Ultrastructure of the Trichomonad Flagellate Stephanonympha nelumbium

ABSTRACT Stephanonympha nelumbium is a large trichomonad measuring 45–60 μm in length and 20–40 μm in width. It is a member of the multinucleate and multiflagellate family Calonymphidae. While the numerous flagella arise in groups of four at the anterior cell pole, the posterior body portion is covered with attached spirochetes and rod-like bacteria. Generally, in the apical body portion of S. nelumbium , 50–100 nuclei are arranged in five to seven circular rows. Each nucleus is associated with a typical mastigont system, comprised of three anterior flagella, one recurrent flagellum being attached to the cell surface for a certain distance, and several typical root structures. Akaryomastigonts and costas do not occur. The fine structure of S. nelumbium corresponds with that of other calonymphids. The main difference to Calonympha is that the axostyle does not embrace the nucleus but passes it in form of a flattened rod.     

Structure of a Nonpathogenic Histomonad from the Cecum of Galliform Birds and Revision of the Trichomonad Family Monocercomonadidae Kirby*

SYNOPSIS. The nonpathogenic flagellate from the cecum of fowl, hitherto known as Histomonas wenrichi Lund, is an actively ameboid organism equipped with 4 flagella. Three of the flagella, subequal in length, are applied to one another for a short distance anterior to their origin in the kinetosomal complex and typically terminate in knob-like expansions; the 4th flagellum originates independently of the others and ends in a fine filament. A short, wide pelta is joined to the anterior part of the broad, spatulate axostylar capitulum applied to the left-ventral surface of the ellipsoidal, spheroidal, or ovoidal nucleus. Posterior to the nucleus the capitulum narrows into a slender axostylar trunk, which does not project beyond the body surface. The parabasal body, associated with a long, stout parabasal filament, is dorsal and to the right of the nucleus. On the basis of the flagellar number, the parasite is placed in a new genus Parahistomonas. A new subfamily, Protrichomonadinae, is created within the appropriately emended trichomonad family Monocercomonadidae for Parahistomonas wenrichi (Lund) comb. nov. as well as for Histomonas meleagridis (Smith) and Protrichomonas legeri (Alexeieff).     

Free kinetosomes in Austrialian flagellates. I. Types and spatial arrangement

An electron microscope study was made of Deltotrichonympha and Koruga, two closely-related hypermastigote flagellates that live in the hindgut of the Australian termite, Mastotermes darwiniensis These symbiotic protozoans have a typical flagellated rostrum and long body flagella. Their "giant centrioles" (centriolar apparatus) are large, fibrillar, and granular bodies which do not resemble typical centrioles in structure. The unique feature of interphase cells is the presence of more than half a million free kinetosomes in the anterior cytoplasm. Two classes of free kinetosomes, differing in length and spatial arrangement, were found. 500,000–750,000 short free kinetosomes are concentrated in a dense column which extends from the centriolar apparatus in the rostrum to the anterior side of the nucleus Most of the short free kinetosomes in the column are arranged end-to-end in chains of varying lengths. Within a kinetosomal chain, all of the individual kinetosomes face in the same direction with respect to their cartwheel ends In most flagellates, the short free kinetosomes are 0 07–0.13 µ long, and are remarkably similar in length within any cell Occasionally, cells with uniformly "longer" short free kinetosomes are found. 70,000–120,000 long free kinetosomes are scattered singly throughout the cytoplasm between the column of short free kinetosomes and the cell surface These long free kinetosomes are 0 4–0 7 µ long, similar in length to the kinetosomes of the body flagella, and are oriented parallel to the anterior-posterior axis of the cell. The significance of this remarkable accumulation of free kinetosomes is discussed.     

The cytology and ultrastructure of zoospores of Hydrurus foetidus (Chrysophyceae)

The unusual tetrahedral shape of Hydrurus foetidus (Vill.) Trev. zoospores is associated with a complex skeletal system of microtubules extending from a broad flagellar root (up to 19 microtubules) into each of three, pointed anterior processes. The posterior end, also pointed and supported by a separate set of microtubules, contains a single large chloroplast with a prominent posterior furrow containing mitochondrial elements. A large immersed pyrenoid is penetrated by paired thylakoids. There is no eyespot. Numerous large Golgi bodies occur immediately anterior to the nucleus and up to 5–6 contractile vacuoles lie near the cell surface at the anterior end. Two terminally inserted flagella extend from the cell surface, a long one serving for cell locomotion, and the other vestigial with an axonemal pattern of 9+0. The flagellar root system consists of: (1) a thin, striated rhizoplast extending from the basal body of the long flagellum and ramifying over the surface of a conspicuous, anteriorly directed, conical projection of the nucleus; (2) a broad microtubular root which emanates from near the basal body of the long flagellum and appears to function as a MTOC; (3) a compound root, consisting of a striated fiber and two associated microtubules, which runs alongside the basal body of the stubby flagellum before terminating at the cell surface; and (4) a short two-membered microtubular root, also associated with the basal body of the stubby flagellum. Other components of the flagellar apparatus include a large dense body near the proximal end of the basal body of the short flagellum, and a small, dense, core-like structure closely associated with one of its triplet fibers. The flagellar apparatus of H. foetidus is remarkably similar in ultrastructure to that of Chrysonebula holmesii Lund.     

The Fine Structure of the Macrogametocytes of Adelina tribolii Bhatia, 1937 (Eucoccidia, Telosporea) from the Fat Body of the Beetle Tribolium castaneum Hbst.

SYNOPSIS. Macrogametocytes of the coccidium Adelina tribolii Bhatia, 1937 are described from the time when they settle in the fat body of the host and form periparasitic vacuoles around them to the stage of microgametocyte occurrence and the beginning of syzygy formation.
The macrogametocyte is surrounded by a 2-layered pellicle 50 mμ thick. Its continuity is interrupted by one or several micropores 40 mμ across and 86 mμ deep.
The cytoplasm of the parasite contains numerous vesicles and lamellae of rough and smooth endoplasmic reticulum. Mitochondria of various sizes have short tubules. The macrogametocyte contains a variable number of dark bodies 1.4-2.4 μ in diameter. It also contains several vacuoles up to 1.2 μ which are covered with a 3-layered membrane and enclose a granular material.
In old macrogametocytes in syzygy multivesicular bodies develop which measure up to 2.4 by 1.6 μ. Several smaller vacuoles containing granular material are also a constituent of the electrondense basic substance of these corpuscles.
Paraglycogen granules 1.4 by 0.9 A occur in old macrogametocytes and are situated inside the vacuoles which are not bordered by a membrane. The numbers and size of these granules increase with the age of the parasite. The Golgi complex lies close to the nucleus.
The nucleus, 6-8.5 μ in diameter, is in the center of the macrogametocyte and contains a large eccentric nucleolus. The nuclear membrane is 2-layered and has many pores.     

The flagellar apparatus of the golden algaSynura uvella: Four absolute orientations

Summary Flagellated vegetative cells of the colonial golden algaSynura uvella Ehr, were examined using serial sections. The two flagella are nearly parallel as they emerge from a flagellar pit near the apex of the cell. The photoreceptor is restricted to swellings on the flagella in the region where they pass through the apical pore in the scale case and the swellings are not associated with the cell membrane or an eyespot. A unique ring-like structure surrounds the axonemes of both flagella at a level just above the transitional helix. The basal bodies are interconnected by three striated, fibrous bands. Four short (<100 nm) microtubules lie between the basal bodies at their proximal ends. Two rhizoplasts extend down from the basal bodies and separate into numerous fine striated bands which lie over the nucleus. Three- and four-membered microtubular roots arise from the rhizoplasts and extend apically together. As the roots reach the cell anterior, the three-membered root bends and curves clockwise to form a large loop around the flagella; the four-membered root bends anticlockwise and terminates under the distal end of the three-membered root as it completes the loop. There are four absolute orientations, termed Types 1–4, in which the flagellar apparatus can occur. With each orientation type the positions of the Golgi body, nucleus, rhizoplasts, chloroplasts and microtubular roots change with respect to the flagella, basal bodies and photoreceptor. Two new basal bodies appear in pre-division cells, and three short microtubules appear in a dense substance adjacent to each new basal body. Based upon the positions of new pre-division basal bodies, a hypothesis is proposed to explain why there are four orientations and how they are maintained through successive cell divisions.     

Chilomastix equi n.sp., from the Intestine of the Indian Horse

Chilomastix equi n.sp., from the intestine of the Indian horse is described. The normal shape of the body is pyriform, posteriorly drawn out into a spike. The spiral groove is U-shaped and runs from one lateral side to the other. The nucleus is either ovoidal or spheroidal with an eccentric endo-some. Four blepharoplasts are present from which arise the anterior flagella, the cytostomal flagellum and the cytostomal fibrils. The arrangement of the four blepharoplasts differs markedly from the other known species, three being on the anterior border located in one row and the fourth adjacent to the nucleus. The three anterior flagella, arising from the first three anterior blepharoplasts, run backwards. The cytostomal flagellum arises from the fourth blepharoplast and rarely protrudes beyond the broad cytostome. The cytostome has a peculiar shape which distinguishes it from other species. The dimensions of the new species range (in microns) 15.7 to 31.5 in total length, 8.5 to 16.5 in body length proper, 6.0 to 15.5 in breadth and 4.0 to 17.0 in the length of the spike.     

Morphology of Diplomonad Flagellates: Spironucleus torosa N. Sp. from Atlantic Cod Gadus morhua L., and Haddock Melanogrammus aeglefinus (L.) and Hexamita salmonis Moore from Brook Trout Salvelinus fontinalis (Mitchill)

ABSTRACT. A diplomonad flagellate, Spironucleus torosa n. sp. is described from Atlantic Cod Gadus morhua and haddock Melanogrammus aeglefinus . This is believed to be the 1st confirmed report of Spironucleus from a marine fish. Organisms swimming in the rectal lumen were broadly pyriform to elongate, and measured 10.5–18.6 μm long and 3.2–13.3 μm wide; other elongate organisms were attached to the rectal epithelium, via apical extensions appearing continuous with the microvilli. The posterior end of the body was extended into a caudal projection, on either side of which was a posteriolateral ring-shaped protrusion or torus, with a recurrent flagellum emerging from its centre. A symmetrical system of microtubules and lamellae, forming a "V" in protargol impregnated specimens, supported the flanges of the body surrounding the tori, the tori themselves and the caudal projection. Supranuclear microtubules were an inverted V to U shape in transverse section, and an electron dense band accompanied the cytostomes. Lightly staining homogenous cytoplasm was usually present in the anterior part of the body, the remainder being highly vacuolated with numerous dark granules. In swimming organisms, rough endoplasmic reticulum (RER) was present around the nuclei and cytostomes, and bacteria were occasionally seen in the cytoplasm. In "attached" organisms, RER was reduced, and bacteria were absent. Hexamita salmonis Moore from Salvelinus fontinalis was studied by light and scanning electron microscopy for comparison; its cytoplasm was not highly vacuolated. The two recurrent flagella emerged close together from the blunt posterior end of the body.     

Structure and Life Cycle of Trypanoplasma beckeri sp. n. (Kinetoplastida), a Parasite of the Cabezon,Scorpaenichthys marmoratus,in Oregon Coastal Waters*,†

SYNOPSIS. Structure of Trypanoplasma beckeri sp. n. from the cabezon, Scorpaenichthys marmoratus (Ayres), is described from living specimens and from both Giemsa-, and protargol-stained smears. Flagellates from fish blood were usually long and slender, averaging 109.0 × 6.5 fan. The anterior flagellum averaged 8.5 μm; the recurrent flagellum bordered the body and terminated as a very short free flagellum, 2.5 μm long on the average. No true undulating membrane was observed, but in living individuals the recurrent flagellum undulated rapidly near its point of origin. The oval nucleus, averaging 8.5 × 4.0 μm, was located near the anterior cad of the body. An argentophilic, aciculum-like structure appeared to connect the nucleus to the area at the base of the flagella. The kinetoplast was not observed in fish blood forms. On the basis of laboratory experiments, the leech, Malmiana diminuta Burreson, was ascertained to be the vector for T. beckeri. Upon entry into the leech, flagellates became rounded, and division commenced within a few hours. Division continued for ?48 h and the flagellates became progressively smaller until reaching a length of ?10.0 μm. After 72 h they were found in high numbers in the proboscis sheath and also in the anterior crop of the leech. When infected leeches fed on an uninfected fish, flagellates were first observed in the fishes’ peripheral circulation 8 days later.     

Sperm and spermatozeugma ultrastructure in the inseminating species Tyttocharax cochui, T. tambopatensis, and Scopaeocharax rhinodus (Pisces: Teleostei: Characidae: Glandulocaudinae: Xenurobryconini)

This article presents the scanning and transmission electron microscopy of the spermatozoa and sperm packets of three inseminating species of the glandulocaudine tribe Xenurobryconini. All three species, Scopaeocharax rhinodus, Tyttocharax cochui, and T. tambopatensis produce unencapsulated sperm packets (= spermatozeugmata) of similar morphology. The external anterior surface of each spermatozeugma is comprised of elongate sperm heads arranged in parallel, and the posterior part is made up of tightly packed flagella. The interior of the anterior portion consists of alternating layers of sperm heads and flagella. The remarkable integrity of each packet appears to be maintained through an electron-dense secretion seen among all parts of the cells. Spermatozeugma formation takes place within the spermatocysts at the end of spermiogenesis and at spermiation fully formed packets are released. Morphology of the mature spermatozoa was similar in all three species. Each nucleus is elongate, flattened along most of its length, and tapers at either end. The two centrioles are nearly parallel to one another and are located just anterior to the nucleus. Elongate mitochondria are located along the nucleus. The single flagellum, which lacks axonemal fins, is initially contained within a short cytoplasmic collar. Accessory microtubules run parallel to the long axis of the nucleus just beneath the plasma membrane. During spermiogenesis, no nuclear rotation occurs and the cytoplasmic canal containing the flagellum elongates along with the nucleus. However, prior to spermiation all but the anterior portion of the collar degenerates. The sperm modifications observed in these species are discussed as adaptations to the unique reproductive habit of insemination.     

A new flagellate <Emphasis Type="Italic">Percolomonas lacustris</Emphasis> sp. n. (Excavata,Percolozoa) from an inland saline lake (Southeastern European Russia)

The external and internal morphology of a new species of excavate flagellate which inhabits the saline Novoe Lake (Orenburg oblast, Russia) is considered. A cell of the flagellate has four naked flagella (one long and three short) extending parallel to each other. The long and one short flagella terminate with an acroneme. Three microtubular bands reinforce the longitudinal ventral groove. The vesicular nucleus lies in the anterior part of the cell. The mithochondria have discoid cristae. The system of submembrane microtubules originates from the fibrils close to the kinetosome and extends to the end of the posterior cell. A small bulbous projection is seen at the tip of the end of the posterior cell. Dictyosomes of the Golgi apparatus are not found. The lifecycle includes spherical thick-walled cysts that have a single hole with a plug. The external structure of the cell and cysts of a similar species, Percolomonas cosmopolitus, is compared to that in P. lacustris. The new species differs from the known ones by the presence of a posterior bulbous projection, a hole and plug inside the cyst, an acroneme of one short flagellum, and other features.     

Cultivation and Lyophilization of Mastigina sp.

SYNOPSIS Mastigina sp. is an amoeboid flagellate isolated from pine frass collected in the Guadarrama Mountains in Spain. It feeds on bacteria and yeasts. It prefers yeasts that produce extracellular polysaccharides, and the 2 species that have been used predominantly for cultivation of the flagellate are Pachysolen tannophilus and Hansenula holstii. Mastigina sp. is easily isolated in axenic culture and grows abundantly therein. Its locomotive form, averaging 27 μ in length, resembles that of a limax amoeba, with a vesiculate nucleus at the anterior end. Cells are capable of simultaneous movement by pseudopodia and flagella. It develops rapidly on dead or living yeast cells in shaken cultures and the trophozoites may convert quantitatively to cysts. The cysts remain viable for long periods of time in refrigerated suspensions and in the lyophilized state. They are spherical or ovoid and smooth-walled cysts; the trophozoite emerges from them by breaking the wall.     

Ultrastructure of Sporozoites and Zoites of Hammondia heydorni

The ultrastructure of sporozoites and zoites of Hammondia heydorni was studied in cultured bovine cells. In addition to ultrastructural features typical of coccidian parasites, H. heydorni sporozoites and zoites contain rhoptries that are located posteriorly as well as anteriorly. Also, sporozoites contain a posteriorly located crystalloid body (1.2 μm in diameter); a small crystalloid body (0.5 μm in diameter) was occasionally seen in the anterior end. Zoites resulting from the 1st division of endodyogeny contain a posteriorly located crystalloid body, which is absent in zoites formed by subsequent divisions. Zoites contain posteriorly located amylopectin granules and a relatively large anterior vacuole which is not present in sporozoites. During penetration, the host cell plasmalemma ballooned laterally around the sporozoite creating a large cavity, which later disappeared. Sporozoites and zoites undergoing cell penetration usually exhibit partially empty anterior rhoptries; no changes occur in posterior rhoptries. Lysosomes fuse with the par-asitophorous vacuole surrounding killed sporozoites but not live sporozoites.     

The spermatogenesis and the sperm structure of Terebrantia (Thysanoptera, Insecta)

Spermatogenesis and the sperm structure of the terebrantian Aeolothrips intermedius Bagnall are described. Spermatogenesis consists of two mitotic divisions; the second is characterized by the loss of half of the spermatids, which have pyknotic nuclei. Early spermatids have two centrioles, but when spermiogenesis starts, a third centriole is produced. The three basal bodies give rise to three flagella; later these fuse into a single flagellum which contains three 9 + 0 axonemes. The basal bodies are surrounded by a large amount of centriole adjunct material. During spermiogenesis this material contributes to the shifting of the three axonemes towards the anterior sperm region parallel to the elongating nucleus, and it is transformed into a dense cylinder. In the mature spermatids the three axonemes amalgamate to create a bundle of 27 doublet microtubules. Near the end of spermiogenesis the dense cylinder of the centriole adjunct lies parallel to the nucleus and the axonemes. It ends where the mitochondrion appears at half-sperm length. We confirm that Terebrantia testes have a single sperm cyst; their sperm are characterized by a cylindrical nucleus, three axonemes fused into one, a small mitochondrion and a short cylindrical centriole adjunct which corresponds to the dense body described in a previous work. The acrosome is lacking. At the midpoint of the anterior half of the sperm the outline of the cross-section is bilobed, with the nucleus contained in a pocket evagination of the plasma membrane. These characters are discussed in light of a comparison between Tubulifera and Terebrantia.     

Dynamic changes in microtubule organization during division of the primitive dinoflagellate Oxyrrhis marina

The marine dinoflagellate Oxyrrhis marina has three major microtubular systems: the flagellar apparatus made of one transverse and one longitudinal flagella and their appendages, cortical microtubules, and intranuclear microtubules. We investigated the dynamic changes of these microtubular systems during cell division by transmission and scanning electron microscopy, and confocal fluorescent laser microscopy. During prophase, basal bodies, both flagella and their appendages were duplicated. In the round nucleus situated in the cell centre, intranuclear microtubules appeared radiating toward the centre of the nucleus from densities located in some nuclear pores. During metaphase, both daughter flagellar apparatus separated and moved apart along the main cell axis. Microtubules of ventral cortex were also duplicated and moved with the flagellar apparatus. The nucleus flattened in the longitudinal direction and became discoid-shaped close to the equatorial plane. Many bundles of microtubules ran parallel to the short axis of the nucleus (cell long axis), between which chromosomes were arranged in the same direction. During ana-telophase, the nucleus elongated along the longitudinal axis and took a dumbbell shape. At this stage a contractile ring containing actin was clearly observed in the equatorial cortex. The cortical microtubule network seemed to be cut into two halves at the position of the actin bundle. Shortly after, the nucleus divided into two nuclei, then the cell body was constricted at its equator and divided into one anterior and one posterior halves which were soon rebuilt to produce two cells with two full sets of cortical microtubules. From our observations, several mechanisms for the duplication of the microtubule networks during mitosis in O. marina are discussed.     

THE FINE STRUCTURE OF GIARDIA MURIS

Giardia is a noninvasive intestinal zooflagellate. This electron microscope study demonstrates the fine structure of the trophozoite of Giardia muris in the lumen of the duodenum of the mouse as it appears after combined glutaraldehyde and acrolein fixation and osmium tetroxide postfixation. Giardia muris is of teardrop shape, rounded anteriorly, with a convex dorsal surface and a concave ventral one. The anterior two-thirds of the ventral surface is modified to form an adhesive disc. The adhesive disc is divided into 2 lobes whose medial surfaces form the median groove. The marginal grooves are the spaces between the lateral crests of the adhesive disc and a protruding portion of the peripheral cytoplasm. The organism has 2 nuclei, 1 dorsal to each lobe of the adhesive disc. Between the anterior poles of the nuclei, basal bodies give rise to 8 paired flagella. The median body, unique to Giardia, is situated between the posterior poles of the nuclei. The cytoplasm contains 300-A granules that resemble particulate glycogen, 150- to 200-A granules that resemble ribosomes, and fusiform clefts. The dorsal portion of the cell periphery is occupied by a linear array of flattened vacuoles, some of which contain clusters of dense particles. The ventrolateral cytoplasm is composed of regularly packed coarse and fine filaments which extend as a striated flange around the adhesive disc. The adhesive disc is composed of a layer of microtubules which are joined to the cytoplasm by regularly spaced fibrous ribbons. The plasma membrane covers the ventral and lateral surfaces of the disc. The median body consists of an oval aggregate of curved microtubules. Microtubules extend ventrally from the median body to lie alongside the caudal flagella. The intracytoplasmic portions of the caudal, lateral, and anterior flagella course considerable distances, accompanied by hollow filaments adjacent to their outer doublets. The intracytoplasmic portions of the anterior flagella are accompanied also by finely granular rodlike bodies. No structures identifiable as mitochondria, smooth endoplasmic reticulum, the Golgi complex, lysosomes, or axostyles are recognized.     

Ultrastructure of In Vivo-Produced Caryocysts Containing the Coccidian Caryospora bigenetica (Apicomplexa: Eimerüdae)

A cotton rat was inoculated orally with oocysts of Caryospora bigenetica from the feces of a rattlesnake. Sixteen days later the rat was euthanized, and portions of the scrotum, foot pad and muzzle were processed for histological sections and transmission electron microscopy. Sporozoites within caryocysts had typical coccidian features such as an anterior and posterior refractile body, centrally located nucleus, micronemes, rhoptries, a conoid, a micropore near the anterior refractile body, a posterior pore, amylopectin granules, lipid bodies, a Golgi-like body, a mitochondrion and subpellicular microtubules. The infected host cell was spherical and surrounded by a fibrous wall-like covering, 0.35–1.00 μm thick. This outer covering, when viewed in stained histological sections, was periodic acid-Schiff (PAS)-positive.     

Ultrastructural Observations on the Spermatozoon,Oocyte and Fertilization Process in Gonapodasmius,a Gonochoristic Trematode (Trematoda: Digenea: Didymozoidae)

Observations were performed in the uterus of a female Gonapodasmius sp., a gonochoristic didymozoid Trematode. The oocyte is a round cell 6 μm in diameter, which shows a ‘nucleolus-like cytoplasmic body’ and cortical granules. The spermatozoon is filiform, mobile and about 50 μm long. There is no acrosome. The anterior tip of the spermatozoon contains two centrioles made up of singlets and cortical microtubules with associated glycocalyx. The centrioles are continued as two axonemes of the classical 9 + ‘1’ pattern of flatworms, accompanied by a mitochondrion and a short row of cortical longitudinal microtubules. It is the posterior part of the sperm cell which contains the nucleus. At the outset of fertilization, the anterior part of the spermatozoon coils around the oocyte and penetrates it by lateral fusion. The posterior region of the spermatozoon, with the nucleus, is the last part to enter the oocyte, after passing through a perforation in the forming eggshell. The whole spermatozoon thus penetrates the female cell.     

Spermiogenesis in Polyplacophora,with special reference to acrosome formation (Mollusca)

Summary The present study examines spermiogenesis, and in particular the formation of the acrosome, in ten species of chitons belonging to four families. This study emphasizes the formation of the acrosome but brings to light several other structures that have received little or no mention in previous studies. The process of spermiogenesis is essentially similar in each species, although Chaetopleura exhibits some significant differences. In early spermiogenesis the Golgi body secretes numerous small pro-acrosomal vesicles that gradually migrate into the apical cytoplasm. The chromatin condenses from granules into fibres which become twisted within the nucleus. A small bundle of chromatin fibres projects from the main nuclear mass into the anterior filament; this coincides with the appearance of a developing manchette of microtubules around the nucleus that originates from the two centrioles. Radiating from the distal centriole is the centriolar satellite complex, which is attached to the plasma membrane by the annulus. The distal centriole produces the flagellum posteriorly and it exits eccentrically through a ring of folded membrane that houses the annulus. Extending from the annulus on one side of the flagellum, in all but one species, is a dense fibrous body that has not been previously reported. The proximal centriole lies perpendicular to the end of the distal centriole and is attached to it by fibro-granular material. Pro-acrosomal vesicles migrate anteriorly through the cytoplasm and move into the anterior filament to one side of the expanding nucleus. Eventually these vesicles migrate all the way to the tip of the sperm, where they fuse to form one of two granules in the acrosome. In mature sperm the nucleus is bullet-shaped with a long anterior filament and contains dense chromatin with occasional lacunae. The mitochondria vary in both number and position in the mature sperm of different species. Both centrioles are housed eccentrically in a posterior indentation of the nucleus, where the membranes are modified. The elongate flagellum tapers to a long filamentous end-piece that roughly corresponds to the anterior filament and may be important in sperm locomotion for hydrodynamic reasons. An acrosome is present in all ten species and stained positively for acid phosphatase in three species that were tested.     

Spermiogenesis in a Scutariellid (Platyhelminthes)

Spermiogenesis and spermatozoa were studied by transmission and scanning electron microscopy in Troglocaridicolasp., a scutariellid epizoic on a cavernicolous freshwater shrimp. Spermiogenesis involves elongation of the spermatid in which the nucleus elongates, but remains close to the common cytoplasmic mass. Flagella first grow in opposite direction and at a right angle to the cytoplasmic shaft, and centrioles show associate structures. Later, the two centrioles rotate and the flagella emerge parallel, but still perpendicular to the shaft. An apical process elongates at the extremity of the spermatid shaft. The spermatozoon shows active flagellar beating and undulations of the sperm body. The spermatozoon comprises an anterior ‘corkscrew’ region, the flagellar insertion region, a cytoplasmic region and a posterior nuclear region. The corkscrew contains an electron dense structure, not membrane-bound, originating from the apical process of the spermatid. The flagella show the 9+‘1’ pattern, usual in Platyhelminthes. The cytoplasmic and nuclear regions show a cortical row of about 50 twisted longitudinal microtubules surrounding a row of electron dense, and not membrane-bound, 25-nm granules. These granules are original structures and seem to be known only in a few Platyhelminthes species in which a non-flagellar movement of the spermatozoon occurs. Thus, it is hypothesised that the 25-nm granules play a role in cellular motility. Sperm ultrastructure in Troglocaridicolashows major differences to that in the temnocephalids. It is therefore concluded that the phylogenetic position of the scutariellids within the Temnocephalidea should be reinvestigated.