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nerve ring in a terminal position, (2) an introvert with scalids, (3) an eversible foregut and (4) tanycytes.The unpublished doctoral thesis of B. Neuhaus [1991 Zur Ultrastruktur, Postembryonalentwicklung und phylogenetischen Verwandtschaft der Kinorhyncha.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 Antygomonas incomitata (Kinorhyncha: Cyclorhagida)

Postembryonic development in the kinorhynch species Antygomonas incomitata was examined using scanning electron microscopy. The morphology of the six juvenile stages, J‐1 to J‐6, varies at numerous details, but they can also be distinguished by a few key characters. Juvenile stage 1 by its composition of only nine trunk segments; J‐2 by the combination of possessing 10 trunk segments, but no cuspidate spines on segment 9; J‐3 by the presence of cuspidate spines on segment 9, but only one pair of cuspidate spines on segment 8; J‐4 by the combination of 10 trunk segments only, but having two pairs of cuspidate spines on segment 8; J‐5 by possessing 11 trunk segments and same spine compositions as adults but is still maintaining postmarginal spiculae; J‐6 specimens closely resemble adults and are most easily identified by their reduced trunk lengths. New segments are formed in a growth zone in the anterior part of the terminal segment. The complete number of segments is reached in J‐5. Development of cuticular head and trunk structures are described through all postembryonic stages and following developmental patterns could be outlined: the mouth cone possesses outer oral styles from J‐1, but in J‐1 to J‐3, the styles alternate in size. Scalids of the introvert are added after each molt, and scalids appear earliest in the anterior rings, whereas scalids in more posterior rings are added in older postembryonic stages. The early J‐1 stage is poor in spines and sensory spots and both structures increase in number after each molt. The complete spine composition is reached in J‐4, whereas new sensory spots appear after all molts, inclusive the final one from J‐6 to adult. Sensory spots in the paraventral positions often appear as Type 3 sensory spots but are through development transformed to Type 2. This transformation happens earliest on the anterior segments. J. Morphol., 2010. © 2010 Wiley‐Liss, Inc.     

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.     

On the morphology of Acanthostomum spiniceps (Looss, 1896) and A. absconditum (Looss, 1901) (Digenea: Cryptogonimidae: Acanthostominae) with particular reference to the juvenile stage

The morphology of juvenile and adult stages of Acanthostomum spiniceps and A. absconditum, from bagrid fish of the river Nile in Egypt, was studied with both light and scanning electron microscopy. In early juveniles, circumoral spines are absent and the entire body surface is covered with tegumental spines. Late juveniles show gradual differentiation of the circumoral tegument into a collar of spines associated with a reduction in density of tegumental spines at the posterior extremity of the body. Genital primordia appear when juveniles are about 1.75 mm long. The distributions of tegumental spines on adult A. spiniceps and A. absconditum are similar. Spines are denser on the dorsal and ventral surfaces of the anterior and middle parts of the body and less dense towards the posterior end. The tegumental fold surrounding the ventral sucker of A. absconditum has spines while the fold of A. spiniceps lacks them. The most important morphological features differentiating both species are the number of circumoral spines, body shape, ratio of body length to width, sucker sizes, and the presence or absence of spines on the ventral sucker.     

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.     

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.     

Taxonomic study of Ascocotyle (Phagicola) longa Ransom, 1920 (Digenea: Heterophyidae) and related taxa

Species of Ascocotyle Looss, 1899 (subgenus Phagicola Faust, 1920) (Digenea: Heterophyidae) related to A. (P.) longa Ransom, 1920, i.e. those with a bipartite gonotyl and a single crown of 16 circumoral spines, were revised on the basis of examination of type and voucher specimens. The species A. (P.) arnaldoi Travassos, 1929, A. (P.) byrdi (Robinson, 1956) and A. (P.) longicollis (Kuntz & Chandler, 1956), all possessing 16 circumoral spines, are synonymised with A. (P.) longa. A. (P.) ascolonga Witenberg, 1929, the types of which were not available, is provisionally retained as a valid species despite its close similarity to A. (P.) longa (identical number of circumoral spines and morphology of the ventrogenital sac). It differs from A. (P.) longa in the presence of uterine loops posterior to the testes and intestinal caeca ending anterior to the ovary (versus the pretesticular uterus and intestinal caeca reaching to the posterior margin of the ovary in A. (P.) longa). A. (P.) inglei (Hutton & Sogandares-Bernal, 1959), found only once in a single specimen, is also tentatively considered a valid species because it differs in number of circumoral spines (19), slightly larger eggs (23–26×13–16 m) and the distribution of refractile bodies of the gonotyl arranged in four separate groups from A. (P.) longa and A. (P.) ascolonga (16 circumoral spines; eggs 15–20×–12 m; refractile bodies in two groups).     

Morphology and ultrastructure of the sensory appendages of a kinorhynch introvert

The structure and arrangement of appendages (scalids) on the head of the homalorhagid kinorhynch Kinorhynchus phyllotropis Brown & Higgins, 1983 are named, described and illustrated. In adults of this species, seven rings of external scalids are separated by segment boundary structures from the oral styles and three rings of internal scalids. All of these appendages contain ciliated receptor cells which pass to pores at the scalid tips, and all but the two anterior rings are jointed. All of these appendages are radially arranged in multiples of five, and closely associated with the ten lobed circumoral nerve ring. The most posterior scalid ring consists of fourteen trichoscalids, of which six are longer than the other eight. The six longer trichoscalids are arranged in bilateral symmetry, two to each of the three facets of the body. Similarities between scalid arrangement in Kinorhyncha and Loricifera are discussed.     

Head sensory organs of Halobiotus stenostomus (Eutardigrada, Hypsibiidae)

The structure and arrangement of sensory organs in the tardigrade Halobiotus stenostomus (Richters 1908) have been studied using transmission and scanning electron microscopy techniques. The sensory organs found on the head of H. stenostomus are as follows: the circumoral sensory field, cephalic papillae, anterolateral and posterolateral sensory fields, and suboral sensory region. Four types of ciliated receptor structures are described in the sensory fields. The lateral sensory fields contain two types of receptor endings, dense and lucent, which differ in the presence or absence of a collar and in the structure of the outer dendrite segment. Two more types of receptor endings, ultrastructurally differing from the lateral sensory field receptors, are located in the suboral sensory region. Receptors with an asymmetric collar have been found, and a receptor ending without a collar is described for the first time in tardigrades. Unlike in other species studied, the sensory organs of H. stenostomus lack the lymph cavity surrounding the outer receptor segment. Similarity and differences in the ultrastructure of receptors between H. stenostomus and other species of Eutardigrada and Heterotardigrada are discussed.     

The evolutionary relationships of the earliest known cycloneuralians and a new record from the Cambrian Fortunian of South China

The Cambrian Fortunian Kuanchuanpu Formation and coeval strata in southern Shaanxi and northern Sichuan Provinces contain phosphatic carbonate that has yielded unambiguous body fossils of cycloneuralians. To date, six genera and species as well as nine indeterminate forms have been reported, and they provided information on the earliest known cycloneuralians and ecdysozoans. Among them, an indeterminate form is critical for understanding the evolution of the Fortunian cycloneuralians, and here I formally name it as Xinliscolex intermedius n. gen. n. sp. It has a series of at least seven longitudinal spinose sclerites on the dorsal side, and sensory structures such as pores and tubules distributed randomly among the annuli. The trunk end has two pairs of bilaterally arranged caudal spines. Micro-CT analysis reveals the preservation of a pharynx with inverted introvert with internally hollow scalids, an esophagus, midgut and hindgut, and a terminal anus. The digestive tract of Xinliscolex intermedius resembles that of modern priapulids, implying that the earliest known cycloneuralians have evolved a relatively derived digestive system, and this lays a foundation for their further diversification and evolution.     

Chemoreceptor sensillum structure in Limulus

The chemoreceptors of Limulus polyphemus (L.) are polyneuronal sensilla found in the spines of the coxal gnathobases of each walking leg, the spines of the chilarial appendages, and the chelae of all the limbs. Each sensillum contains 6–15 bipolar sensory cells that share a single pore in the cuticle. The dendrites of the sensory cells of each sensillum course to the cuticle together. These attenuate sharply and enter a canal in the cuticle as a very narrow terminal thread. The dendrites retain their identity in the thread, but with the light microscope, they are usually not visible individually. Each thread, consisting of 6–15 dendrites, is accompanied to the cuticular surface by a cuticular tubule found within the canal. The chemoreceptor sensilla of the gnathobase, chilarium, and chela, the temperature organs of Patten, and the flabellar receptor organs all have the same basic organization. In general this is the same structural plan shown by chemoreceptors of other arthropods. Several different mechanisms of peripheral physiological interaction among receptor cells are possible with a sensillum organization like that described here for Limulus.     

Acanthostomum macroclemidis n. sp. (Digenea: Cryptogonimidae: Acanthostominae) from the alligator snapping turtle,Macroclemys temmincki

Acanthostomum macroclemidis n. sp. is described from specimens found in the intestine of an alligator snapping turtle Macroclemys temmincki from southern Mississippi. The most important diagnostic features of the new species are the general shape and proportions of the body, the position of the pharynx (relative length of the prepharynx and esophagus), the egg size, the relative length and position of the vitelline fields, and the number, shape, and size of the circumoral spines. The new species has a very elongated body (length-width ratio, 8.9-13.0:1), 26 circumoral spines, which are almost oval in shape, a long prepharynx and a very short (shorter than the pharynx) esophagus, a seminal receptacle situated between the ovary and the anterior testis, a uterus not extending posterior to the anterior margin of the ovary, a long-stemmed and short-armed excretory vesicle, and 2 anal openings. Some features of the external morphology, such as the suckers, circumoral spines, sensory papillae, tegumental spines, and morphology of the posterior end, are examined using scanning electron microscopy. A diagnosis differentiating A. macroclemidis n. sp. from some other acanthostomine digeneans is provided. Acanthostomum macroclemidis n. sp. is the first digenean reported from an alligator snapping turtle and represents the northernmost record of an acanthostomine from turtles.     

Ultrastructure of the circumoral nerve ring and the radial nerve cords in holothurians (Echinodermata)

The circumoral nerve ring and the radial nerve cords (RNCs) of Eupentacta fraudatrix and Pseudocnus lubricus (Holothuroidea) were examined as an example of holothurian nervous tissue. The RNC is composed of outer ectoneural and inner hyponeural layers, which are interconnected with one another via short neural bridges. The circumoral nerve ring is purely ectoneural. Both ectoneural and hyponeural components are epithelial tubes with a thick neuroepithelium at one side. A thin ciliated non-neuronal epithelium complements the neuroepithelium to form a tube, thereby enclosing the epineural and hyponeural canals. The whole of the ectoneural and hyponeural subsystems is separated from the surrounding tissue by a continuous basal lamina. The nerve ring and the ectoneural and hyponeural parts of the radial nerves are all neuroepithelia composed of supporting cells and neurons. Supporting cells are interpreted as being glial cells. Based on ultrastructural characters, three types of neurons can be distinguished: (1) putative primary sensory neurons, whose cilium protrudes into the epineural or hyponeural canal; (2) non-ciliated neurons with swollen rough endoplasmic reticulum cisternae; (3) monociliated neurons that are embedded in the trunk of nerve fibers. Different types of synapses occur in the neuropile area. They meet all morphological criteria of classical chemical synapses. Vacuolated cells occur in the neuroepithelium of E. fraudatrix, but are absent in P. lubricus; their function is unknown. The cells of the non-neuronal epithelia that overlie the ectoneural and hyponeural canals are hypothesized to belong to the same cell type as the supporting cells of the neuroepithelium.     

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.     

Ultrastructure of the “statocysts” inProtodrilus species (Polychaeta): Reconstruction of the cellular organization with morphometric data from receptor cells

Summary The statocysts inProtodrilus ciliatus, P. oculifer, P. haurakiensis andP. helgolandicus are situated in the prostomium anterior to the palps and have been investigated by electron microscopy. The sensory organs were reconstructed from serial sections, volumes were calculated from areas of consecutive section profiles, and additional data on surface area of distal receptor elements have been determined. In spite of variations in size (diameter 8–20 m) their structure is nearly identical. The organs consist of one cup-shaped supportive cell, one large bi- or multiciliated sensory cell and two small uni- or biciliated sensory cells forming an extracellular cavity. This cavity is completely filled with microvillus-like or paracrystalline structures and there are no signs of statoliths composed of extracellular material. The most striking feature is the occurrence of paracrystals made up of undulating ciliary membranes extending from the large sensory cell and occupying 75–90% of the cavity inP. ciliatus, P. oculifer andP. haurakiensis. The remaining space is filled with microvilli or dendritic processes of the sensory cells. InP. helgolandicus the ciliary paracrystals are almost completely replaced by microvillus-like branches of cilia of the corresponding sensory cell. Paracrystals fill less than 10% of the cavity and are formed of flattened membranes. These sensory organs enclose large surface areas of membranes (15,000–38,000 m²). The surface areas of the paracrystals composed of undulating membranes is almost identical to that of densely arranged arrays of microvilli (about 25 m² per m³). These sensory organs are so different from all known statocysts that it is likely that they have another function. Their greatest structural correspondence is to light-receptive organs, especially in the structure and arrangement of microvilli. The role the paracrystals play is discussed: they might bear photopigments or simply represent a lens — a transparent, refractile and crystalline structure. These sensory organs are completely different from pigmented ocelli and phaosomes occurring in some protodrilids and represent a type of sensory organ thus far undescribed in polychaetes.     

The Mouth Cone and Mouth Ring of Echiniscus viridissimus Peterfi, 1956 (Heterotardigrada) with Comparisons to Corresponding Structures in Other Tardigrades

The mouth cone and mouth ring of the tardigrade Echiniscus viridissimus are described and their primary homology is assessed by comparing them to structures considered to be homologous in other heterotardigrades, eutardigrades, and ecdysozoans. In E. viridissimus the mouth cone is divided into anterior and posterior regions that differ in the ultrastructure of their cuticle. The mouth ring of the buccopharyngeal apparatus lies just anterior to the buccal tube. It is connected to it by a narrow flange or ridge of cuticle that allows the rostral end of the mouth tube to telescope part way into the mouth ring. Sensilla innervating the mouth cone and mouth ring in E. viridissimus correspond to sensilla of the circumoral and suboral sensory regions described for eutardigrades. The sensory fields, of both the mouth cone and mouth ring, exhibit two planes of mirror image or biradial symmetry. Although the relative dimensions of mouth ring differ markedly between E. viridissimus and eutardigrades, homologous areas can be identified easily. For example, the supporting rods of the mouth ring in E. viridissimus appear to be homologous to the supporting plates in the lamellae of eutardigrades. The widespread occurrence of the mouth cone and mouth ring in hetero- and eutardigrades suggests that these structures were present in the last common ancestor of all tardigrades. Expression of these characters in other ecdysozoans is more problematic. Tentatively the mouth cone and mouth ring are considered to be homologous to the circumoral plates of onychophorans and the ring of radiating plates found in the ‘Peytoia’ apparatus of Cambrian arthropods. These structures are not thought to be homologous to the introverts of scalidophorans or the circumoral structures of nematodes or nematomorphs, and are considered to be absent in these taxa. Thus, the mouth cone and mouth ring are potentially key synapomorphies for Panarthropoda.     

The Nymphal Eyes of Parabuthus transvaalicus Purcell, 1899 (Buthidae): An Accessory Lateral Eye in a Scorpion

In P. transvaalicus nymphs, 5 pairs of lateral ocelli each composed of a corneal lens, R-cell units forming a latticed rhabdom, arhabdomeric cells and pigment cells are present. In addition, we found a pair of unpigmented accessory sense organs situated ventroposteriorly to the lateral ocelli in prenymphs as well as in first nymphs. They are composed of primary, rhabdomeric sensory cells, and we infer that they represent a second type of lateral eye. They also comprise sensory units, but lenses and screening pigment are lacking. Their position and cellular architecture corresponds well with that of the “rudimentary” lateral eye of the xiphosuran, Limulus. The occurrence of a bipartite lateral visual system in Chelicerata and Arthropoda is discussed.     

The morphology and physiology of CAP organs in Jasus novaehollandiae (crustacea,decapoda, reptantia,macrura)

The location and external anatomy of the CAP organs of Jasus novaehollandiae were examined and found to be similar to those in Homarus gammarus (Laverack, 1978a). Histological examination of the organs showed threads or filaments arising from the internal surface of the spines to run through canals in the cuticle and join with dendrites of CAP sensory cells in the region of the hypodermis. The CAP neuron cell bodies lie in the nearby chordotonal organ strand. It is demonstrated that flexion of the limb causes the articulating membrane to deflect the spines of the sensillae distally. A variety of experimental techniques used to investigate the physiology of the organs reveals why previously reported attempts to record from the receptors failed. Direct stimulation of the sensillae with an analogue of the membrane and summation of many traces revealed phase constant responses at points corresponding to the covering and uncovering of the sensillae.     

Anatomy and Ultrastructure of Ventral Pharyngeal Organs and their Phylogenetic Importance in Polychaeta (Annelida). IV. The Pharynx and Jaws of the Dorvilleidae

An anatomical and ultrastructural investigation of the ventral pharyngeal organ, jaws and replacement of jaws was carried out in Ophryotrocha gracilis and Protodorvillea kefersteini (Dorvilleidae). The pharynx exhibits the following features: jaw apparatus present, consisting of paired mandibles and rows of maxillary plates, the latter are fused to form a single piece; cuticular jaws electron-dense, in P. kefersteini with collagen fibres; muscle bulbus solid, composed of muscle cells only; parallel running myofilaments, centrally located mitochondria and nuclei, bulbus epithelium containing the mandibles and gland cells, maxillary plates lying on folds corresponding to a tongue-like organ, connected with mandibles by longitudinal investing muscles; numerous gland cells not united to distinct salivary glands. Development of jaw replacements occurs in epithelial cavities beside the functional maxillae. Shape of maxillary plates is preformed by microvilli carrying cell processes. Maxilloblasts change their shape during the development. Synapomorphic structures occurring in ventral pharyngeal organs of other species outside the Eunicea are not present and even the closely related Dinophilidae exhibit a completely different pharyngeal organ. Therefore, convergent evolution of these organs is the most probable explanation. These findings do not agree with the hypothesis of the homology of the ventral pharyngeal organs in the Polychaeta.