Ultrastructure of the introvert and associated structures of the larvae of Halicryptus spinulosus (Priapulida)

Summary Based on new SEM and TEM observations, the arrangement and ultrastructure of the scalids and adhesive tubuli of the larvae of Halicryptus spinulosus are described. The scalids are arranged in 25 longitudinal rows. Transversally they form seven circlets which alternate. The first circlet consists of 8 and the second of 9 scalids. All following circlets consist of three subrings with 8+8+9 scalids each. New scalids develop in a sub frontal growing zone posterior to the second circlet. The ultrastructure of all different receptor cells and sense organs of the scalids is described. Whereas the receptor cell type 1 occurs in nearly all scalids, the verticalrootlet receptor is restricted to the first circlet, the basal receptor to the second and the coiled-cilium receptor to the third and fourth circlet. Based on these new data, the scalid arrangement and the equipment with different receptor cells can be postulated for the ground pattern of the priapulid larvae. Also the larval development, moulting and the structure of the cuticle are discussed in comparison with the Kinorhyncha and Loricifera.     

Introvert,mouth cone,and nervous system of Echinoderes capitatus (Kinorhyncha,Cyclorhagida) and implications for the phylogenetic relationships of Kinorhyncha

Summary The introvert, mouth cone, and nervous system of Echinoderes capitatus were examined by transmission and scanning electron microscopy. The introvert bears seven rings of primarily quincunxial sensory scalids, including type 1 and 2 spinoscalids as well as trichoscalids; the latter two types are additionally provided with glandular cells. The mouth cone bears one ring of decamerous sensory oral styles and three rings of quincunxial sensory pharyngeal styles. The intra- to basiepithelial, bilateral nervous system consists of a circumentric nerve ring in the introvert, a terminal and proximal nerve ring in the mouth cone, a ventral chain of ganglia, one in each trunk zonite, and a caudal ganglion. The introvert, the neck, and the trunk zonites are innervated from the forebrain; the mouth cone and the pharyngeal bulb are innervated from the hindbrain. The monophyly of the Kinorhyncha is based upon the following autapomorphic characters: (1) a mouth cone, (2) a neck with 16 placids, (3) a trunk with 11 zonites, (4) scalids of three types: type 1 and type 2 spinoscalids, and trichoscalids, (5) an anteriormost ring of ten type 1 spinoscalids (sensory organs divided into a basal and a terminal part), (6) a posteriormost ring of 14 trichoscalids (glandular sensory organs which are undivided), (7) rings in between the anteriormost and posteriormost are type 2 spinoscalids (glandular sensory organs divided into a basal and a terminal part), (8) a mouth cone with a terminal and a proximal nerve ring, (9) nine sensory oral styles with decamerous symmetry (the dorsal style is missing) and (10) three rings of sensory pharyngeal styles with, from anterior to posterior, ten, five, and five styles with quincunxial arrangement. The following characters are assumed to be autapomorphic for the taxon Nematoda+Gastrotricha+Kinorhyncha+Loricifera+Priapulida: (1) a basiepithelial circumentric brain and (2) a neuropileous nerve ring in a subterminal position. The following characters are assumed to be autapomorphic for the taxon Kinorhyncha+Loricifera+Priapulida: (1) a neuropileous nerve ring in a terminal position, (2) an introvert with scalids, (3) an eversible foregut and (4) tanycytes.The unpublished doctoral thesis of B. Neuhaus [1991 Zur Ultrastruktur, Postembryonalentwicklung und phylogenetischen Verwandtschaft der Kinorhyncha. PhD thesis. University of Götingen, Germany] was finished simultaneously with the completion of this study.     

Postembryonic development of Paracentrophyes praedictus (Homalorhagida): neoteny questionable among the Kinorhyncha

The postembryonic ontogeny of the kinorhynch Paracentrophyes praedictus is described by light microscopy and scanning electron microscopy. The development of head scalids and trunk Characteristics has been studied. The first of 6 juvenile stages possesses 11 segments already. One segment each is added both in stages 2 and 5. The number of head scalids increases during development. Every scalid becomes visible as a spinose anlage in one stage and differentiates in the next stage. All head scalids in stage 1 represent anlagen of scalids. Late juvenile stages J-5 and J-6 express two different scalid patterns which do not seem to depend on the gender. Scalids and trunk segments develop in a subfrontal and a subcaudal growing zone, respectively. The presumed neotenic status of P. praedictus is rejected because this species exhibits the same number of juvenile stages as observed in all other Kinorhyncha studied. It is suggested that a weakly sclerotized, juvenile-like trunk cuticle as found, e.g. in adult P. praedictus represents the plesiomorphic characteristic within the Kinorhyncha; a thickened cuticle has probably evolved independently within both the cyclorhagid and the homalorhagid Kinorhyncha.     

Ultrastructural observations of the larva ofTubiluchus corallicola (Priapulida)

Larvae ofTubiluchus corallicola van der Land 1968 were investigated by scanning and transmission electron microscopy. The scalids are sensory organs, each has a bipolar receptor cell with a single apical cilium similar to the scalid in the adult. Muscle cells of the larva are more differentiated than previously reported for other Priapulida; the larval arrangement of circular and longitudinal musculature differs from that of the adult, and a diaphragm is reported for the first time in Priapulida. The diaphragm may function in hydrostatic control of eversion and inversion of the introvert and mouth cone. The functional morphology of these two structures is discussed and contrasted with the Kinorhyncha.     

Hatching and earliest larval stages of the priapulid worm Priapulus caudatus

Abstract. Here we describe the hatching and morphology of the earliest larval stages of the priapulid worm Priapulus caudatus for the first time. The hatching larva differs considerably from previously described larvae not only in its general body shape but also in its lack of a proper lorica including the typical lorica tubuli. Furthermore, no mouth opening or pharyngeal teeth have formed as yet, and the number and arrangement of scalids differ from that of later larvae. The hatching larva molts and emerges as the first lorica larva. This larva partially resembles earlier described lorica larvae, but there are a number of important differences; the first lorica larva is smaller, and the mouth opening as well as pharyngeal teeth are still yet to form. The second lorica larva is equipped with four rings of pharyngeal teeth; it shows striking similarity to the previously described larva of P. caudatus , i.e., the larva-type 2 , only differing in the scalid pattern. We conclude that the first two larval stages of P. caudatus have not been described previously. We suggest that discrepancies between the earliest lorica larvae described here and in earlier publications might depend on sub-speciation or ecophenotypic modification of larvae collected from different localities. Our findings highlight the importance of studying the development of non-model organisms such as priapulids under controlled laboratory conditions.     

Structure of the nervous system in Tubiluchus troglodytes (Priapulida)

Abstract. The nervous system of the meiobenthic priapulid species Tubiluchus troglodytes is described by immunohistochemistry and confocal laser scanning microscopy. The brain is circumpharyngeal, consisting of a central ring of neuropil and both anterior and posterior somata. From the brain emerges a ventral nerve cord, which shows ganglion-like swellings in the neck and caudal region. The introvert includes longitudinal neurite bundles running below and between the rows of scalids, with a small cluster of sensory cells under each scalid. In the body wall of the neck and trunk region, longitudinal and circular neurite bundles are present in an orthogonal pattern. The tail is innervated from the caudal swelling of the ventral nerve cord; it also includes longitudinal and circular bundles in an orthogonal pattern. The pharynx has a reticulated system of neurite bundles running between the pharyngeal teeth and fimbrillae. Below each tooth and fimbrilus is a ganglion-like cluster of somata. The intestine is surrounded by a nerve net. The data on the nervous system are compared within other priapulids and with other species of Scalidophora (Kinorhyncha and Loricifera).     

Microscopic anatomy and ultrastructure of the introvert of Priapulus caudatus and P. tuberculatospinosus (Priapulida)

Introverts of Priapulus caudatus and P. tuberculatospinosus bear 25 rows of scalids, as well as 8 spines and scattered papillae in the region the circumoral lip. These, as well as the first ring of pharyngeal teeth in P. tuberculatospinosus, are sensory organs. Although superficially they differ between species, they are all characterized by apical and/or subapical openings which are located on tiny cuticular tubules. All sensory organs contain cilia bearing bipolar receptor cells. The 8 sensory spines situated between the circumoral area and the beginning of the scalids are ultrastructurally similar to the scalids. The introvert and pharyngeal body walls, and associated muscles are described. © 1994 Wiley-Liss, Inc.     

Electron microscopic study of cell surface rings during cell division and morphogenesis of Arthrobacter crystallopoietes.

The whole cell ultrastructure during cell division and morphogenesis of Arthrobacter crystallopoietes was monitored using electron microscopic techniques. Glucose-grown spherical cells were inoculated into succinate-based medium. In this medium, the organism undergoes a morphogenetic cycle consisting of elongation of spheres to rods, exponential growth as rods, and fragmentation of rods to spherical cells. Raised bands or rings that encircled the cells were evident on the cell surface of both sphere- and rod-shaped cells. Many rod-shaped cells possessed two or more rings arranged adjacent to each other in a parallel orientation. At each cell division a new ring was formed on both siblings. However, as predicted by the proposed model of unidirectional cell growth and by maintaining a ring from the previous generation, unequal numbers of rings were observed on sibling cells. Only one ring was visible on most of the spherical inoculum cells, but in some cases a second ring perpendicular to the other ring was observed. Parallel rings were found on spherical cells resulting from fragmentation or reductive cell division of rods during the stationary growth phase. Thus, these spheres could be distinguished from inoculum spheres containing a single ring or perpendicular orientation of rings. The number of rings per cell and arrangement of rings on the cell surface of sibling cells after cell division, but before cell separation, are discussed with respect to cell age, cell division, and sphere-rod-sphere morphogenesis of A. crystallopoietes.     

The structure of dynein-c by negative stain electron microscopy

Dynein ATPases contain six concatenated AAA modules within the motor region of their heavy chains. Additional regions of sequence are required to form a functional ATPase, which a previous study suggested forms seven or eight subdomains arranged in either a ring or hollow sphere. A more recent homology model of the six AAA modules suggests that these form a ring. Therefore both the number and arrangement of subdomains remain uncertain. We show two-dimensional projection images of dynein-c in negative stain which reveal new details of its structure. Initial electron cryomicroscopy shows a similar overall morphology. The molecule consists of three domains: stem, head, and stalk. In the absence of nucleotide the head has seven lobes of density forming an asymmetric ring. An eighth lobe protrudes from one side of this heptameric ring and appears to join the elongated cargo-binding stem. The proximal stem is flexible, as is the stalk, suggesting that they act as compliant elements within the motor. A new analysis of pre- and post-power stroke conformations shows the combined effect of their flexibility on the spatial distribution of the microtubule-binding domain and therefore the potential range of power stroke sizes. We present and compare two alternative models of the structure of dynein.     

Ultrastructure of introvert and pharynx of Halicryptus spinulosus (priapulida)

The introvert of Halicryptus spinulosus bears three kinds of sensilla: buccal papillae, ordinary scalids, and dentoscalids. They are all characterized by bipolar monociliary receptor cells. The former two have apical openings at which the sensory cilia are in close contact with the ambient sea water. The pharyngeal teeth are composed of slender epithelial cells the tips of which are devoid of organelles and a thick cuticle. The anatomy of the muscle arrangement of the pharynx is described. Glands occur at the junction of the pharynx and midgut.     

Two new species of the genus Pliciloricus (Loricifera, Pliciloricidae) from the Faroe Bank, North Atlantic

Five cruises to the Faroe Bank in the northeast Atlantic revealed a rich loriciferan fauna inhabiting shell gravel to fine carbonate sand. This paper describes two new species of Pliciloricus found at depths of 120-260 m. Currently, only six species of this genus have been described from various localities around the world, but several new species are currently being described. The diagnosis of the genus Pliciloricus is emended to include the two new taxa. The first of the two new species Pliciloricus leocaudatus sp. n. is characterized by having four new kinds of scalids in the second row and an additional row of alternating plates in the eighth row. The second species Pliciloricus shukeri sp. n. is characterized in having a secondary double organ in the third row. The morphology of the two species, including the new structures, has been investigated using LM and scanning electron microscopy. Conclusively, the scalid pattern of the introvert of Pliciloricus has been re-evaluated, due to the finding of the new structures. Additionally, a discussion of the life cycle of the genus Pliciloricus is given, since the postlarval stage is reduced in both new species.     

Scanning and Transmission Electron Microscopical Observations on the Larvae of Priapulus caudatus (Priapulida)

Abstract Scanning electron microscopical studies revealed four distinct morphological larva-types (not instars) of Priapulus caudatus whose lorica-length measured 82–860 μm. The smallest of the larvae are round to oval, have 20 longitudinal ridges, a series of transverse ridges, and have two pairs of laterally situated tubuli near the posterior limits. The second larva-type is dorsoventrally compressed, has a single dorsal and single unsculptured ventral plate each with a prominent midridge near the anterior limits, three infolded lateral plates with a pair of tubuli near the posterior limits. The third and fourth larva-types remain dorsoventrally flattened; the third larva-type has less pronounced sculpturing than the fourth larva-type and is smaller; two pairs of tubuli are situated as in the previous stage. The number and arrangement of scalids on the introvert and pharyngeal teeth differ according to each of the four larva-types and are described. The ultrastructure of all organ systems is described. All scalids are sensilla equipped with ciliated receptor cells. The cuticle is similar to the cuticle in the adult and differs fundamentally from the cuticle of larval Halicryptus. The trunk is richly supplied with sensory structures. Trunk tubuli contain secretory cells. Posteriorly, a gland complex was found which presumably is the precursor of the equivalent gland in postlarvae and adults. The implications of the first larva-type's structural similarity to the larva of Tubiluchus are discussed.     

Segmental mismatch in crustacean appendages: the naupliar antennal exopod of Artemia (Crustacea, Branchiopoda, Anostraca)

Based on traditional techniques and confocal laser scanning microscopy for external morphology, and immunohistochemistry for the muscular system, we describe here the segmental features of the antennal exopod of Artemia nauplii. Two kinds of serial elements are present, i.e. setae (with cuticular folds at their base) and ringlets (serially arranged sclerites separated by joint-like cuticular folds not extending to form complete rings around the appendage). The two series are usually not in register. The cuticular folds of the setae and of the ringlets are also sites of intermediate insertions of the three exopod muscles: as the two tegumentary structures are discordant in periodicity, this is also mirrored in the pattern of muscle insertions on the two sides of the appendage. Similar cases of segmental mismatch are known for the trunk of several arthropods, but segmental mismatch along the appendages has received very little attention. The occurrence of segmental mismatch in the naupliar appendages of both extant and fossil crustaceans is reviewed and it is suggested here to be a primitive feature of the exopods of both second antennae and mandibles. Problems in the interpretation of morphological evidence are discussed, also in relation to development and evolution of segmentation of naupliar appendages.     

Ultrastructural features of the apical complex, pellicle, and membranes investing the gamonts of Haemogregarina magna (Apicomplexa: Adeleina)

Mature gamonts of Haemogregarina magna lie within a type of parasitophorous vacuole (Pv) apparently unique to the haemogregarines. The cytoplasm of infected erythrocytes was separated from the parasite by two Pv membranes. An additional membrane, coated on both sides with electron-dense material, closely invested the gamonts. The apical complex of the gamonts includes a conoid, two preconoidal rings, and an elaborate polar ring complex. The latter consisted of the polar ring and approximately 78 posteriorly directed, radially arranged, "tine-like" structures which fuse as they merge anteriorly into the polar ring. Freeze fracture replicas demonstrated that the pellicle of gamonts of H. magna was structurally similar to that of other apicomplexans. The closely apposed inner membranes of the pellicle formed plates which were arranged into strips along the long axis of the gamont. Calculations indicated that 13 such strips are found around the circumference of the gamonts with about six subpellicular microtubules associated with the inner surface of each strip. Gamonts of H. magna share many structural similarities with the kinetes, ookinetes, and sporokinetes of other apicomplexans. We propose that the conoid and polar ring complex are fundamental features of all apicomplexan "kinetes."     

Symmetry of priapulids (Priapulida). 2. Symmetry of larvae

Larvae of priapulids are characterized by radial symmetry evident from both external and internal characters of the introvert and lorica. The bilaterality appears as a result of a combination of several radial symmetries: pentaradial symmetry of the teeth, octaradial symmetry of the primary scalids, 25-radial symmetry of scalids, biradial symmetry of the neck, and biradial and decaradial symmetry of the trunk. Internal radiality is exhibited by musculature and the circumpharyngeal nerve ring. Internal bilaterality is evident from the position of the ventral nerve cord and excretory elements. Externally, the bilaterality is determined by the position of the anal tubulus and two shortened midventral rows of scalids bordering the ventral nerve cord. The lorical elements define the biradial symmetry that is missing in adult priapulids. The radial symmetry of larvae is a secondary appearance considered an evolutionary adaptation to a lifestyle within the three-dimensional environment of the benthic sediment.     

Three-dimensional reconstructions of calcium/calmodulin-dependent (CaM) kinase IIalpha and truncated CaM kinase IIalpha reveal a unique organization for its structural core and functional domains

Studies of the structural organization of calcium/ calmodulin-dependent protein kinase IIalpha (CaM KIIalpha) and truncated CaM KIIalpha by three-dimensional electron microscopy and protein engineering show that the structures consist of 12 subunits that are organized in two stacked hexameric rings with 622 symmetry. The body of CaM KIIalpha is gear-shaped, consisting of six slanted flanges, and has six foot-like processes attached by narrow appendages to both ends of the flanges. Truncated CaM KIIalpha that lacks functional domains has a structure that is very similar to the body of CaM KIIalpha. Thus, the functional domains reside in the foot-like processes, and the association domain comprises the gear-shaped core. The ribbon diagram of the bilobate structure of CaM KI fits nicely in the envelope of the foot-like component and indicates that the crevice between the two lobes comprising the functional domains is near the middle portion of the foot. The clustering of the functional domains provides a favorable arrangement for the autophosphorylation reaction, and the unusual arrangement of the catalytic domain on extended tethers appears to be significant for the remarkable functional diversity of CaM KIIalpha in cellular regulation.     

Ultrastructural Features of the Apical Complex,Pellicle, and Membranes Investing the Gamonts of Haemogregarina magna (Apicomplexa: Adeleina)1

ABSTRACT. Mature gamonts of Haemogregarina magna lie within a type of parasitophorous vacuole (Pv) apparently unique to the haemogregarines. The cytoplasm of infected erythrocytes was separated from the parasite by two Pv membranes. An additional membrane, coated on both sides with electron-dense material, closely invested the gamonts. The apical complex of the gamonts includes a conoid, two preconoidal rings, and an elaborate polar ring complex. The latter consisted of the polar ring and approximately 78 posteriorly directed, radially arranged, “tine-like” structures which fuse as they merge anteriorly into the polar ring. Freeze fracture replicas demonstrated that the pellicle of gamonts of H. magna was structurally similar to that of other apicomplexans. The closely apposed inner membranes of the pellicle formed plates which were arranged into strips along the long axis of the gamont. Calculations indicated that 13 such strips are found around the circumference of the gamonts with about six subpellicular microtubules associated with the inner surface of each strip. Gamonts of H. magna share many structural similarities with the kinetes, ookinetes, and sporokinetes of other apicomplexans. We propose that the conoid and polar ring complex are fundamental features of all apicomplexan “kinetes.”