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

Revisiting kinorhynch segmentation: variation of segmental patterns in the nervous system of three aberrant species

BACKGROUND

Kinorhynch segmentation differs from the patterns found in Chordata, Arthropoda and Annelida which have coeloms and circulatory systems. Due to these differences and their obsolete status as 'Aschelminthes', the microscopic kinorhynchs are often not acknowledged as segmented bilaterians. Yet, morphological studies have shown a conserved segmental arrangement of ectodermal and mesodermal organ systems with spatial correspondence along the anterior-posterior axis. However, a few aberrant kinorhynch lineages present a worm-like body plan with thin cuticle and less distinct segmentation, and thus their study may aid to shed new light on the evolution of segmental patterns within Kinorhyncha.

RESULTS

Here we found the nervous system in the aberrant Cateria styx and Franciscideres kalenesos to be clearly segmental, and similar to those of non-aberrant kinorhynchs; hereby not mirroring their otherwise aberrant and posteriorly shifted myoanatomy. In Zelinkaderes yong, however, the segmental arrangement of the nervous system is also shifted posteriorly and misaligned with respect to the cuticular segmentation.

CONCLUSIONS

The morphological disparity together with the distant phylogenetic positions of F. kalenesos, C. styx and Z. yong support a convergent origin of aberrant appearances and segmental mismatches within Kinorhyncha.

Ancestral morphology of Ecdysozoa constrained by an early Cambrian stem group ecdysozoan

BACKGROUND

Ecdysozoa are the moulting protostomes, including arthropods, tardigrades, and nematodes. Both the molecular and fossil records indicate that Ecdysozoa is an ancient group originating in the terminal Proterozoic, and exceptional fossil biotas show their dominance and diversity at the beginning of the Phanerozoic. However, the nature of the ecdysozoan common ancestor has been difficult to ascertain due to the extreme morphological diversity of extant Ecdysozoa, and the lack of early diverging taxa in ancient fossil biotas.

RESULTS

Here we re-describe Acosmia maotiania from the early Cambrian Chengjiang Biota of Yunnan Province, China and assign it to stem group Ecdysozoa. Acosmia features a two-part body, with an anterior proboscis bearing a terminal mouth and muscular pharynx, and a posterior annulated trunk with a through gut. Morphological phylogenetic analyses of the protostomes using parsimony, maximum likelihood and Bayesian inference, with coding informed by published experimental decay studies, each placed Acosmia as sister taxon to Cycloneuralia + Panarthropoda-i.e. stem group Ecdysozoa. Ancestral state probabilities were calculated for key ecdysozoan nodes, in order to test characters inferred from fossils to be ancestral for Ecdysozoa. Results support an ancestor of crown group ecdysozoans sharing an annulated vermiform body with a terminal mouth like Acosmia, but also possessing the pharyngeal armature and circumoral structures characteristic of Cambrian cycloneuralians and lobopodians.

CONCLUSIONS

Acosmia is the first taxon placed in the ecdysozoan stem group and provides a constraint to test hypotheses on the early evolution of Ecdysozoa. Our study suggests acquisition of pharyngeal armature, and therefore a change in feeding strategy (e.g. predation), may have characterised the origin and radiation of crown group ecdysozoans from Acosmia-like ancestors.

Understanding the effects of coastal defence structures on marine biota: The role of substrate composition and roughness in structuring sessile, macro- and meiofaunal communities

The increasing deployment of artificial structures into the marine environment is creating new hard substrates that differ from natural ones in physical and biological aspects. However, studies of macrofaunal and meiofaunal communities associated with artificial structures are very limited. Seawalls, cubes, acropods and rip-raps in Algeciras Bay (southern Spain) were each compared with the nearest natural hard substrate and their community structure was related to substrate roughness, composition, carbonates content, crystallinity and age, using db-RDA. The results showed clear differences between substrates for the three community levels (sessile, macro- and meiofauna). Overall, rip-raps were the most similar to natural substrates. Under similar environmental conditions, substrate roughness, composition (only for sessile) and age of the structures seemed to play important roles in structuring those communities. They especially affected the sessile community, initiating strong cascading effects that were detectable at high taxonomic level in the associated fauna.

Neuroanatomy of mud dragons: a comprehensive view of the nervous system in Echinoderes (Kinorhyncha) by confocal laser scanning microscopy

Background

The Scalidophora (Kinorhyncha, Loricifera and Priapulida) have an important phylogenetic position as early branching ecdysozoans, yet the architecture of their nervous organ systems is notably underinvestigated. Without such information, and in the absence of a stable phylogenetic context, we are inhibited from producing adequate hypotheses about the evolution and diversification of ecdysozoan nervous systems. Here, we utilize confocal laser scanning microscopy to characterize serotonergic, tubulinergic and FMRFamidergic immunoreactivity patterns in a comparative neuroanatomical study with three species of Echinoderes, the most speciose, abundant and diverse genus within Kinorhyncha.

Results

Neuroanatomy in Echinoderes as revealed by acetylated α-tubulin immunoreactivity includes a circumpharyngeal brain and ten neurite bundles in the head region that converge into five longitudinal nerves within the trunk. The ventral nerve cord is ganglionated, emerging from the brain with two connectives that converge in trunk segments 2–3, and diverge again within segment 8. The longitudinal nerves and ventral nerve cord are connected by two transverse neurites in segments 2–9. Differences among species correlate with the number, position and innervation of cuticular structures along the body. Patterns of serotoninergic and FMRFamidergic immunoreactivity correlate with the position of the brain neuropil and the ventral nerve cord. Distinct serotonergic and FMRFamidergic somata are associated with the brain neuropil and specific trunk segments along the ventral nerve cord.

Conclusions

Neural architecture is highly conserved across all three species, suggesting that our results reveal a pattern that is common to more than 40% of the species within Kinorhyncha. The nervous system of Echinoderes is segmented along most of the trunk; however, posterior trunk segments exhibit modifications that are likely associated with sensorial, motor or reproductive functions. Although all kinorhynchs show some evidence of an externally segmented trunk, it is unclear whether external segmentation matches internal segmentation of nervous and muscular organ systems across Kinorhyncha, as we observed in Echinoderes. The neuroanatomical data provided in this study not only expand the limited knowledge on kinorhynch nervous systems but also establish a comparative morphological framework within Scalidophora that will support broader inferences about the evolution of neural architecture among the deepest branching lineages of the Ecdysozoa.

Electronic supplementary material

The online version of this article (10.1186/s12862-019-1405-4) contains supplementary material, which is available to authorized users.

The neuromuscular system of Pycnophyes kielensis (Kinorhyncha: Allomalorhagida) investigated by confocal laser scanning microscopy

Background

Kinorhynchs are ecdysozoan animals with a phylogenetic position close to priapulids and loriciferans. To understand the nature of segmentation within Kinorhyncha and to infer a probable ancestry of segmentation within the last common ancestor of Ecdysozoa, the musculature and the nervous system of the allomalorhagid kinorhynch Pycnophyes kielensis were investigated by use of immunohistochemistry, confocal laser scanning microscopy, and 3D reconstruction software.

Results

The kinorhynch body plan comprises 11 trunk segments. Trunk musculature consists of paired ventral and dorsal longitudinal muscles in segments 1–10 as well as dorsoventral muscles in segments 1–11. Dorsal and ventral longitudinal muscles insert on apodemes of the cuticle inside the animal within each segment. Strands of longitudinal musculature extend over segment borders in segments 1–6. In segments 7–10, the trunk musculature is confined to the segments. Musculature of the digestive system comprises a strong pharyngeal bulb with attached mouth cone muscles as well as pharyngeal bulb protractors and retractors. The musculature of the digestive system shows no sign of segmentation. Judged by the size of the pharyngeal bulb protractors and retractors, the pharyngeal bulb, as well as the introvert, is moved passively by internal pressure caused by concerted action of the dorsoventral muscles. The nervous system comprises a neuropil ring anterior to the pharyngeal bulb. Associated with the neuropil ring are flask-shaped serotonergic somata extending anteriorly and posteriorly. A ventral nerve cord is connected to the neuropil ring and runs toward the anterior until an attachment point in segment 1, and from there toward the posterior with one ganglion in segment 6.

Conclusions

Segmentation within Kinorhyncha likely evolved from an unsegmented ancestor. This conclusion is supported by continuous trunk musculature in the anterior segments 1–6, continuous pharyngeal bulb protractors and retractors throughout the anterior segments, no sign of segmentation within the digestive system, and the absence of ganglia in most segments. The musculature shows evidence of segmentation that fit the definition of an anteroposteriorly repeated body unit only in segments 7–10.

Electronic supplementary material

The online version of this article (doi:10.1186/s13227-016-0062-6) contains supplementary material, which is available to authorized users.

The larval nervous system of the penis worm Priapulus caudatus (Ecdysozoa)

The origin and extreme diversification of the animal nervous system is a central question in biology. While most of the attention has traditionally been paid to those lineages with highly elaborated nervous systems (e.g. arthropods, vertebrates, annelids), only the study of the vast animal diversity can deliver a comprehensive view of the evolutionary history of this organ system. In this regard, the phylogenetic position and apparently conservative molecular, morphological and embryological features of priapulid worms (Priapulida) place this animal lineage as a key to understanding the evolution of the Ecdysozoa (i.e. arthropods and nematodes). In this study, we characterize the nervous system of the hatching larva and first lorica larva of the priapulid worm Priapulus caudatus by immunolabelling against acetylated and tyrosinated tubulin, pCaMKII, serotonin and FMRFamide. Our results show that a circumoral brain and an unpaired ventral nerve with a caudal ganglion characterize the central nervous system of hatching embryos. After the first moult, the larva attains some adult features: a neck ganglion, an introvert plexus, and conspicuous secondary longitudinal neurites. Our study delivers a neuroanatomical framework for future embryological studies in priapulid worms, and helps illuminate the course of nervous system evolution in the Ecdysozoa.

Fine Structure of the Integumentary Cuticles and Alimentary Tissues of Pycnophyid Kinorhynchs Pycnophyes oshoroensis and Kinorhynchus yushini (Kinorhyncha, Homalorhagida)

Integumentary and alimentary tissues were ultrastructurally examined in two pycnophyid kinorhynchs, Pycnophyes oshoroensis and Kinorhynchus yushini, to elucidate some aspects of their ecology. The body is entirely enveloped by an epicuticle layer with no gaps between cuticle plates and joints. The cuticular layer has a structure dense enough to prevent invasion by foreign organisms. The cuticular surface is overlaid by a mucus layer that may form a hydrophilic surface. The alimentary contents were heterogeneous, probably including some cellular components, such as chloroplast-like structures. Kinorhynchs likely break down food particles in the pharyngeal bulb by pressing it between the cuticulated epithelia. The pharyngeal crown was located in front of the pharyngeal bulb and had a thick wall with a striated sub-structure. Contraction of the pharyngeal bulb probably increases the internal pressure of the pharyngeal crown; this may be one reason for the thick wall of the pharyngeal crown. Nutrients appear to be taken up by midgut epithelial cells through both absorption via microvilli and endocytosis. Additionally, sperm tails in the testis of P. oshoroensis have unusual axonemes; i.e., an 18+9+2 pattern.

Phylogeny of Kinorhyncha Based on Morphology and Two Molecular Loci

The phylogeny of Kinorhyncha was analyzed using morphology and the molecular loci 18S rRNA and 28S rRNA. The different datasets were analyzed separately and in combination, using maximum likelihood and Bayesian Inference. Bayesian inference of molecular sequence data in combination with morphology supported the division of Kinorhyncha into two major clades: Cyclorhagida comb. nov. and Allomalorhagida nom. nov. The latter clade represents a new kinorhynch class, and accommodates Dracoderes, Franciscideres, a yet undescribed genus which is closely related with Franciscideres, and the traditional homalorhagid genera. Homalorhagid monophyly was not supported by any analyses with molecular sequence data included. Analysis of the combined molecular and morphological data furthermore supported a cyclorhagid clade which included all traditional cyclorhagid taxa, except Dracoderes that no longer should be considered a cyclorhagid genus. Accordingly, Cyclorhagida is divided into three main lineages: Echinoderidae, Campyloderidae, and a large clade, 'Kentrorhagata', which except for species of Campyloderes, includes all species with a midterminal spine present in adult individuals. Maximum likelihood analysis of the combined datasets produced a rather unresolved tree that was not regarded in the following discussion. Results of the analyses with only molecular sequence data included were incongruent at different points. However, common for all analyses was the support of several major clades, i.e., Campyloderidae, Kentrorhagata, Echinoderidae, Dracoderidae, Pycnophyidae, and a clade with Paracentrophyes + New Genus and Franciscideres (in those analyses where the latter was included). All molecular analyses including 18S rRNA sequence data furthermore supported monophyly of Allomalorhagida. Cyclorhagid monophyly was only supported in analyses of combined 18S rRNA and 28S rRNA (both ML and BI), and only in a restricted dataset where taxa with incomplete information from 28S rRNA had been omitted. Analysis of the morphological data produced results that were similar with those from the combined molecular and morphological analysis. E.g., the morphological data also supported exclusion of Dracoderes from Cyclorhagida. The main differences between the morphological analysis and analyses based on the combined datasets include: 1) Homalorhagida appears as monophyletic in the morphological tree only, 2) the morphological analyses position Franciscideres and the new genus within Cyclorhagida near Zelinkaderidae and Cateriidae, whereas analyses including molecular data place the two genera inside Allomalorhagida, and 3) species of Campyloderes appear in a basal trichotomy within Kentrorhagata in the morphological tree, whereas analysis of the combined datasets places species of Campyloderes as a sister clade to Echinoderidae and Kentrorhagata.

Comparative myoanatomy of Echinoderes (Kinorhyncha): a comprehensive investigation by CLSM and 3D reconstruction

INTRODUCTION

Kinorhyncha is a clade of marine invertebrate meiofauna. Their body plan includes a retractable introvert bearing rings of cuticular spines, and a limbless trunk with distinct segmentation of nervous, muscular and epidermal organ systems. As derived members within the basal branch of Ecdysozoa, kinorhynchs may provide an important example of convergence on the evolution of segmentation within one of three bilaterian superclades. We describe the myoanatomy of Echinoderes, the most specious kinorhynch genus, and build upon historical studies of kinorhynch ultrastructure and gross morphology. This is the first multi-species comparison of a complete organ system by confocal microscopy and three-dimensional reconstruction within Kinorhyncha.

RESULTS

Myoanatomy of adult Echinoderes is composed of the following: Head with two mouth cone circular muscles, nine pairs of oral style muscles, ten introvert retractors, one introvert circular muscle, and fourteen introvert circular muscle retractors; Neck with one circular muscle; Trunk showing distinct pairs of ventral and dorsal muscles within segments 1-10, dorsoventral muscles within segments 3-10, diagonal muscles within segments 1-8, longitudinal fibers spanning segments 1-9, three pairs of terminal spine muscles, and one pair of male penile spine muscles; Gut showing a pharynx with ten alternating rings of radial and circular muscle fibers enclosed in a complex sheath of protractors and retractors, an orthogonal grid of longitudinal and circular fibers surrounding the intestine, and paired hindgut dilators.

CONCLUSIONS

Myoanatomy is highly conserved between species of Echinoderes. Interspecific variation is observed in the arrangement and number of introvert fibers and the composition of pharyngeal muscles. Segmented trunk musculature facilitates the movements of articulated cuticular plates along the anterior-posterior axis. Intersegmental muscle fibers assist with dorsoventral and lateral trunk movements. Protractors, retractors and circular muscles coordinate eversion and retraction of the introvert and mouth cone, and relocation of the pharynx during locomotion and feeding behaviors. Pairs of posterior fibers suggest independent movements of terminal spines, and male penile spines. Within Scalidophora, myoanatomy is more similar between Kinorhyncha and Loricifera, than either group is to Priapulida. Kinorhynch myoanatomy may reflect a convergent transition from vermiform to segmented body plans during the early radiation of Ecdysozoa.

Selective neuronal staining in tardigrades and onychophorans provides insights into the evolution of segmental ganglia in panarthropods

BACKGROUND

Although molecular analyses have contributed to a better resolution of the animal tree of life, the phylogenetic position of tardigrades (water bears) is still controversial, as they have been united alternatively with nematodes, arthropods, onychophorans (velvet worms), or onychophorans plus arthropods. Depending on the hypothesis favoured, segmental ganglia in tardigrades and arthropods might either have evolved independently, or they might well be homologous, suggesting that they were either lost in onychophorans or are a synapomorphy of tardigrades and arthropods. To evaluate these alternatives, we analysed the organisation of the nervous system in three tardigrade species using antisera directed against tyrosinated and acetylated tubulin, the amine transmitter serotonin, and the invertebrate neuropeptides FMRFamide, allatostatin and perisulfakinin. In addition, we performed retrograde staining of nerves in the onychophoran Euperipatoides rowelli in order to compare the serial locations of motor neurons within the nervous system relative to the appendages they serve in arthropods, tardigrades and onychophorans.

RESULTS

Contrary to a previous report from a Macrobiotus species, our immunocytochemical and electron microscopic data revealed contralateral fibres and bundles of neurites in each trunk ganglion of three tardigrade species, including Macrobiotus cf. harmsworthi, Paramacrobiotus richtersi and Hypsibius dujardini. Moreover, we identified additional, extra-ganglionic commissures in the interpedal regions bridging the paired longitudinal connectives. Within the ganglia we found serially repeated sets of serotonin- and RFamid-like immunoreactive neurons. Furthermore, our data show that the trunk ganglia of tardigrades, which include the somata of motor neurons, are shifted anteriorly with respect to each corresponding leg pair, whereas no such shift is evident in the arrangement of motor neurons in the onychophoran nerve cords.

CONCLUSIONS

Taken together, these data reveal three major correspondences between the segmental ganglia of tardigrades and arthropods, including (i) contralateral projections and commissures in each ganglion, (ii) segmentally repeated sets of immunoreactive neurons, and (iii) an anteriorly shifted (parasegmental) position of ganglia. These correspondences support the homology of segmental ganglia in tardigrades and arthropods, suggesting that these structures were either lost in Onychophora or, alternatively, evolved in the tardigrade/arthropod lineage.

Molecular phylogeny of kinorhynchs

We reconstructed kinorhynch phylogeny using maximum-likelihood and Bayesian analyses of nuclear 18S and 28S rRNA gene sequences from 30 species in 13 genera (18S) and 23 species in 12 genera (28S), representing eight families and both orders (Cyclorhagida and Homalorhagida) currently recognized in the phylum. We analyzed the two genes individually (18S and 28S datasets) and in combination (18S+28S dataset). We detected four main clades (I-IV). Clade I consisted of family Echinoderidae. Clade II contained representatives of Zelinkaderidae, Antygomonidae, Semnoderidae, Centroderes, and Condyloderes, the latter two currently classified in Centroderidae; within Clade II, Zelinkaderidae, Antygomonidae, and Semnoderidae comprised a clade with strong nodal support. Clade III contained only two species in Campyloderes, also currently classified in the Centroderidae, indicating polyphyly for this family. Clades I-III, containing all representatives of Cyclorhagida included in the analysis except for Dracoderes abei, formed a clade with high nodal support in the 28S and 18S+28S trees. Clade IV, resolved in the 18S and 18S+28S trees with high nodal support, contained only species in order Homalorhagida, with the exception of the cyclorhagid Dracoderes abei. Order Cyclorhagida as it currently stands is thus polyphyletic, and order Homalorhagida paraphyletic. Our results indicate that Dracoderidae has been misplaced in Cyclorhagida based on homoplasious characters. Our analyses did not resolve the relationships among Clades I-III within Cyclorhagida. Neither gene alone nor the combined dataset resolved all nodes in trees, indicating that additional markers will be needed to reconstruct kinorhynch phylogeny.

Comparative morphology of serotonergic-like immunoreactive elements in the central nervous system of kinorhynchs (Kinorhyncha, Cyclorhagida)

Cycloneuralian taxa exhibit similar organ system architectures, providing informative characters of metazoan evolution, yet very few modern comparative descriptions of cellular and molecular homologies within and among those taxa are available. We immunolabeled and characterized elements of the serotonergic nervous system in the kinorhynchs Echinoderes spinifurca, Antygomonas paulae, and Zelinkaderes brightae using confocal laser scanning microscopy. Fluorescent markers targeting DNA were combined with observations of auto-fluorescent structures to guide interpretations of the internal and external anatomy in each species. Results show a common pattern of the central nervous system with a circumenteric brain divided into ring-shaped anterior and posterior neuronal somata and a central neuropil connected to a multi-stringed, longitudinal ventral nerve cord. Structural similarities and differences in the nervous systems of these species were observed and described, stressing the incomplete ring nature of the anterior region of the kinorhynch brain, the functional relationship between the brain and the movable introvert, and the number and arrangement of nerve strings and somata of the ventral nerve cord. The ventral cord ends in two ventrolateral cell bodies in E. spinifurca, and forms a terminal loop associated with a midterminal spine in A. paulae and Z. brightae. The possible functional and phylogenetic significance of these features and arrangements are discussed.

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.

A new species of Condyloderes (Cyclorhagida, Kinorhyncha) from Korea

A new kinorhynch species, Condyloderes megastigma sp. nov., is described from the Korea Strait. The new species is characterized by the presence of 16 placids with either eight, four, or two knobby projections, middorsal and lateroventral acicular spines on segments 1 to 9, lateroventral cuspidate spines on segment 2 in females only, but otherwise lateroventral cuspidate spines on segments 4 and 5, and 8 and 9 in both sexes. Unique for the new species is furthermore the presence of paired ventromedial appendages on segments 7 and 8, giant ventromedial sensory spots on segment 9, and a terminal segment consisting of one tergal and one sternal plate. The mouth cone and introvert armature are described in detail for the first time for the genus Condyloderes Higgins, 1969. This study reveals similarities in several morphological characters between this genus and species of Campyloderes Zelinka, 1913.

Neural development in Onychophora (velvet worms) suggests a step-wise evolution of segmentation in the nervous system of Panarthropoda

A fundamental question in biology is how animal segmentation arose during evolution. One particular challenge is to clarify whether segmental ganglia of the nervous system evolved once, twice, or several times within the Bilateria. As close relatives of arthropods, Onychophora play an important role in this debate since their nervous system displays a mixture of both segmental and non-segmental features. We present evidence that the onychophoran "ventral organs," previously interpreted as segmental anlagen of the nervous system, do not contribute to nerve cord formation and therefore cannot be regarded as vestiges of segmental ganglia. The early axonal pathways in the central nervous system arise by an anterior-to-posterior cascade of axonogenesis from neuronal cell bodies, which are distributed irregularly along each presumptive ventral cord. This pattern contrasts with the strictly segmental neuromeres present in arthropod embryos and makes the assumption of a secondary loss of segmentation in the nervous system during the evolution of the Onychophora less plausible. We discuss the implications of these findings for the evolution of neural segmentation in the Panarthropoda (Arthropoda+Onychophora+Tardigrada). Our data best support the hypothesis that the ancestral panarthropod had only a partially segmented nervous system, which evolved progressively into the segmental chain of ganglia seen in extant tardigrades and arthropods.

Ultrastructure of the protonephridia of larvalRugiloricuscf.cauliculus, maleArmorloricus elegans, and femaleNanaloricus mysticus(Loricifera)

The protonephridial system of several Loricifera was studied by transmission electron microscopy. A larval specimen of Rugiloricus cf. cauliculus possesses two protonephridia, which are "capped" frontally by a compact mass of still undifferentiated gonadal cells. Each protonephridium consists of four monociliary terminal cells and four canal cells with a diplosome but no cilia. Because of incomplete series of sections and unsatisfactory fixation, the outleading cell(s) could not be detected. In a male specimen of Armorloricus elegans, each gonad contains two protonephridia that open into the gonadal lumen. Each protonephridium consists of two monociliary terminal cells, each forming a filter, two nonciliated canal cells, and two nephroporus cells. The protonephridial lumina of the latter cells fuse to one common lumen, which unites with the gonadal lumen. Preliminary observations on the protonephridia of a female Nanaloricus mysticus reveal a more complicated arrangement of interdigitating terminal and canal cells. One or two terminal cells form their own individual filter or four cells form a common compound filter. The cilium of the terminal cells of all species investigated are surrounded by a palisade of nine microvilli that support the filter barrier made of an extracellular matrix. An additional filter diaphragm could be traced between the pores in the cell wall of each terminal cell of A. elegans. The urogenital system of the Loricifera differs from that of the Priapulida in that the protonephridia of the former are completely integrated into the gonad, whereas the excretory organs of the latter open into the urogenital duct caudally of the gonads.

Terminal addition, the Cambrian radiation and the Phanerozoic evolution of bilaterian form

We examine terminal addition, the process of addition of serial elements in a posterior subterminal growth zone during animal development, across modern taxa and fossil material. We argue that terminal addition was the basal condition in Bilateria, and that modification of terminal addition was an important component of the rapid Cambrian evolution of novel bilaterian morphology. We categorize the often-convergent modifications of terminal addition from the presumed ancestral condition. Our focus on terminal addition and its modification highlights trends in the history of animal evolution evident in the fossil record. These trends appear to be the product of departure from the initial terminal addition state, as is evident in evolutionary patterns within-fossil groups such as trilobites, but is also more generally related to shifts in types of morphologic change through the early Phanerozoic. Our argument is contingent on dates of metazoan divergence that are roughly convergent with the first appearance of metazoan fossils in the latest Proterozoic and Cambrian, as well as on an inference of homology of terminal addition across bilaterian Metazoa.

Reconstruction of the muscle system in Antygomonas sp. (Kinorhyncha, Cyclorhagida) by means of phalloidin labeling and cLSM

In the present investigation the entire muscle system of the cyclorhagid kinorhynch Antygomonas sp. was three-dimensionally reconstructed from whole mounts by means of FITC-phalloidin labeling and confocal scanning microscopy. With this technique, which proved to be especially useful for microscopically small species, we wanted to reinvestigate and supplement the findings obtained by histological and electron microscopical methods. The organization of the major muscle systems can be summarized as follows: 1) All muscle fibers, apart from the intestinal ones, the spine, and the mouth cone muscles, show a cross-striated pattern; 2) Dorsal longitudinal muscle fibers as well as segmentally arranged dorsoventral fibers occur from segment III to XIII; 3) Diagonal muscle fibers are located laterally in segments III to X; 4) Two rings of circular fibers are present in segment II, forming the closing apparatus in Cyclorhagida. Further circular muscles are present in segment I, forming the mouth cone and the eversible introvert, and in the pharyngeal bulb.

Molecules, development and fossils in the study of metazoan evolution; Articulata versus Ecdysozoa revisited

Two conflicting hypotheses of protostome relationships, Articulata and Ecdysozoa, are reviewed by evaluating the evidence in favor and against each one of them. Understanding early embryonic development and segmentation in non-arthropod non-annelid protostomes seems crucial to the debate. New ways of coding metazoan matrices, avoiding ground-patterns and higher taxa, and incorporating fossil evidence seems the best way to avoid circular debates. Molecular data served as the catalyzer for the Ecdysozoa hypothesis, although morphological support had been implicitly suggested. Most molecular analyses published so far have shown some support for Ecdysozoa, whereas none has ever supported Articulata. Here, new analyses of up to four nuclear loci, including 18S rRNA, myosin heavy chain II, histone H3 and elongation factor 1-alpha are conducted to test the molecular support for Ecdysozoa, and, at least under some parameter sets, most data sets show a clade formed by the molting animals. In contrast, support for Articulata is not found under any analytical conditions.

Animal phylogeny and the ancestry of bilaterians: inferences from morphology and 18S rDNA gene sequences

Insight into the origin and early evolution of the animal phyla requires an understanding of how animal groups are related to one another. Thus, we set out to explore animal phylogeny by analyzing with maximum parsimony 138 morphological characters from 40 metazoan groups, and 304 18S rDNA sequences, both separately and together. Both types of data agree that arthropods are not closely related to annelids: the former group with nematodes and other molting animals (Ecdysozoa), and the latter group with molluscs and other taxa with spiral cleavage. Furthermore, neither brachiopods nor chaetognaths group with deuterostomes; brachiopods are allied with the molluscs and annelids (Lophotrochozoa), whereas chaetognaths are allied with the ecdysozoans. The major discordance between the two types of data concerns the rooting of the bilaterians, and the bilaterian sister-taxon. Morphology suggests that the root is between deuterostomes and protostomes, with ctenophores the bilaterian sister-group, whereas 18S rDNA suggests that the root is within the Lophotrochozoa with acoel flatworms and gnathostomulids as basal bilaterians, and with cnidarians the bilaterian sister-group. We suggest that this basal position of acoels and gnathostomulids is artifactal because for 1,000 replicate phylogenetic analyses with one random sequence as outgroup, the majority root with an acoel flatworm or gnathostomulid as the basal ingroup lineage. When these problematic taxa are eliminated from the matrix, the combined analysis suggests that the root lies between the deuterostomes and protostomes, and Ctenophora is the bilaterian sister-group. We suggest that because chaetognaths and lophophorates, taxa traditionally allied with deuterostomes, occupy basal positions within their respective protostomian clades, deuterostomy most likely represents a suite of characters plesiomorphic for bilaterians.

Cuticular structures and epidermal glands of Echinoderes cantabricus and E. hispanicus (Kinorhyncha, Cyclorhagida) with special reference to their taxonomic value

The body wall of two species of kinorhynchs, Echinoderes cantabricus and E. hispanicus, was examined with transmission and scanning electron microscopy, to determine accurately the nature, arrangement, and consistency of characters used for taxonomic purposes. Structural details of cuticular hairs, pectinate fringes, spines, tubules, and different cuticular scars are described and discussed. Two kinds of epidermal glands, types 1 and 2, are characterized according to the nature of their secretory products, their exact location and distribution in the trunk segments, and their possible value as taxonomic characters. The generally assumed function of tubules for the release of secretory product is analyzed and finally rejected in light of the different distribution of tubules and glands along the trunk and because of the absence of a clear anatomic relation between both structures. The cuticular features studied can be useful for taxonomic purposes because of their consistency, but some of them are difficult to access and evaluate and therefore must be used with caution.