of the bovine seminiferous tubule

Summary The boundary tissue of bovine testicular seminiferous tubules exhibits remarkable regional differences at the level of the seminiferous tubule proper, as compared with its terminal segment. The basal lamina of the seminiferous tubule proper is multilayered and possesses knob-like protrusions. At the level of the terminal segment the basal lamina is highly specialized; in the region of the terminal plug candelabrum-like projections of the tubular basal lamina invade the bases of the modified supporting cells up to a depth of 3.5 m. The adjoining surface of these supporting cells is densely studded with hemidesmosomes. The elongated peritubular cells are arranged in 3–5 concentric layers around the tubulus seminiferus proper but form a loose association at the level of the terminal segment. Where the terminal segment joins the testicular straight tubule, peritubular cells may assemble to constitute a contractile spiral. Elastic tissue is situated mainly subjacent to the tubular basal lamina and to a lesser degree between the peritubular cell layers. A peritubular space lined by endothelium-like cells may surround the seminiferous tubule proper and also the transitional zone of the terminal segment.Supported by a grant from the Deutsche Forschungsgemeinschaft     

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.     

Feinstruktur der Protonephridien von Paromalostomum proceracauda (Plathelminthes,Macrostomida)

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

---

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

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

---

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

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.     

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

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

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

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

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

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