Die Aufnahme partikulärer Nahrung beiReniera sp. (Porifera)

The choanocyte chambers of the marine spongeReniera sp. protrude with their curved outer surface free into the incurrent canals. The water is sucked into the chambers by cavities between the choanocytes. Particles up to 1 µm in diameter may enter the chambers with the water current. These particles are trapped on the outer surface of the choanocyte collars and are ingested by the choanocytes and processes of the pinacocyte epithelium of the incurrent canal system, which project into the chambers. Bigger particles are retained in the incurrent canals mainly on the outer surface of the choanocyte chambers. They are ingested by pinacocytes of the canal wall and transported to cells of the mesenchyme. The present investigation shows the great importance of the pinacocyte epithelium of the incurrent canal system for suspension feeding inReniera sp.     

Body structure of marine sponges

Summary Specimens of Haliclona elegans (Bowerbank, 1866) are covered by a thin, double layered dermal membrane extending over large subdermal spaces. The pores in the dermal membrane are formed by single porocytes with one or sometimes several pores in the center of the cell. The subjacent tissue shows a faintly developed mesenchyme and numerous big choanocyte chambers projecting into lacunar spaces of the incurrent canal system. The outer surface of the chambers is directly covered by the pinacocyte epithelium of the incurrent canal wall, which also separates them completely from the mesenchyme. Water influx into the chambers is guaranteed by prosopylar openings in the pinacocyte cover at the outer chamber surface. The chambers are connected to the excurrent canal system in the eurypylous way by wide apopyles, each of which is surrounded by a small ring of flagellated cone cells. About 15% of the choanocyte chambers in H. elegans contain central cells, which are thought to derive from migrating pinacocytes of the canal systems.     

Canal systems and choanocyte chambers in freshwater sponges (Porifera,Spongillidae)

Summary The spongillid species Spongilla lacustris and Ephydatia fluviatilis possess choanocyte chambers of the classical eurypylous type. They are surrounded by the mesenchymal tissue and connected to the incurrent canal system by prosopyles and to the excurrent canal system by wide apopyles. Each apopyle is sealed against spaces between the basal choanocyte collar parts by a ring of uniflagellated cone cells. The functional aspects of the choanocyte chamber and canal structure are discussed.Dedicated to Prof. Dr. K.E. Wohlfarth-Bottermann, Bonn, in honor of his 65th birthday     

Hydrodynamics in early animal evolution

Choanoflagellates and sponges feed by filtering microscopic particles from water currents created by the flagella of microvillar collar complexes situated on the cell bodies of the solitary or colonial choanoflagellates and on the choanocytes in sponges. The filtering mechanism has been known for more than a century, but only recently has the filtering process been studied in detail and also modelled, so that a detailed picture of the water currents has been obtained. In the solitary and most of the colonial choanoflagellates, the water flows freely around the cells, but in some forms, the cells are arranged in an open meshwork through which the water can be pumped. In the sponges, the choanocytes are located in choanocyte chambers (or choanocyte areas) with separate incurrent and excurrent canals/pores located in a larger body, which enables a fixed pattern of water currents through the collar complexes. Previous theories for the origin of sponges show evolutionary stages with choanocyte chambers without any opening or with only one opening, which makes separation of incurrent and excurrent impossible, and such stages must have been unable to feed. Therefore a new theory is proposed, which shows a continuous evolutionary lineage in which all stages are able to feed by means of the collar complexes.     

The architecture of the canal systems of Petrosia ficiformis and Chondrosia reniformis studied by corrosion casts (Porifera,Demospongiae)

Summary The three-dimensional organization of the canal system in two sponge species, Petrosia ficiformis and Chondrosia reniformis, was studied using corrosion casts. Casts were made of live animals, in situ, and canal replicas were analzyed by scanning electron microscopy (SEM). In P. ficiformis the incurrent system consists of a superficial canal network giving rise to large radial canals, which ramify and anastomosize forming an internal web. Excurrent canals are arranged into modular ramified systems radiating from atrial cavities opening to the exterior. Main excurrent canals run at various depths within the sponge, even through the superficial incurrent network. Both incurrent and excurrent canal replicas show smooth, blind-ending capillaries. Some large incurrent canals merge with excurrent ones, thus bypassing choanocyte chambers. In C. reniformis there is a cortical collagen layer crossed by three-like incurrent canals, the twigs of which communicate with groups of inhalant pores. The stems of tree-like canals penetrate into the sponge medulla where they ramify and anastomosize to form a web. Main excurrent canals arise from large cloacal ducts leading to the oscular openings. They give rise to a sequence of branches intersecting the incurrent web. Both incurrent and excurrent canals have sharp, blind-ending capillaries. Morphometric data functions show that diameter scaling in canal branches is exponential in Petrosia and linear in Chondrosia. Structural differences and homologies between the two species are discussed.     

Oogenesis and larval development of Scypha ciliata (Porifera,Calcarea)

Summary Scypha ciliata is a syconoid sponge. Its oocytes differentiate from choanocytes located near the apopyle of a flagellated chamber, and initially they remain in that location, in a trophic complex with neighbouring choanocytes. When this first growth phase is completed, the oocyte migrates to the periphery of the sponge. There it undergoes a second growth phase, in which it phagocytizes choanocytes and mesenchyme cells.Fertilization of the mature egg is assisted by a converted choanocyte, the sperm carrier cell. This cell penetrates the oocyte and transfers to it the sperm contained in a carriercell vacuole. No meiotic events have yet been observed.Cleavage is asynchronous, with holoblastic, approximately equal divisions. After the first cleavage steps the blastomeres often contain multiple nuclei. The single-layered blastoderm of the stomoblastula consists of many micromeres with flagella that project into the blastocoel, a few macromeres and four cruciform cells. There is no development of a follicle epithelium.The stomoblastula develops into the amphiblastula by inversion; with the assistance of the maternal choanocyte epithelium, the hollow sphere turns inside out, simultaneously moving out of the mesoderm and into the lumen of the adjacent flagellated chamber. In this process, the blastocoel of the stomoblastula is lost. The flagellated cells that form the wall of the amphiblastula now have their flagella extending outward; the amphiblastula also comprises four cruciform cells, macrogranular and agranular cells. The larval cavity of the amphiblastula is a newly formed structure.Abbreviations AB amphiblastula - AP apopyle - BC blastocoel - aC agranular cell - maC macrogranular cell - miC microgranular cell - CB crystalline body - CC central cavity - Ch choanocyte - fCh flat choanocyte - gCh granulate choanocyte - CM cell membrane - Co collar of choanocyte - CrC cruciform cell - DM dense material - EM electron micrograph - F flagellum - FC flagellated cell - FCm flagellated chamber - FL free larva - FV food vacuole - IR interior region - LC larval cavity - M mesenchyme - Ma macromere - MC mesenchyme cell - Mi micromere - N nucleus - Nu nucleolus - O opening - OC oocyte - P psudopodium - PC pinacocyte - PhM phase-contrast micrograph - Po pore - PP prosopyle - S sperm - SB stomoblastula - SC segmentation cavity - SCC sperm-carrier cell - SV sperm vacuole - lT large trophocyte - sT small trophocyte - V vacuole - VC vesicular cytoplasm - VM vacuole membrane     

Formation and construction of asexual buds of the freshwater spongeRadiospongilla cerebellata (Porifera,Spongillidae)

Summary The buds ofRadiospongilla cerebellata are formed asexually. Budding can be induced experimentally by injuring the sponge. The first sign of budding is a slight elevation of some surface areas, which proceed to rise rapidly so that they soon protrude conspicuously from the surface of the sponge. As a bud develops, the broad base joining it to the mother sponge narrows to a stalk, which finally breaks. The free buds drift in the water for 15–20 min and then settle, forming new sessile sponges. The buds, 1.5–2.5 mm in diameter, have an internal organization identical with that of the mother sponge. They are enclosed in a layer of pinacoderm perforated by dermal pores. Under the pinacorderm there is a shallow subdermal space, which is in communication with the incurrent canals leading to the choanocyte chambers. The water sucked into these chambers proceeds into the excurrent canal system and emerges from the sponge through the oscular tube. Spicules projecting radially from the bud bear apical tufts of microscleres. The skeletal spicules of the buds, like their choanocyte chambers, are smaller than those in the mother sponge. The chambers expand to their mature size by choanocyte mitosis. Buds and sponges are colored green by intracellular symbiotic algae of the genusChlorella.     

The aquiferous system of two <Emphasis Type="Italic">Oceanapia</Emphasis> species (Porifera,Demospongiae) studied by corrosion casts

The aquiferous systems of two Indopacific Oceanapia species (Oceanapiidae) were studied by corrosion casts: O. amboinensis living in shallow lagoons and O. fistulosa living at the base of the reef slope. Both species show a massive, entirely buried body, emerging from the sediment only by long, completely close fistules. Particularly in O. fistulosa the corrosion casts revealed a complex, grape-like structure of the choanosome organised in anatomical and functional units composed by an incurrent web whose anastomosed meshes are crossed by a central excurrent canal. A system of thin canals connects the two systems giving rise to an area of choanocyte chambers. The corrosion casts revealed that in both species incurrent water penetrates into the sponge body by the fistules and that it is expelled through specialised structures buried in the sediment. This observation is in accordance with field experiments performed on O. fistulosa. In some specimens of this species, a solution of china ink injected into plastic bags enveloping the external fistules was observed, after waiting for a while, to flow through the buried structures.     

A new type of central cell in the choanocyte chambers of Pellina fistulosa (Porifera,Demospongiae)

Central cells of a hitherto unknown type, forming a continuous, perforated layer at the level of the distal collar ends in each choanocyte chamber, have been found in the choanocyte chambers of Pellina fistulosa. The collars project through the pores of the perforated central cell layer. The spaces between the collar ends and between the collars and the cone cell ring in the apopyle region are sealed by the central cell cytoplasm. The latter represents an impermeable barrier for particulate material as well as for water and thus enhances the filtration efficiency by preventing a bypass of water and particles between the collar apices.     

Ultrastructure and embryonic development of a syconoid calcareous sponge

Abstract. Recent molecular data suggest that the Porifera is paraphyletic (Calcarea+Silicea) and that the Calcarea is more closely related to the Metazoa than to other sponge groups, thereby implying that a sponge‐like animal gave rise to other metazoans. One ramification of these data is that calcareous sponges could provide clues as to what features are shared among this ancestral metazoan and higher animals. Recent studies describing detailed morphology in the Calcarea are lacking. We have used a combination of microscopy techniques to study the fine structure of Syconcoactum Urban 1905, a cosmopolitan calcareous sponge. The sponge has a distinct polarity, consisting of a single tube with an apically opening osculum. Finger‐like chambers, several hundred micrometers in length, form the sides of the tube. The inner and outer layers of the chamber wall are formed by epithelia characterized by apical–basal polarity and occluding junctions between cells. The outer layer—the pinacoderm—and atrial cavity are lined by plate‐like cells (pinacocytes), and the inner choanoderm is lined by a continuous sheet of choanocytes. Incurrent openings of the sponge are formed by porocytes, tubular cells that join the pinacoderm to the choanoderm. Between these two layers lies a collagenous mesohyl that houses sclerocytes, spicules, amoeboid cells, and a progression of embryonic stages. The morphology of choanocytes and porocytes is plastic. Ostia were closed in sponges that were vigorously shaken and in sponges left in still water for over 30 min. Choanocytes, and in particular collar microvilli, varied in size and shape, depending on their location in the choanocyte chamber. Although some of the odd shapes of choanocytes and their collars can be explained by the development of large embryos first beneath and later on top of the choanocytes, the presence of many fused collar microvilli on choanocytes may reflect peculiarities of the hydrodynamics in large syconoid choanocyte chambers. The unusual formation of a hollow blastula larva and its inversion through the choanocyte epithelium are suggestive of epithelial rather than mesenchymal cell movements. These details illustrate that calcareous sponges have characteristics that allow comparison with other metazoans—one of the reasons they have long been the focus of studies of evolution and development.     

The spermatogenesis ofHalichondria panicea (Porifera,Demospongiae)

Summary Spermatogenesis of the marine spongeHalichondria panicea begins with the break up of choanocyte chambers, choanocytes constituting the origin of spermatogonia. The transition from choanocytes to spermatogonia is direct, without cell division. Already the spermatogonia are flagellated. The ensuing large aggregates of spermatogonia are enclosed by spermatocyst-building cells. Further development takes place within the spermatocysts, mostly arranged in fields which, however, lack any developmental gradient. Within a single spermatocyst development is mostly synchronous. Spermatogonia transform into first order spermatocytes directly. The transition from spermatid to spermatozoon is characterized by an unusual prolongation of the chromatin, often resulting in a helical form of the chromosome material and a strong enlargement of the mitochondria which align with the nucleus, leading to an irregular shape of the spermatozoon. Another exceptional feature is the virtual absence of a Golgi apparatus during all stages of spermatogenesis. TheH. panicea investigated here contained only male reproductive elements, thus appear to be gonochorists. Some features of the spermatogenesis ofH. panicea, such as dissolving choanocyte chambers, the enclosure of spermatogonia by spermatocyst-building cells and the formation of a synaptonemal complex in first order spermatocytes occur in other sponge species as well; however, the early presence of flagella in spermatogonia, the absence of the Golgi apparatus and the later irregular development of nuclei, mitochondria and the spermatozoa themselves represent features hitherto not observed in sponges.     

Continuous cell movements rearrange anatomical structures in intact sponges.

Time-lapse cinemicrography was used to record the active movements of cells in living intact sponges. Each of the three main cell types (pinacocytes, mesohyl cells, and choanocytes) continuously moved and rearranged themselves so that the internal anatomy of the sponge was continuously remodeled. The shape and appearance of the sponges anatomical structures often changed substantially within a few hours. The most motile were the mesohyl cells, with many moving as fast as one cell-length per minute (15 microns/min). Mesohyl cell locomotion was often accompanied by displacements of spicules, canals, and choanocyte chambers; the patterns of these displacements suggested that the mesohyl cells were providing the motive forces for these rearrangements. The locomotion of the pinacocytes varied according to position: those along the outer sponge margins were most active, whereas those in other parts of the surface moved relatively little. Choanocytes were never observed to undergo independent locomotion but were always found grouped together in choanocyte chambers. These choanocyte chambers interacted with pinacocytes and mesohyl cells to form excurrent canals, which continuously moved, fused with, and branched from one another. These observations suggest that the experimental phenomenon of sponge cell-reaggregation and reconstitution, discovered by H. V. Wilson, represents an extreme version of morphogenetic processes that normally go on continuously within intact sponges. The results from the present study also suggest that these cellular rearrangements are controlled by active cell movements and behavioral responses that include but are not limited to selective cell adhesion.     

Ultrastructural study of spermatogenesis in Oscarella lobularis (Porifera,Demospongiae)

Summary

The various phases of spermatogenesis in the demosponge Oscarella lobularis were studied by electron microscopy. Spermatogenesis occurs within spermatic cysts, which are presumed to derive from choanocyte chambers by transformation of choanocytes into spermatogonia. Germ cells develop asynchronously within spermatocysts, and cytoplasmic bridges, indicating incomplete cells division, connect several germ cells. Attached spermatogonia suggest gonial generations. Spermatocytes I typically show the presence of synaptonemal complexes indicating meiotic divisions. Spermatocytes II have a small size probably because of the meiotic divisions of spermatocytes I. Spermatids are characterized by an acrosome, a big mitochondrion and a peripheral sheath of condensed chromatin surrounding a clearer central area in the nucleus. The mature spermatozoon shows a lateral flagellum and a flattened acrosome capping the nucleus. The phylogenetic implications of some features of the spermatozoon are suggested.     

Hydroxyurea: An Inhibitor of the Differentiation of Choanocytes in Fresh-Water Sponges and a Possible Agent for the Isolation of Embryonic Cells

During the development of a fresh-water sponge from its gemmules, most cell types originate from the undifferentiated archaeocytes through a few divisions, whereas each choanocyte chamber, composed of several tens of choanocytes, arises from a single archaeocyte through repeated mitoses.
This process was studied on gemmules incubated in various concentrations of hydroxyurea.
A concentration of 100 μg/ml postponed the hatching by about two days, and blocked the differentiation of the choanocytes and the morphogenesis of the aquiferous system. The resulting organism was a hollow dome of pinacoderm, stretched on spicules, the bottom of which was strewn with embryonic archaeocytes. After washing and incubation in mineral medium, the sponge differentiated its choanocytes and achieved normal development.
The incorporation of ³H-thymidine into DNA was compared throughout the development of normal and hydroxyurea-treated gemmules. Hydroxyurea delayed the first peaks of incorporation and abolished the large peak that normally occurs around 90 h, just before the formation of choanocyte chambers.
When added after 96 h incubation, hydroxyurea did not affect the differentiation of the choanocytes.
These results suggest that the differentiation of the choanocytes and the further morphogenesis of the aquiferous system depend on the repetitive divisions of the archaeocytes that normally occur around 90 h.
Furthermore, hydroxyurea-blocked sponges provide a suitable source for the isolation of pure populations of embryonic archaeocytes.     

Ultrastructural investigation of spermatogenesis in Spongilla lacustris and Ephydatia fluviatilis (Porifera,Spongillidae)

Summary Spermatogenesis of the spongillids investigated here is similar in Spongilla lacustris and Ephydatia fluviatilis and proceeds, on the whole, as in other Eumetazoa. Sponges however lack true sex organs, the germ cells developing from somatic cells. The male germ cells originate in spongillids from choanocytes and the female ones from archaeocytes. In Spongilla lacustris single choanocytes leave the flagellated chambers and transform into spermatogonia; in Ephydatia fluviatilis they result from differential cell division. The spermatogonia gather in distinct mesenchyme regions and are surrounded by cyst-building cells. Thus spermatocysts are built in which spermatogenesis proceeds. The spermatogonia in the spermatocysts differentiate into flagellated spermatocytes of I. order. In this process, the early appearance of the flagellum and its mode of formation are uncommon. The following meiotic divisions generate spermatocytes of II. order in the first step and spermatids in the second. In both developmental stages the cells remain connected by cytoplasmic bridges. In the subsequent spermiocytogenesis the cytoplasm of the spermatids is reduced. The reduced parts of the cytoplasm appear as cell fragments in the lumen of the spermatocysts and are eventually ingested by the cystwall cells. The mature spermatozoa arrange in the spermatocysts in a characteristic pattern. Later the spermatocysts open into the excurrent canal system and the spermatozoa leave the sponge with the egestive water stream.     

Choanocyte ultrastructure in Halisarca dujardini (Demospongiae,Halisarcida)

Understanding poriferan choanocyte ultrastructure is crucial if we are to unravel the steps of a putative evolutionary transition between choanoflagellate protists and early metazoans. Surprisingly, some aspects of choanocyte cytology still remain little investigated. This study of choanocyte ultrastructure in the halisarcid demosponge Halisarca dujardini revealed a combination of minor and major distinctive traits, some of them unknown in Porifera so far. Most significant features were 1) an asymmetrical periflagellar sleeve, 2) a battery of specialized intercellular junctions at the lateral cell surface complemented with an array of lateral interdigitations between adjacent choanocytes that provides a particular sealing system of the choanoderm, and 3) a unique, unexpectedly complex, basal apparatus. The basal apparatus consists of a basal body provided with a small basal foot and an intricate transverse skeleton of microtubules. An accessory centriole, which is not perpendicular to the basal body, is about 45°. In addition, a system of short striated rootlets (periodicity = 50–60 nm) arises from the proximal edge of the basal body and runs longitudinally to contact the nuclear apex. This is the first flagellar rootlet system ever found in a choanocyte. The accessory centriole, the rootlet system, and the nuclear apex are all encircled by a large Golgi apparatus, adding another distinctive feature to the choanocyte cytology. The set of distinct features discovered in the choanocyte of H. dujardini indicates that the ultrastructure of the poriferan choanocyte may vary substantially between sponge groups. It is necessary to improve understanding of such variation, as the cytological features of choanocytes are often coded as characters both for formulation of hypotheses on the origin of animals and inference of phylogenetic relationships at the base of the metazoan tree. J. Morphol., 2009. © 2008 Wiley‐Liss, Inc.     

Bau und Funktion des Süßwasserschwamms Ephydatia fluviatilis L. (Porifera)

Zusammenfassung Die Kragengeißelkammern von Ephydatia fluviatilis entstehen frei im Mesenchym. An den Entstehungsorten trifft man auf Anhäufungen rundlicher Zellen, die allem Anschein nach von Archäocyten stammen, jedoch kleiner sind als diese und einen nukleoluslosen Kern besitzen. Hierbei handelt es sich um Choanoblasten, die zunächst eine Geißel, später den Kragen ausbilden und sich als Choanocyten zu Kragengeißelkammern zusammenfügen.Die im Mesenchym vorläufig fertiggestellten Kragengeißelkammern gelangen an das Endopinacocytenepithel des ausführenden Kanalysystems. Daraufhin bilden sich die tangierten Choanocyten zu Konuszellen um. Das Endopinacocytenepithel antwortet seinerseits mit der Ausbildung einer Poruszelle pro Kragengeißelkammer. Die Porocyten gehen mittels der konfrontierten Konuszellen dauerhafte Verbindungen mit den zugehörigen, nunmehr funktionstüchtigen Kragengeißelkammern ein.

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Structure and function of the fresh-water sponge Ephydatia fluviatilis L. (Porifera)VIII. The origin and development of the flagellated Chambers and their junction with the excurrent canal system

Summary The flagellated chambers of Ephydatia fluviatilis arise at scattered sites within the mesenchyme. Each such site is marked by an accumulation of rounded cells, which appear to be derived from archaeocytes in most respects except that they are smaller than the latter and have no nucleoli in the nucleus. These are choanoblasts, which first develop a flagellum and later a collar; eventually, as choanocytes, they become arranged so as to form a flagellated chamber.Having reached this preliminary stage of completion in the mesenchyme, the flagellated chambers migrate to the endopinacocyte epithelium of the excurrent canal system. Then the choanocytes at the contact point are converted to cone cells. The endopinacocyte epithelium in turn responds by developing one pore cell for each flagellated chamber. The porocytes become permanently joined to the chamber by way of the adjacent cone cells, and from this time on the flagellated chamber is functional.

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Abkürzungen A Archäocyte - aK ausführender Kanal - B Bakterien - Ch Choanocyte - eK einführender Kanal - G Geißel - GK Kragengeißelkammer - GK-A Anlage von Kragengeißelkammern - K Zellkern - Kr Kragen - KZ Konuszelle - M Mesenchym - N SiO₂-Nadel - PC Endopinacocyten - PD Pinacoderm - PV pulsierende Vakuole - PZ Porenzelle - S Gemmulaschale - Sk Skleroblast - Sp Spongin - SR Subdermalraum     

Condensation rhythm of fresh-water sponges (Spongillidae, Porifera)

The mesenchyme continuum of spongillids exhibits rhythmic changes that at first glance appear to be contractions. Actually, however, the process is a condensation initiated by the formation of punctate cell contacts and a rapid swelling of all mesenchymal cells. As the cells come into closer contact and the spaces between them are constricted, the volume of the mesenchyme shrinks, giving the impression of a contraction. It seems likely that rhythmic mesenchyme condensation assists the choanocyte chambers in pumping water through the sponge.     

Ultrastructural study of differentiation processes during aggregation of purified sponge archaeocytes

Archaeocytes from the spongeEphydatia fluviatilis were dissociated and then isolated on Ficoll density gradients. Their aggregation and reconstitution processes were studied by transmission electron microscopy to determine their capabilities for differentiation.Archaeocyte aggregates follow a well defined sequence of differentiation to generate the characteristic structures of a sponge. Pinacoderm is the first structure to be regenerated and appears progressively at the surface of the 12 h aggregates. Pinacocytes which have differentiated in archaeocyte aggregates are identical to native ones except that the nucleolus remains in most cells. The choanocytes appear only after 24 h by a two step process. First, small cells (choanoblasts) are formed from archaeocytes by mitosis. These cells then transform into fully differentiated choanocytes possessing collars and flagella. The early choanocyte chambers are small, irregular and randomly dispersed in the aggregates. Finally, collencytes and sclerocytes begin to appear just before the aggregates spread on the substrate.The differentiation of a suspension of pure archaeocytes is a unique model system to study sponge cell differentiation and has allowed us to demonstrate that archaeocytes isolated from developed sponges maintain the capacity to differentiate even though this capacity is not usually expressed.     

Untersuchungen zum Körperbau von Meeresschwämmen. II. Das Wasserleitungssystem vonHalichondria panicea

The water-conducting system ofHalichondria panicea (Pallas) shows the classical eurypylous type. The excurrent system consists of a multiple anastomosing network of channels, within which numerous flagellated chambers are embedded in such a way that their apopyles open into the excurrent canals. Each flagellated chamber communicates with the incurrent system by way of several prosopyles.