Ultrastructural studies of epidermis, sense receptors and sperm of Craspedella sp. and Didymorchis sp. (Platyhelminthes, Rhabdocoela)

Craspedella has a non-ciliated epidermis with nuclei located in the epidermis and with short microvilli. There is a thin basal lamina and thick underlying fibrous matrix. Rhabdites are secreted through ducts lined by microtubules. Multiciliate sense receptors consist of bundles of dendrites in a depression of the epidermis. Each dendrite has a cilium with a cross-striated rootlet; there are no electron-dense collars. Spermatozoa have peripheral microtubules which in cross-section are arranged in a ring-like or spiral fashion, numerous electron-dense granules, mitochondria and a nucleus; axonemes of the 9 +'1'type are free for most of their length. Centrioles occur in some nerve fibres. In Didymorchis parts of the epidermis are ciliated and epidermal perikarya are 'insunk', connected to the surface part of the epidermis by a single cytoplasmic process. Epidermal cilia have cross-striated vertical and horizontal rootlets. In the ciliary tips a short electron-dense rod along the central pair of tubules extends to the tip, where it widens to become a terminal plate; peripheral doublets gradually disappear by losing their microtubules. Receptors observed are uniciliate. Spermatozoa are as in Craspedella . Ultrastructural evidence indicates that Craspedella and Didymorchis arc closely related and belong to the Rhabdocoela.     

Ultrastructural study of the epidermis in two free-living Platyhelminthes, Mesostoma viaregginum and M. productum (Rhabdocoela, Typhloplanida)

Abstract. The epidermis of the free-living typhloplanids Mesostoma viaregginum and M. productum (Mesostominae) is described. In both species, the epidermis has polarized cells with nuclei located at the basal part of the cell, whereas mitochondria are in the apical one. The epidermis is entirely covered by microvilli and locomotory cilia anchored in the cytoplasm by vertical and horizontal rootlets. Rootlets exhibit distinct length and periodic structure in the two species. Furthermore, in each species vertical and horizontal rootlets possess different periodic structure. The pattern of termination of microtubules in epidermal cilia is described for the first time in the Typhloplanida; central microtubules shift along one axonemal side, doublets 1 and 6–9 lose their microtubule B, and gradually peripheral doublets become singlets. Finally, an electron-dense material caps the tip of the cilia. This pattern of termination closely resembles that of Temnocephalida, Kalytorhynchia, and Dalyelliida examined so far, but differences exist.     

Epidermal ultrastructure of the adult parasitic phase female and encapsulated larva of Kronborgia isopodicola (Platyhelminthes, Fecampiidae): Phylogenetic implications

The parasitic phase female K. isopodicola possesses a ciliated epidermis of polyhedral cells. Adjacent lateral plasma membranes are separated at intervals creating intercellular spaces. Epidermal cilia are anchored by a horizontal rootlet, opposite which a spur projects from the basal body, and a narrow vertical rootlet. The cytoplasm contains coated vesicles, and coated pits lie between microvilli. Large granular and vesicular bodies (rhabdoids) are scattered through the epidermal epithelium; in the epidermis of the encapsulated larva, granular rhabdoids are densely packed and slender, more compact bodies also occur. The compact, granular and vesicular bodies are probably morphological variants of the same epidermal structures, suggested to undergo sequential changes accomplished in later stages by lysosomal activity. Morphologically similar epidermal bodies are found in triclads. They are also characteristic of the parasitic genus Urastoma, which shares other ultrastructural features with K. isopodicola. The Neodermata may have arisen from parasitic turbellarian forms, at a more “primitive” level of organization than ancestors of the contemporary Rhabdocoela.     

Epidermis and protonephridia of a free-living platyhelminth, Mesocastrada führmanni Voltz, 1898 (Rhabdocoela, Typhloplanoida): An ultrastructural study

The ultrastructure of the epidermis and the protonephridia of the free-living rhabdocoel Mesoscastrada führmanni is described. The epidermis consists of polarized cells, the nucleus located in the basal part of the cell and the mitochondria in the apical part. The surface is entirely covered by cilia anchored in the cytoplasm by horizontal and vertical striated rootlets. Cilia of the flame bulbs also have horizontal and vertical striated rootlets. The weir apparatus of the cyrtocyte is composed of a single row of ribs connected by a thin “membrane” of extracellular material. Bundles of microtubules, located in the ribs originate in the centrioles. Epidermal cells and flame bulbs of M. führmanni closely resemble those of the other Typhloplanoida examined so far.     

Ultrastructural studies of Austramphilina elongata (Cestoda,Amphilinidea)

Summary The fine structure of larval Austramphilina elongata is described using serial semithin and ultrathin sections. Densely packed germ cells with many ribosomes and mitochondria and with large Golgi complexes fill the middle third of the body. Some necrotic nuclei were observed near the anterior end. The neodermis consists of a subepidermal syncytium connected to pericarya in the parenchyma by means of cytoplasmic processes containing peripheral microtubules; electron-dense ovoid bodies condense in these processes. Myoblasts are connected to muscle fibres by means of cytoplasmic connections rich in mitochondria. Twelve (exceptionally eleven) type I gland cells containing large secretory granules and extensive granular endoplasmic reticulum are located in the dorso-posterior part of the body; they open through 12 (or 11) discrete ducts into an anterior invagination of the tegument which is covered by epidermis and not connected to the outside. Ten type II gland cells containing elongate secretory granules with regularly arranged longitudinal microtubules are located ventral to the type I cell bodies; they open on a ventral papilla a short distance behind the anterior end. Ten type III gland cells containing irregularly round-oval secretory granules with coiled microtubules are located anterior and ventral to the type I gland cells; they open through five discrete ventro-anterior openings on each side of the body. Ducts of all gland cells have mitochondria and microtubules. The spermatozoon has a basic pattern of two axonemes, each with a single central filament, a mitochondrion (mitochondria), and a row of surface microtubules interrupted by the axonemes. In the tips of epidermal cilia, doublet 1 and doublets adjacent to it lose their microtubules B first and close in on the central pair of filaments in a spiral fashion, enclosing an electron-dense rod. Presence of a neodermis and ultrastructure of the spermatozoon support the validity of the taxa Neodermata Ehlers and Trepaxonemata Ehlers and are strong evidence against a phylogenetic relationship of the cestodarians — cestodes with the Acoelomorpha; this is also indicated by the ultrastructure of sense receptors and epidermal ciliary rootlets.     

The spermatogenesis and sperm structure of Acerentomon microrhinus (Protura, Hexapoda) with considerations on the phylogenetic position of the taxon

The spermatogenesis of the proturan Acerentomon microrhinus Berlese, (Redia 6:1–182, 1909) is described for the first time with the aim of comparing the ultrastructure of the flagellated sperm of members of this taxon with that of the supposedly related group, Collembola. The apical region of testes consists of a series of large cells with giant polymorphic nuclei and several centrosomes with 14 microtubule doublets, whose origin is likely a template of a conventional 9-doublet centriole. Beneath this region, there are spermatogonial cells, whose centrosome has two centrioles, both with 14 microtubule doublets; the daughter centriole of the pair has an axial cylinder. Slender parietal cells in the testes have centrioles with nine doublet microtubules. Spermatocytes produce short primary cilia with 14 microtubule doublets. Spermatids have a single basal body with 14 microtubule doublets. Anteriorly, a conical dense material is present, surrounded by a microtubular basket, which can be seen by using an α-anti-tubulin antibody. Behind this region, the basal body expresses a long axoneme of 14 microtubule doublets with only inner arms. An acrosome is lacking. The nucleus is twisted around the apical conical dense structure and the axoneme; this coiling seems to be due to the rotation of the axoneme on its longitudinal axis. The posterior part of the axoneme forms three turns within the spermatid cytoplasm. Few unchanged mitochondria are scattered in the cytoplasm. Sperm consist of encysted, globular cells that descend along the deferent duct lumen. Some of them are engulfed by the epithelial cells, which thus have a spermiophagic activity. Sperm placed in a proper medium extend their flagellar axonemes and start beating. Protura sperm structure is quite different from that of Collembola sperm; and on the basis of sperm characters, a close relationship between the two taxa is not supported.     

Ultrastructure of gill cilia and ciliary rootlets of Chaetoderma nitidulum Lovén 1844 (Mollusca, Chaetodermomorpha)

Cilia and associated structures on the gill lamellae on the ctenidum of Chaetoderma nitidulum were studied. The gill cilia are very long and have a whip-like narrow portion distally, where only three microtubule doublets continue to the distal tip. In the transition zone between the cilium and the centriolar triplet section of the basal body there is a dense plate, an aggregation of granules and a ciliary necklace with four strands. Further down there is a short cross-striated basal foot and two conical cross-striated ciliary rootlets. The first rootlet is flattened and directed forward. It connects distally with the basal feet of other adjacent cilia. The second rootlet is rounded in cross-section and vertically directed. The epithelial structures of Chaetoderma show similarities with other Mollusca. We found no structural characters that could support the current hypothesis of a close relationship of Xenoturbella to the Mollusca.     

Ultrastructure of the epidermis of adult and embryonic Paravortex species (Turbellaria,Eulecithophora)

The epidermis and associated structures of adult and embryonic Paravortex cardii and Paravortex karlingi, internal parasites of Cerastoderma edule, have been examined using scanning and transmission electron microscopy. The cellular epidermis of adult Paravortex bears cilia and microvilli which differ in number and distribution between P. karlingi and P. cardii. Cellular organelles include mitochondria, lipid bodies, Golgi bodies, and ultrarhabdites. Epidermal nuclei are located in the proximal portion of the cells. The development of the tegument of embryo Paravortex has been described and a possible origin for the embryo capsule is suggested. These findings are discussed in relation to the phylogenetic status of the Turbellaria in relation to other Platyhelminthes and in the functional adaptation of the epidermis for a parasitic mode of life.Abbreviations bb- basal bodies - bl- basal lamella - c- cilia - cp- capsule - dc- dark cells - e- embryos - ep- epidermis - g- Golgi bodies - int- interdigitation (of cells) - l- lipid - lf- lamellar fold - mc- migrating cell - mf- membranous folds - mt- mitochondria - mv- microvilli - n- nucleus - nb- neoblasts - p- projections of epidermis - par- parenchyma of mother - pr- primary rootlet - rc- rhabditogen cells - sr- secondary rootlet - ur- ultrarhabdites - vt- vitelline material     

The spermatogenesis and sperm structure of Timema poppensis (Insecta: Phasmatodea)

The ultrastructure of spermatogenesis and spermatozoa was studied in Timema poppensis Vickery & Sandoval, 1999, a putative basal taxon of Phasmatodea. The apical portion of testis follicles consists of spermatogonial cells with polymorphic nuclei. Primary spermatocytes display very short primary cilia originating from the peripheral centrosomes. Early spermatids develop a conspicuous “nebenkern” consisting of fused mitochondria. They have a single peripheral centriole with microtubular triplets, which expresses a 3.6-μm-long cilium featuring a 9?+?2 axonemal pattern. In a later stage, the centriole and the ciliary shaft displace toward the inner part of the cytoplasm by an infolding of the plasma membrane. Mature spermatids exhibit a derived centriole with microtubule doublets devoid of dynein arms, which is equipped with a dense arc-like outer structure. Ciliary degeneration was not observed during spermiogenesis. Spermatozoa are short flagellate cells about 55–60?μm in length. They are characterized by a three-layered acrosomal complex. The distinctive bell-shaped morphology of the acrosome vesicle is likely an autapomorphic trait of Timema. The flagellum has a 9?+?9?+?2 axoneme, two accessory bodies, two flattened cisterns, and two elongated mitochondrial derivatives. Results support the hypothesis that Phasmatodea, comprising Timema?+?Euphasmatodea, form a monophyletic group. The presence of 17 protofilaments in the wall of accessory microtubules and the flattened configuration of the flagellum are potential apomorphic groundplan features of the order. Within Phasmatodea, a key evolutionary divergence was from the conventional insect spermiogenesis and sperm structure of Timema, to the unusual spermiogenetic process and peculiar sperm structure of Euphasmatodea. As a result, Timema retains more sperm character states found in the polyneopteran ground pattern, while Euphasmatodea have evolved outstanding sperm autapomorphies, like the loss of mitochondria and flattened cisterns, and the presence of strongly expanded accessory bodies.     

False Vertical Rootlets of Epidermal Cilia in the Oncomiracidia of Neoheterocotyle rhinobatidis and Monocotyle spiremae (Platyhelminthes,Monogenea, Monopisthocotylea)

Epidermal cilia of the oncomiracidium of Neoheterocotyle rhinobatidis (Monogenea, Monopisthocotylea, Monocotylidae) have long cross-striated vertical rootlets that are not extensions of the basal bodies as are the vertical rootlets in all catenulid and rhabditophoran turbellarians examined to date. Instead, they originate in the basal part of the horizontal rootlet a short distance from the basal bodies. In Monocotyle spiremae (Monocotylidae), the vertical rootlets are less distinct, with no apparent cross-striation, but they also originate from the basal part of the horizontal rootlets. Epidermal cilia of the oncomiracidium of Zeuxapta seriolae (Monogenea, Polyopisthocotylea, Axinidae) lack vertical rootlets like all other neodermatans examined, but bundles of fibres extend from the basal bodies a short distance into the cytoplasm of the epidermal cells. Monopisthocotylean Monogenea would be intermediate between rhabditophorans and the other neodermatans (in having weakly developed vertical rootles), if these structures were homologous in the two groups. However, in view of the different origin of vertical rootlets in turbellarians and monopisthocolylean oncomiracidia, it is suggested that they are not homologous, and vertical rootlets in the Monopisthocotylea are therefore named “false vertical rootlets”.     

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?).     

Structural biology of cytoplasmic and axonemal dyneins

Dyneins are microtubule-based, ATP-driven motor proteins with six tandemly linked AAA+ domains, a long N-terminal tail and a coiled-coil stalk. Cytoplasmic dyneins function as individual homodimers and are responsible for minus-end-oriented transport along microtubules. Axonemal dyneins of flagella/cilia are anchored in arrays to peripheral microtubule doublets by their N-terminal tails, and generate sliding motions of adjacent microtubule doublets toward the plus end. The coiled-coil stalk is responsible for communication between the AAA+ domains and the microtubule binding domain. A number of isoforms of axonemal dyneins are integrated to generate bending motion. In this article I will review recent structural studies and address the question as to how dyneins generate force and cause bending in flagella/cilia.     

Unciliated body surface in three species of the Umagillidae (Dalyellioida,Platyhelminthes)

Anoplodiera voluta Westblad, Seritia elegans (Westblad) and Wahlia macrostylifera Westblad are species of the family Umagillidae living in the intestine of the holothurian Parastichopus tremulus. In all three species, part of the epidermis is unciliated, but unlike unciliated epidermis in the major parasitic flat-worm groups, it is cellular and has intraepithelial nuclei. The surface of the unciliated cells in A. voluta is convex with the cells separated by lateral gaps. The cells have two distinct regions: the basal, organelle-rich part and the apical part which contains few identifiable organelles except vesicles. In W. macrostylifera the unciliated cells have a flat surface and, between them, narrower gaps that in some cases widen to paracellular compartments below the cell surface. Apically the cells contain electron-dense vesicles, often in contact with the surface. S. elegans has unciliated cells separated by gaps. In all these species, apical vesicles indicate secretory activity. Comparison of the epidermis of these three species with the neodermis of the major parasitic flatworm groups within the Neodermata does not support a close relationship of the three to the Neodermata.     

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

Summary The fine structure of the ciliated epidermis, the body musculature and the neodermis anlage cells of the free-swimming lycophora larva of Gyrocotyle urna Grube and Wagener, 1852, is described. The epidermis is syncytial and covers the whole body including a caudal cavity into which the larval hooks protrude. It contains several types of vesicles, mitochondria and membrane whorls but lacks nuclei, dictyosomes and endoplasmic reticulum. The locomotory cilia exhibit single rostrally directed rootlets. The body musculature consists of about 25 longitudinal and 42 circular muscles. Their nuclei are located proximally to the contractile elements. The neodermis anlage cells show numerous dictyosomes, elaborated cisternae of endoplasmatic reticulum, typical coated vesicles and membranous bodies. Extrusions of these cells do not penetrate the epidermis but contact it by desmosoms.The evolution of epidermal and neodermal structures of Gyrocotyle and other parasitic Platyhelminthes is discussed. The probable consequences of the lack of some types of organelles in the epidermis of Neodermata are considered.Abbreviations bb basal body - bl basal lamina - ci locomotory cilia - Ce epidermis of the caudal cavity - cr ciliary rootlet - di dictyosome - Ep epidermis - er endoplasmic reticulum - Hm hook musculature - ld lipid droplet - Lh larval hook - Lm longitudinal musculature - mi mitochondria - mt microtubule - mv microvilli - mw membrane whorl - Ne neodermis anlage cell - nu nucleus - Re receptor - Rm circular musculature - ve vesicles     

The ciliated epidermis of Xenoturbella bocki (Platyhelminthes, Xenoturbellida) with some phylogenetic considerations

The epidermis of Xenoturbella bocki Westblad was studied by scanning and transmission electron microscopy. Two cell types predominate in the epidermis: multiciliated epidermal cells and non-ciliated or monociliated gland cells. A conspicuous feature is the dense ciliary coverage and the numerous gland cell openings. Xenoturbella has a characteristic pattern of axonemal filament termination in the distal tips of their cilia. Each epidermal cilium has the typical 9 + 2 patten through the major part of its shaft. Near the tip there is a shelf at which doublets 4–7 terminate. Doublets 1, 2, 3, 8 and 9 continue into the thinner distal part of the cilium. A similar shelf in cilia is known only from the turbellarian orders Nemertodermatida and Acoela, and hence may be an apomorphic feature which indicates a close relationship between Xenoturbellida, Nemertoder-matida and Acoela. The basal body is provided with a so-called basal foot which has a cross-striated appearance and an expanded distal plate that seems to act as a microtubule organizing center. Approximately 15–25 microtubuli radiate from the endplate of the basal foot to the basal bodies caudally. The arrangement of basal foot and ciliary rootlets in Xenoturbella differs from that of Acoela and related orders in that there are two striated rootlets only (an anterior and a posterior one), rather than one main rootlet and two lateral rootlets.     

Fine Structure and Function of Pharynx Cilia in Glossobalanus minutus Kowalewsky (Enteropneusta)

Two kinds of cilia have been observed in the pharynx of Glossobalanus minutus Kowalewsky. From the present study, a ciliary specialization can be found in order to move a determinate substance, i.e. mucus or water. Mucus-moving cilia (type I cilia) have a single basal centriole and poorly developed ciliary rootlets. Their tips are rounded, bearing an inner, asymmetrical cap attached to some tubules. Water-moving cilia (type II cilia) are exclusively located at lateral epithelia of branchial bars, giving rise to the water current through the gills. They have two basal centrioles, proximal and distal, and a complex system of ciliary rootlets made up of a principal rootlet, a secondary or accessory rootlet and a 'fan' rootlet. The tips of type II cilia have a long process with some tubules inside. All basal structures are precisely orientated in order to assure a good coordination of ciliary beat. The possible functional significance of ciliary substructure is also discussed. From these observations a model for mucus and water currents through gill slits is postulated.     

Spermatozoa and spermatogenesis in Genostoma kozloffi (Plathelminthes, Rhabdocoela)

Genostoma kozloffi Hyra, 1993, is a symbiotic plathelminth living beneath the carapace of Nebalia pugettensis (Clark, 1932) (Leptostraca Crustacea). Because of similarities in the structure of its epidermis with that of the major group of parasitic flatworms, the Neodermata, and because of similarity in body form to a member of the Revertospermata, which includes the Neodermata and taxa that may be the sister group to the Neodermata, Genostoma was recently classified in the Revertospermata. By electron microscopy we found, however, that spermiogenesis in G. kozloffi does not occur in the manner characteristic of the Revertospermata. Instead, positioning of the axoneme and nucleus takes place as in free-living turbellarians in which the axoneme is fully incorporated, that is, in a distal-proximal fashion. Mature spermatozoa of G. kozloffi are filiform, possess an elongate rod-like nucleus, and one short single, fully incorporated axoneme. A rod of multiple, fused mitochondria accompanies the nucleus and axoneme, and an array of cortical microtubules with thickened walls runs the length of the sperm. Neither dense bodies nor acrosomal vesicles could be found. These features of its spermatozoa as well as the presence of a tunica surrounding its testes are reminiscent of the free-living group Kalyptorhynchia, specifically the Schizorhynchia. It is unlike the Kalyptorhynchia in other respects, however, and its systematic position remains uncertain, albeit removed from the Revertospermata.     

Comparative ultrastructure of the epidermal ciliary rootlets and associated structures in species of the Nemertodermatida and Acoela (Plathelminthes)

 The fine morphology of epidermal ciliary structures in four species of the Nemertodermatida and four species of the Acoela was studied, with emphasis on Meara stichopi (Nemertodermatida). The cilium of M. stichopi has a distal shelf and is proximally separated from the basal body by a cup-shaped structure. The bottom of the cup consists of a bilayered dense plate, or basal plate. The basal body consists of peripheral microtubule doublets continuous with those of the cilium. In the upper part of the basal body, the doublets are set at an angle and are anchored to the enclosing cell membrane by Y-shaped structures. The lower part of the basal body tapers eventually. The striated main rootlet arises on the anterior face of the basal body, initially like a flattened strap, and continues along the basal body shaped as a tube which further down becomes solid. The hour-glass-shaped posterior rootlet arises on the posterior face of the basal body. Contrary to the main rootlet, the striations in the proximal part of the posterior rootlet run parallel to the microtubule doublets of the basal body. A pair of microtubule bundles lead from the posterior rootlet to the two main rootlets in the hind ciliary row, and follow these to their lower tip. In the other species of the Nemertodermatida studied, the structure of the ciliary basal body and the ciliary rootlets is similar to that of M. stichopi. Structural differences in the species of the Acoela are that the lowermost end of the basal body is narrow and bent forwards, the proximal part of the main rootlet is trough-shaped, the main rootlet is accompanied by a pair of lateral rootlets and the posterior rootlet with associated microtubule bundles is thin. The epidermal ciliary structures in species of the Nemertodermatida and Acoela have a number of shared characters which are unique within the Plathelminthes. However, almost all of these characters are found in Xenoturbella bocki (Xenoturbellida), and some even in species of other ”phyla” of the ”lower” Metazoa. Hence, these characters cannot be considered apomorphic for the Acoelomorpha. A character seemingly present only in species of the Nemertodermatida and Acoela is the bilayered dense plate. This feature might represent an autapomorphic character state for the Acoelomorpha. Accepted: 7 March 1997     

Ultrastructural studies on the reproductive system of gyrocotylidea and amphilinidea (Cestoda): Spermatogenesis, spermatozoa, testes and vas deferens of Gyrocotyle

The ultrastructure of the vas deferens, testes, spermatogenesis and spermatozoa of Gyrocotyle urna and G. parvispinosa is described. The vas deferens is ciliated and syncytial. Within the testes primary spermatocytes arise from the primary spermatogonia by incomplete mitotic divisions; the primary spermatocytes undergo two meiotic divisions leading to spermatids. In early spermatids microtubules are formed at the cell periphery. Later the spermatozoal cytoplasm (the ‘middle-piece’) grows out and the two spermatozoal flagella with their typical 9 + ‘1’ axonemes are formed. During ciliogenesis the flagella are at an angle of about 60° to the axis of the middle-piece. The flagella are inserted into basal bodies terminating in striated rootlets. Subsequently, the nucleus and isolated mitochondria migrate into the central axis. The angle between the flagella and the axis decreases; the flagella are incorporated to form the spermatozoon. In mature spermatozoa no basal body or rootlet elements were found. The phylogeny of parasitic Platyhelminthes is discussed with respect to the evolution of spermatozoa. The reduction of the acrosinoid granules which are found in spermatozoa of free-living Platyhelminthes and the incorporation of the spermatozoal flagella into the sperm body constitute autapomorphies of the Neodermata (the parasitic Platyhelminthes). Included in the Cestoda because of several common derived characters, Amphilinidea and Gyrocotylidea are the only cestodes with spermatozoa containing mitochondria. Their absence in Cestoidea—all taxa with a six-hooked larva and other characteristics—is an autapomorphy of this group.     

Light and electron microscopic investigation of Pelomyxa prima (Gruber, 1884) (Peloflagellatea, Pelobiontida)

Cell organization of a multinuclear pelobiont Pelomyxa prima has been studied at the light and electron microscopic levels. Motile individuals demonstrate a characteristic drop-like or pyriform shape and reach 550 microkm in length. The cell cover is represented by a well-developed, morphologically differentiated glycocalyx 80-100 nm thick. The cytoplasm contains many structural vacuoles. The nuclei are of vertical type, numbering up to several nuclei in large individuals. Numerous cytoplasmic microtubules are associated with the external membrane of the nuclear envelope. Separate non-motile flagella are distributed throughout the cell surface, being more numerous in the posterior body end and uroidal zone of the protist. Basal bodies of the flagella are extremely long, being deeply immersed into the cytoplasm. These bodies are surrounded by a muff of electron-dense material, with numerous microtubules radiating from it. A compact bundle of microtubules starts from the base of a basal body axially into the cytoplasm. Besides, a band-like lateral microtubular rootlet is present. The number of microtubules in the axoneme of undulipodia is unstable. Neither mitochondria, nor Golgi complex were found. Two species of bacterial endocytobionts are present in the cytoplasm in considerable numbers.