Ultrastructure of the lycophora larva of Gyrocotyle urna (Cestoda,Gyrocotylidea)

Summary The ultrastructure of the protonephridial system of the lycophore larva of Gyrocotyle urna Grube and Wagener, 1852, is described. It consists of six terminal cells, at least two proximal canal cells, two distal canal cells and two nephridiopore cells. The terminal cells and the proximal canal cell build up the filtration weir with its two circles of weir rods. The proximal canal cell constitutes a solid, hollow cylinder without a cell gap and desmosome. The distal canal cell is characterized by a strong reduction of the canal lumen by irregularly shaped microvilli. The nephridiopore region is formed by a nephridiopore cell; its cell body is located at some distance proximally within the larva. The connection among different canal cells is brought about by septate desmosomes. Morphological, evolutionary and functional aspects of the protonephridial system within Platyhelminthes are discussed. The structure of the proximal canal cells without a desmosome is considered an autapomorphy of Cestoda.Abbreviations ci cilia of the terminal cell - Co distal canal cell - col lumen of the distal canal cell - Ep epidermis - er outer rods of the filtration weir - il inner leptotriches - ir inner rods of the filtration weir - ld lipid droplets - mt microtubule - mv microvilli - Nc nephridiopore cell - Ne neodermis anlage cells - nu nucleus - pC proximal canal cell - ro ciliary rootlets - sd septate desmosome - Tc terminal cell     

Ultrastructure of the protonephridial system of regenerating Stenostomum sp. (Plathelminthes,Catenulida)

Summary The ultrastructure of the flame bulbs, protonephridial capillaries and duct of fully developed and regenerating Stenostomum sp. is described. Flame bulbs are formed by a single cell whose nucleus is located basally or laterally to the weir. The weir is formed by a single row of transverse ribs connected by a thin membrane, apparently of extracellular matrix. Internal leptotriches arise from the proximal cytoplasm and extend in a (usually) single row along the weir and into the lumen of the distal cytoplasmic tube. Many or all leptotriches do not fuse with the distal cytoplasm. Two cilia are each anchored in the proximal cytoplasm by a cross-striated vertical and lateral rootlet, the latter bent forward and extending for some distance into one of the two cytoplasmic cords along the weir. Each cord contains the lateral rootlet in its proximal part, as well as many microtubules. The distal cytoplasmic tube contains two (longitudinal) junctions, i.e. lines of contact between cell processes of the same, terminal cell. Occasionally, more than two junctions were seen, apparently due to branches of the terminal cell in contact with each other. Flame bulbs join capillaries lined by several canal cells type I, containing few or no microvilli but lateral flames. Such capillaries join a duct (or ducts?) lined by canal cells type II with many long microvilli. The large protonephridial duct is lined by numerous cells with lateral flames and many long microvilli. In regenerating tissue (10.5 hours after cutting) some flame bulbs were free, i.e. not connected to capillaries, and some capillaries openly communicated with the surrounding intercellular space. In the presence of a single row of ribs in the weir, of internal leptotriches, and of vertical and lateral ciliary rootlets, the flame bulb of Stenostomum sp. resembles that of other Plathelminthes much more closely than hitherto thought. The species differs from non-catenulid plathelminths mainly in the large number of glandular cells lining the large protonephridial ducts, in the transverse orientation of the ribs in the weir and in the presence of only two cilia in the flame.     

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 of the Flame Bulbs and Protonephridial Capillaries of Artioposthia sp. (Platyhelminthes,Tricladida, Geoplanidae)

The protonephridial terminal complex of Artioposthia is formed by one or two terminal cells, each with a nucleus located in the lateral wall of the flame bulb, and probably two proximal canal cells forming the wall of the protonephridial capillary. The weir is restricted to the proximal parts of the flame bulbs and consists of convoluted slits separated by thick cytoplasmic columns. Cross-striated ciliary rootlets running parallel with and obliquely or transversely to the longitudinal axis of the flame bulbs strengthen the walls of the flame bulbs and, to a lesser degree, that of the capillary. Numerous cristate mitochondria are present in the terminal and proximal canal cells. Cytoplasmic processes extend from the terminal cells into the adjacent tissue, and narrow internal leptotriches extend from the cytoplasm of the terminal cells into the lumen of the flame bulbs. The wall of the capillary contains many interconnected, liquid filled spaces that communicate with the lumen of the capillary, and two septate junctions. Phylogenetic implications of the findings are discussed.     

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.     

Ultrastructure of the flame bulbs and protonephridial capillaries of Prorhynchus (Lecithoepitheliata, Prorhynchidae, Turbellaria)

The flame bulb of Prorhynchus is formed by a single cell. Its nucleus is not located in the cytoplasm at the base of the flame. Cilia of the flame have cross-striated hollow ciliary rootlets converging towards their tips. The maximum number of cilia counted was 13. The weir consists of a single row of longitudinal ribs that contain longitudinal filaments and possess regularly arranged protrusions along their surface appearing as transverse bands in horizontal section. A 'membrane' of extracellular material extends between the ribs. and loose material fills the places between the ribs, with a denser layer midway between adjacent ribs. Distally, the ribs fuse to form a continuous tube without a junction. Small protonephridial capillaries lack junctions, larger ducts have lateral flames and patches of long microvilli. Large excretory ducts open into a ciliated and lamellated cavity which is connected by a narrow canal to the excretory pore. The terminal part of the canal close to the pore possesses many cilia and microvilli. Phylogenetic implications of the findings are discussed.     

The evolution of protonephridia of the Platyhelminthes

Three types of flame bulbs are distinguished in the Platyhelminthes: type 1 has two cilia arising from a terminal cell and rootlets extending along the weir; type 2 has many cilia arising from a terminal cell and the proximal canal cell closely aligned with it; and type 3 has a non-terminal perikaryon forming many flame bulbs, each with many cilia and a single row of longitudinal ribs. Each type appears in various structural forms. Type 1 is found in the Catenulida; type 2 in the Macrostomida, Polycladida, Prolecithophora, Proseriata, Tricladida, Fecampiidae, and Neodermata; and type 3 in the Rhabdocoela and Lecithoepitheliata. The most likely evolutionary sequence is that type 3 is derived from type 2 and, perhaps, that type 2 is derived from type 1. Characters of the protonephridia show that the Rhabdocoela and the Neodermata form separate phylogenetic lineages; other similarities between these taxa are due to convergent evolution (or horizontal gene transfer?).     

扩张莫尼茨绦虫(绦虫纲:圆叶目)的原肾管及原肾管概念的评述

本研究应用透射电子显微镜研究了扩张莫尼茨绦虫原肾管的细胞学特征 ,莫尼茨绦虫原肾管的焰茎球为一个过滤器结构 ,类似于“挡河坝”样构造 ,此构造由端细胞和近管细胞外突形成的肋条 (或称杆 )相互交错排列而成。肋条之间由细胞外物质构成的“膜”结构连接 ,过滤作用通过该“膜”发生。焰细胞与近管细胞交界处有裂缝或孔与细胞外的结缔组织 (实质组织 )相通 ;原肾管的毛细排泄管细胞质索之间没有隔状联结 ;毛细排泄管及排泄管的管腔内有大量珠状微绒毛突起以增加表面积。从扩张莫尼茨绦虫及其它一些无脊椎动物原肾管的研究结果表明 ,原原肾管概念将焰细胞作为封闭的盲端已不再合适 ,需要进行修订 ,建议修订为 :原肾管是一种焰细胞系统 ,通常由焰细胞、管细胞和肾孔细胞组成 ,焰茎球作为过滤装置与周围的结缔组织 (实质组织 )有或没有裂缝 (孔 )相通     

The excretory organs in Sphaerodorum flavum (Phyllodocida, Sphaerodoridae): a rare case of co-occurrence of protonephridia, coelom and blood vascular system in Annelida

The excretory organs of Sphaerodorum flavum (Sphaerodoridae) were investigated by TEM and reconstructed from serial ultrathin sections. These organs are segmentally arranged paired protonephridia, which are in close association with a well-developed blood vascular system. Each protonephridium consists of a terminal part made up of two monociliary terminal cells (solenocytes), and a nephridioduct, formed by two cells. The two solenocytes lie close together. Each cilium is surrounded by 12 microvillar rods projecting from the perikaryon of each solenocyte. These rods form a weir-like structure in the coelomic space. The distal part of the weir is embedded in the proximal nephridioduct. The largest part of the cell bodies of the solenocytes, containing the nucleus, is lateral or basal to the weir-like structures. The lumen of the nephridioduct is formed by two multiciliated cells, which enclose the extracellular nephridial canal one behind the other. The canal opens through the nephropore beneath the cuticle without penetrating the cuticle. Both nephridioduct cells are surrounded by a blood vessel, which is partially folded into several layers. The significance of a simultaneous occurrence of protonephridial excretory organs and a well-developed blood vascular system as well as coelomic cavities is discussed. The results of this investigation indicate a close relationship of Sphaerodoridae to Phyllodocidae instead of to Syllidae within the Phyllodocida. Accepted: 27 November 2000     

Ultrastructure of excretory system in <Emphasis Type="Italic">Bothrioplana semperi</Emphasis> (Platyhelminthes,Turbellaria)

The ultrastructure of flame bulbs and epithelium of excretory canals in Bothrioplana semperi (Turbellaria, Seriata) have been studied. The flame bulbs consist of two cells, the terminal cell and the proximal canal cell. The weir is formed by two rows of longitudinal ribs. The ribs of the internal row originate from the flame cell, external ribs are formed by the proximal canal cell. Each external rib has a remarkable bundle of microfilaments, originating in the cytoplasm of the first canal cell distally to the bases of external ribs. Membrane of internal ribs is marked by small electrondense granules, separate or fused to an electron-dense layer, continuous to dense “membrane,” connecting both external and internal ribs. Sparse internal leptotrichs originate from the bottom of the flame bulb cavity. External leptotrichs are lacking. Septate junction is present only in proximal canal cell at the level of tips of cilia. The apical surface of the canal cell bears rare short microvilli. The basal membrane of canal cells forms long invaginations that may reach nearly the apical membrane. The epithelium of excretory canals lacks the cilia. The ultrastructure of flame bulbs and epithelium of the excretory canals in B. semperi shares representatives of suborder Proseriata (Seriata). The contradiction exists in interpretation of the structure of flame bulbs in Proseriata. Ehlers and Sopott-Ehlers assumed that the external ribs are derivatives of the proximal canal cell and internal ones are outgrowths of the terminal cell, while Rohde has found conversely: the external ribs are outgrowths of the terminal cell, the internal ones are outgrowths of the proximal canal cell. However, the illustrations provided by Rohde do not enable to ascertain what cells the internal and external ribs derive from, while illustrations provided by Ehlers justify his interpretation. The order of weir formation in B. semperi confirms the viewpoint of Ehlers. The implication of ultrastructure of flame bulbs in Proseriata, especially of the order of flame bulb formation, in the Platyhelminthes phylogeny has been discussed.     

Ultrastructure of the Protonephridial Terminal Organ in the Terrestrial Nemertean Geonemertes pelaensis (Rhynchocoela,Enopla, Hoplonemertini)

The protonephridial terminal organ of Geonemertes pelaensis consists of two cells that are equal in both size and shape and form mirror images of each other. From the perinucleate lump-shaped region basally in each cell arises a cytoplasmic column which branches at regular intervals to form 3–4 nearly circular bars. The opposed columns and alternating bars of the two cells are arranged in such a way that they form an obviously rigid, cylindrical structure which both supports and gives rise to the thin-walled weir. The fenestration of the weir consists of a single, enormously extended and sinuous cleft which represents part of the boundary between the two terminal cells. The ciliary flame comprises 92–118 cilia that originate basally in the two cells and is surrounded by a palisade of long, straight microvilli, positioned immediately within the weir's wall. The structure is very similar to that of the terminal organ of the land nemertean Pantinonemertes, except that the cytoskeleton which supports columns and bars consists of a fibro-granular substance, not, as in Pantinonemertes, of an abundance of oriented microtubules.     

Protonephridial organs in Myzostoma cirriferum (Myzostomida)

Myzostoma cirriferum Leuckart, 1836 possesses five paired, serially arranged, blindending nephridial organs which are described for the first time. Ultrastructural investigations reveal that each nephridium is composed of three terminal cells and one tubular cell that forms the emission tubule. The central lumen of the individual terminal cells contains six to nine flagella, each of which is surrounded regularly by cytoplasmic rods arranged in parallel. Weir-like fenestrations in the peripheral wall of the terminal cells make up the connection between the central lumina and the extracellular space around the nephridial organ. The canal of the emission tubule possesses cilia, microvilli and cytoplasmic structures, suggesting involvement of this cell with active transport and storage. It opens into the cuticle at the ventral surface of the animal.     

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.     

Ultrastructure of the copulatory organ of Haplopharynx quadristimulus and its phylogenetic significance (Plathelminthes,Haplopharyngida)

Summary The male reproductive system in Haplopharynx quadristimulus consists of paired testes, sperm ducts, seminal vesicles, seminal ducts, a copulatory organ containing prostatic vesicle and stylet apparatus, and the male canal. By electron microscopy all components appeared to be regional specializations of a canal extending from the testes to the body wall and lined by a multiciliated epithelium. The epithelium of the stylet apparatus contained six different cell types. One cell type (matrix syncytium) formed the stylet and the other five were located distal to the stylet/prostatic-vesicle junction along the male system epithelium. Each cell type was attached to the supporting intercellular matrix at a different level along the stylet apparatus. All cell types extended to the distal end of the stylet apparatus regardless of where they originated along its length. The cells in the apparatus lacked cilia, but one of the cell types contained rootlets. Modified rootlets or rootlet derivatives were possibly present in another cell type in the form of rootlet-like ribbons. The findings support the monophyly of the Macrostomida Haplopharyngida (by common occurrence of a matrix syncytium) and at the same time suggest their separation as two distinct taxa (by differences in the structure of the prostatic vesicle and other parts of the stylet apparatus).Abbreviations a accessory spine - c circular muscle - ce centriole - ci cilium - di dictyosome - e epithelial cell - ed ejaculatory duct - ep epidermal cell - f rootlet-like ribbon - g prostatic gland cell neck - g1 type I gland cell granules - g2 type II gland cell granules - g3 type III gland cell granules - h hemidesmosome - i intercellular matrix - im internal muscle - j septate junction - l stylet apparatus lumen - le spine lateral extension - lm longitudinal muscle - m matrix syncytium - mc male-canal epithelial cell - me male canal - mp male pore - mt microtubules - mv microvilli - n nucleus - nc nerve cell body - np nerve process - om oblique muscle - p prostatic vesicle epithelial cell - pv prostatic vesicle - r rootlet - s stylet - sa stylet apparatus - sc sensory receptor - sd sperm duct - se seminal duct - sl stylet lumen - sp spot desmosome - sr sperm - sv seminal vesicle - t terminal web - te testis - u ultrarhabdite - z zonula adhaerens - 2 cell type 2 - 3 cell type 3 - 4 cell type 4 - 6 cell type 6     

Photoreceptors of Bryozoan Larvae (Cheilostomata, Cellularioidea)

The ultrastructure of potential photoreceptors in larvae of Tricellaria occidentalis and four species of Bugula is described and compared with previously reported photoreceptors in larvae of Bugula neritina and Scrupocellaria bertholetti. A single sensory cell forms the functional unit of each photoreceptor. This cell is distinguished by a concentration of pigment vesicles in its apical part, a direct connection with the nervous system, and a large number of cilia that form the photoreceptoral organelle. These cilia have axonemes morphologically identical to those of motile cilia. The membranes of sensory cilia are unbeaded and qualitatively less osmophilic than those of the motile cilia of adjacent accessory and coronal cells. Three photoreceptor types are designated based on topological complexity: Type I, in which the sensory cell is flush with adjacent coronal cells and the photoreceptoral organelle is unprotected; Type II, in which the apical surface of the sensory cell is invaginated, forming a lumen containing the photoreceptoral organelle; and Type III, in which the sensory cell is at the base of an epidermal invagination and the photoreceptoral organelle is protected in a lumen formed by the sensory cell and accessory cells. There is a greater range of morphological variation among photoreceptors in larvae of Bugula spp. than between those of two species of the related genera Scrupocellaria and Tricellaria.     

Investigations on the protonephridial system of post-larval Gyrocotyle urna and Amphilina foliacea (Cestoda).

The gross morphology and ultrastructure of the different parts of the protonephridial system of the monozoic tapeworms Gyrocotyle urna and Amphilina foliacea are described. The terminal cell in both species has numerous cilia which are interconnected and extend into the lumen of the first canal cell. The filtration area is built up from projections of two cells, the terminal cell and the first canal cell. The first canal cell forms a solid hollow cylinder without a cell gap and a desmosome as found in Neodermata other than cestodes and Udonella. In Gyroctyle the nucleus of the first canal cell is located in the wall cytoplasma whereas more distally located ductules of both species have subepithelial cell bodies containing the nuclei. In both taxa the protonephridial canal system is reticulate. In Amphilina the distal canals lack non-terminal ciliary flames, such ciliary tufts can be found in the larger capillaries of Gyrocotyle. The capillary cilia have rootlets and the ultrastructure of the duct wall cytoplasm containing large numbers of vesicles indicates highly active transport processes. The morphology of the protonephridial systems is discussed with regard to the evolution of Neodermata (especially of the Cestoda) and the function of the protonephridial system in cestodes as a probable organ of nutrient distribution.     

Ultrastructure of flame cells and protonephridial capillaries of Craspedella and Didymorchis (Plathelminthes,Rhabdocoela)

Summary The ultrastructure of the flame cells and protonephridial capillaries of the Rhabdocoela Craspedella sp. and Didymorchis sp., ectocommensals on the freshwater crayfish Cherax destructor in eastern Australia is described. The flame cells of both species have variable numbers of cilia without distinct rootlets and with decreasing numbers of axonemal tubules towards the ciliary tips. Bundles of microtubules extend from the cytoplasm adjacent to the ciliary rootlets through the ribs of the weir apparatus into the distal cytoplasmic tube, where the numbers of microtubules gradually decrease. The weir apparatus is formed by a single row of longitudinal ribs connected by a membrane. In Craspedella, but not in Didymorchis, the ribs have external branched leptotriches. Mitochondria are common in the wall of the flame cell of both species. The protonephridial capillary just above the end of the ciliary tuft narrows in both species and bends sharply in Craspedella. The lumen of the flame cell and the capillary is lined by a dark layer of cytoplasm; there is no enlargement of the surface area by microvilli or lamellae. Centrioles were seen in the capillary wall of Craspedella, and in Didymorchis the cytoplasm around the capillaries has a very loose and light appearance. The ultrastructure of the flame cells and capillaries of both species corresponds closely to that of Temnocephala sp.Abbreviations in the figures BB basal body - CE centriole - L leptotrich - M microtubules - ME membrane of weir apparatus - MI mitochondrion - PC protonephridial capillary - R rib (rod) of weir apparatus     

The development of bioluminescence in the ctenophore Mnemiopsis leidyi

The photocytes of the ctenophore Mnemiopsis have a discontinuous distribution along the radial canal between the sites where the comb plate cilia cells are located on the side of the canal which contains the testes. They are separated from the lumen of the canal by a population of gastric cells. Cytologically these cells are characterized by a condensed nucleus and cytoplasm which stains lightly with basophilic dyes.The ability of the ctenophore embryo to produce light appears at the developmental stage when the comb plate cilia first begin to grow out. At this stage four light-producing areas are present; each area corresponds to one quadrant of the adult animal. At the sites of light production, a population of cells can be identified that have some of the cytological properties of the photocytes of the adult animal. Within 8–10 hr after light production begins there is a 10-fold increase in the amount of light produced by an embryo and a cytological maturation of its photocytes; during this time period there is no increase in photocyte number. At about the time the embryo begins to feed, each light-producing region splits into two regions, each of which corresponds to a radial canal.During the process of embryogenesis the photocyte cell lineage is first segregated from non-photocytes at the differential division which gives the 8-cell stage embryo. The M macromere lineage goes on to form photocytes, but the E macromere lineage does not. The M macromeres form a micromere at the aboral pole of the embryo at each of the next two cleavages; during these cleavages the potential for photocyte differentiation continues to segregate with the M macromeres. During the division which gives the 64-cell stage the M macromeres divide equally; the potential for photocyte differentiation segregates with the M macromeres nearest the oral-aboral axis. M macromeres which are isolated from the embryo at the 8-, 16-, or 32-cell stage of development will continue to cleave as though they were part of a normal embryo and differentiate to form photocytes.The events that are responsible for the differential division during the formation of the 8-cell stage embryo have been studied by centrifuging eggs to produce fragments of different cytoplasmic composition. Egg fragments which contain only cortical cytoplasm differentiate comb plate cilia cells, but do not produce photocytes. Cortical fragments with a small amount of yolk differentiate comb plate cilia cells and photocytes. Both the M and E macromeres from cortical fragments with no yolk produce comb plate cilia. Only M macromeres containing yolk form photocytes; if an M macromere forms photocytes it does not form comb plate cilia.     

The fine structure of the protonephridial system in the land nemertean Pantinonemertes californiensis (Rhynchocoela,Enopla, Hoplonemertini)

Summary The protonephridial terminal organ in the nemertean Pantinonemertes californiensis is composed of two cells that are similar in size and shape and are mirror images of each other. Basally in the organ the two cells combine to form a binucleate cytoplasmic mass. Apically they are intimately joined to form a subcylindrical thin-walled weir apparatus; this part is supported by two opposed cytoplasmic columns running the length of the weir region, one originating from each of the two cells, and by a number of regularly spaced circular bars that arise from the two columns. The ciliary flame consists of 94–114 cilia that originate in the bases of the two cells, and it is surrounded by a palisade of incomplete circlets of long, straight microvilli. The convoluted protonephridial tubule is rich in structures that indicate intensive reabsorption from the primary urine. It is argued that the terminal organs in Pantinonemertes and Geonemertes are fundamentally similar and differ only in the amount of microtubules present in the longitudinal supports.Abbreviations BL basal lamina - CF ciliary flame - CT connective tissue - CV coated vesicle - E endocytotic pit - FM filtration membrane - G Golgi complex - LC longitudinal cytoplasmic column - M mitochondrion - MT microtubules - MV microvilli - N nucleus - NPC nucleus of protonephridial capillary cell - PC protonephridial capillary cell - R rootlets - TB transverse bar - TC terminal cell - WE weir, exterior of fenestrated wall - WI weir, interior of same     

The ultrastructure of the statocysts in the pediveliger larvae of Pecten maximus (L.) (Bivalvia)

The pediveliger of Pecten maximus (L.) has a pair of statocysts situated at the base of the foot on either side of a bilobed pedal ganglion. The statocysts consist of a spherical sac connected to the mantle cavity by a cylindrical ciliated canal. Within the sac there are statoconia which are variable both in shape and structure. The cells of the sac are joined by septate desmosomes. There is a non-ciliated cell in each sac containing a variety of granules some of which resemble certain of the statoconia. The remainder of the sac is composed of hair cells, which bear a circular array of radiating cilia. The basal bodies and horizontal striated roots of these cilia are directed radially. The hair cells give rise to thin processes which probably join together to form the static nerve. This nerve runs from the static canal to the pleural ganglion.