On the Ultrastructure and Function of Rhogocytes from the Pond Snail Lymnaea stagnalis

Rhogocytes, also termed “pore cells”, occur as solitary or clustered cells in the connective tissue of gastropod molluscs. Rhogocytes possess an enveloping lamina of extracellular matrix and enigmatic extracellular lacunae bridged by cytoplasmic bars that form 20 nm diaphragmatic slits likely to act as a molecular sieve. Recent papers highlight the embryogenesis and ultrastructure of these cells, and their role in heavy metal detoxification. Rhogocytes are the site of hemocyanin or hemoglobin biosynthesis in gastropods. Based on electron microscopy, we recently proposed a possible pathway of hemoglobin exocytosis through the slit apparatus, and provided molecular evidence of a common phylogenetic origin of molluscan rhogocytes, insect nephrocytes and vertebrate podocytes. However, the previously proposed secretion mode of the respiratory proteins into the hemolymph is still rather hypothetical, and the possible role of rhogocytes in detoxification requires additional data. Although our previous study on rhogocytes of the red-blooded (hemoglobin-containing) freshwater snail Biomphalaria glabrata provided much new information, a disadvantage was that the hemoglobin molecules were not unequivocally defined in the electron microscope. This made it difficult to trace the exocytosis pathway of this protein. Therefore, we have now performed a similar study on the rhogocytes of the blue-blooded (hemocyanin-containing) freshwater snail Lymnaea stagnalis. The intracellular hemocyanin could be identified in the electron microscope, either as individual molecules or as pseudo-crystalline arrays. Based on 3D-electron microscopy, and supplemented by in situ hybridization, immunocytochemistry and stress response experiments, we provide here additional details on the structure and hemocyanin biosynthesis of rhogocytes, and on their response in animals under cadmium and starvation stress. Moreover, we present an advanced model on the release of synthesized hemocyanin molecules through the slit apparatus into the hemolymph, and the uptake of much smaller particles such as cadmium ions from the hemolymph through the slit apparatus into the cytoplasm.     

Rhogocytes (pore cells) as the site of hemocyanin biosynthesis in the marine gastropod Haliotis tuberculata

Rhogocytes (pore cells) are specific molluscan cell types that are scattered throughout the connective tissues of diverse body parts. We have identified rhogocytes in large numbers in tissue taken from mantle, foot and midgut gland of the abalone Haliotis tuberculata (Vetigastropoda). Within cisternae of the endoplasmic reticulum, particles are visible that resemble, in shape and size, hemocyanin molecules, the respiratory protein of many molluscs. Immunohistochemical experiments using hemocyanin-specific antibodies demonstrated that these cells contain hemocyanin. In situ hybridization with a cDNA probe specific for Haliotis hemocyanin showed that hemocyanin-specific mRNA is present in rhogocytes, which confirmed that they are the site of hemocyanin biosynthesis in this gastropod. A possible path of hemocyanin release into the hemolymph is discussed. Also in the vetigastropod Megathura crenulata, many rhogocytes could be detected. However, they lacked hemocyanin molecules which, together with published data, indicates a seasonal expression of hemocyanin in this animal.     

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.     

Synthesis of keyhole limpet hemocyanin by the rhogocytes of Megathura crenulata

Rhogocytes are morphologically distinct cells distributed throughout connective tissues of crustaceans and molluscs. Using light microscopy, rhogocytes of the vetigastropod Megathura crenulata were identified by their ovoid shape, and their cytoplasm filled with spherical inclusions which contained lysosomal enzymes, based on uptake of neutral red and staining with LysoTracker dye. Rhogocytes were most abundant in the digestive gland (2,824 rhogocytes/mm²), followed by the connective tissue layer surrounding the middle and posterior esophagus and intestine (1,431 rhogocytes/mm², 872 rhogocytes/mm², and 1,190 rhogocytes/mm², respectively), and were lowest in abundance in the foot (154 rhogocytes/mm²). At the transmission electron microscopy level, characteristic features of rhogocytes were inclusions showing a variety of electron densities, abundant vesicles, and rough endoplasmic reticulum in the cytoplasm, and regions of plasma membrane folded to produce slits connected by thin diaphragms. Although several functions have been proposed for gastropod rhogocytes, much attention has been focused on their possible role in the synthesis of the respiratory pigment hemocyanin. In M. crenulata, this molecule exists in several isoforms called keyhole limpet hemocyanin (KLH). One isoform, KLH1, is a large didecamer and has been used extensively in studies on vertebrate immunology and cancer therapy. We present four lines of evidence indicating rhogocytes in M. crenulata synthesize KLH1. First, at the transmission electron microscopy (TEM) level, dilated cisternae of RER containing material similar in size and shape to KLH were observed in rhogocytes examined throughout the year. Second, KLH1 mRNA was identified exclusively in tissue samples that contained rhogocytes; no mRNA for KLH1 was identified in samples containing only hemocytes. Third, immunoperoxidase staining with antibodies specific to KLH was localized only to rhogocytes. Fourth, in situ hybridization with a probe specific for M. crenulata KLH1 demonstrated KLH1‐specific mRNA was present only in rhogocytes. Identification of the cells responsible for the synthesis of KLH is important because of the clinical significance of this molecule.     

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     

Poecilogony in the caenogastropod Calyptraea lichen (Mollusca: Gastropoda)

In marine invertebrates, polymorphism and polyphenism in mode of development are known as “poecilogony.” Understanding the environmental correlates of poecilogony and the developmental mechanisms that produce it could contribute to a better understanding of evolutionary transitions in mode of development. However, poecilogony is rare in marine invertebrates, with only ten recognized, well‐documented cases. Five examples occur in sacoglossan gastropods, and five occur in spionid polychaetes. Here, we document the eleventh case, and the first in a caenogastropod mollusc. Females of Calyptraea lichen collected in the field or reared in the laboratory often produce broods of planktotrophic larvae. They can also be collected with mixed broods, in which each capsule contains planktotrophic larvae, nurse embryos, and adelphophagic embryos. Adelphophages eat the nurse embryos and hatch as short‐lived lecithotrophic larvae, or even as juveniles. Mitochondrial COI and 16S DNA sequences for females with different types of broods differ by less than 0.5%, supporting conspecific status. Some females collected in the field with mixed broods subsequently produced planktotrophic broods, demonstrating that females can produce two different kinds of broods. Calyptraea lichen is therefore polyphenic in two ways: mode of development can vary among embryos within a capsule, and females can change the types of broods they produce.     

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

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

Ultrastructure 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.     

Sequential developmental programmes for retractor muscles of a caenogastropod: reappraisal of evolutionary homologues

Evolutionary changes in the development of shell-attached retractor muscles in gastropods are of fundamental importance to theories about the early evolution and subsequent diversification of this molluscan class. Development of the shell-attached retractor muscle (columellar muscle) in a caenogastropod has been studied at the ultrastructural level to test the hypothesis of homology with the post-torsional left retractor muscle (larval velar retractor) in vetigastropod larvae. The vetigastropod muscle has been implicated in the generation of ontogenetic torsion, a morphogenetic twist between body regions that is important to theories about early gastropod evolution. Two shell-attached retractor muscles develop sequentially in the caenogastropod, Polinices lewisii, which is a pattern that has been also identified in previous ultrastructural studies on a vetigastropod and several nudibranch gastropods. The pattern may be a basal and conserved characteristic of gastropods. I found that the first-formed retractor in larvae of P. lewisii is comparable to the larval velar retractor that exists at the time of ontogenetic torsion in the vetigastropod, Haliotis kamtschatkana. However, the post-metamorphic columellar muscle of P. lewisii is derived exclusively from part of the second-formed muscle, which is comparable to the second-formed pedal muscle system in the vetigastropod. I conclude that the post-metamorphic columellar muscle of P. lewisii, is not homologous to the larval velar retractor of the vetigastropod, H. kamtschatkana.     

THE MOLLUSCAN RHOGOCYTE (PORE-CELL, BLASENZELLE, CELLULE NUCALE), AND ITS SIGNIFICANCE FOR IDEAS ON NEPHRIDIAL EVOLUTION

The rhogocyte (Leydig's cell, cellule nucale, Blasen-zelle,pore cell) is a specific molluscan cell type that occurs throughoutthe animal's primary body cavity, i.e free in the haemocoelor embedded in connective tissue. Rhogocytes closely resemblecyrtocytes and podocytes jn having slit-Uke diaphragms withan encoating extracellular matrix at their surface which probablyacts as a molecular sieve However, rhogocytes are solitary cells,whereas cyrtocytes and podocytes form epitheha Occurrence, variability,naming, and possible functions of the rhogocyte among the MoUuscaare reviewed and discussed. In general, rhogocytes play an importantrole in metal ion metabolism of molluscs, they are possiblyinvolved in the recycling of respiratory pigments and may alsoact in detoxification. Up to now direct homologues of the molluscan rhogocyte havenot been described in any other bi-laterian phylum, so-called‘pore cells’ of other phyla show entirely differentfine-structures However, at least partial (serial) homologycan be accepted between rhogocytes (free cells), arthropod nephro-cytes(cell aggregates or solitary cells) and podocytes (true epithelialcells), and there is also a cytological and functional continuumof both rhogocytes and podocytes to protonephndial cyrto- respectivelysolenocytes. Implications of this postulated ‘cell-family’with slit-diaphragms and their respective ultrafiltration systemsfor theories on the evolution of filtration nephndia are discussed. (Received 10 February 1995; accepted 10 October 1995)     

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     

GABA is an inhibitory neurotransmitter in the neural circuit regulating metamorphosis in a marine snail

The marine mud snail, Tritia (=Ilyanassa) obsoleta, displays a biphasic life cycle. During the initial phase of early development, embryos hatch from benthic egg capsules to become weakly swimming veliger larvae. In the second phase, adult T. obsoleta are facultative carnivores and major agents of community disturbance. Metamorphosis is the irreversible developmental event that links these two life history stages. When physiologically competent, larvae can respond to appropriate environmental cues by settling onto their mudflat habitat and transforming themselves into miniature adult snails. Two neurotransmitters—serotonin and nitric oxide—have opposing effects on the metamorphic process in this species. In multiple other species of gastropod and bivalve molluscs, a third neurotransmitter, the classically inhibitory compound γ‐aminobutyric acid (GABA), can induce settlement or metamorphosis upon external application to competent larvae. In this situation, GABA is presumed to mimic the action of ligands from the juvenile environment that bind to larval chemosensory receptors and activate the metamorphic pathway. Results of our experiments contradict this commonly reported action of GABA on molluscan larvae. External application of GABA to competent larvae of T. obsoleta elicited no response, but instead attenuated the action of serotonin (5‐HT), a metamorphic inducer. Our investigations into the responses of larval T. obsoleta to multiple GABAergic reagents support our hypothesis that GABA functions internally as a neurotransmitter in the pathway that controls the initiation of metamorphosis. Our results also suggest that GABA acts directly on or downstream from serotonergic neurons to regulate the metamorphosis‐inducing effects of this neurotransmitter. © 2018 Wiley Periodicals, Inc. Develop Neurobiol 78: 736–753, 2018     

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.     

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

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

Ultrastructure of the 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 structure of a functional unit from the wall of a gastropod hemocyanin offers a possible mechanism for cooperativity

Structure-function relationships in a molluscan hemocyanin have been investigated by determining the crystal structure of the Rapana thomasiana (gastropod) hemocyanin functional unit RtH2e in deoxygenated form at 3.38 A resolution. This is the first X-ray structure of an unit from the wall of the molluscan hemocyanin cylinder. The crystal structure of RtH2e demonstrates molecular self-assembly of six identical molecules forming a regular hexameric cylinder. This suggests how the functional units are ordered in the wall of the native molluscan hemocyanins. The molecular arrangement is stabilized by specific protomer-to-protomer interactions, which are probably typical for the functional units building the wall of the cylinders. A molecular mechanism for cooperative dioxygen binding in molluscan hemocyanins is proposed on the basis of the molecular interactions between the protomers. In particular, the deoxygenated RtH2e structure reveals a tunnel leading from two opposite sides of the molecule to the active site. The tunnel represents a possible entrance pathway for dioxygen molecules. No such tunnels have been observed in the crystal structure of the oxy-Odg, a functional unit from the Octopus dofleini (cephalopod) hemocyanin in oxygenated form.     

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     

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.