Symmetry of priapulids (Priapulida). 1. Symmetry of adults

Priapulids possess a radial symmetry that is remarkably reflected in both external morphology and internal anatomy. It results in the appearance of 25-radial (a number divisible by five) symmetry summarized as a combination of nonaradial, octaradial, and octaradial (9+8+8) symmetries of scalids. The radial symmetry is a secondary appearance considered as an evolutionary adaptation to a lifestyle within the three-dimensional environment of bottom sediment. The eight anteriormost, or primary, scalids retain their particular position because of their innervation directly from the circumpharyngeal brain. As a result of a combination of the octaradial symmetry of primary scalids, pentaradial symmetry of teeth, and the 25-radial symmetry of scalids, the initial bilateral symmetry remains characterized by the single sagittal plane.     

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).     

Recent Priapulidae and their Early Cambrian ancestors: comparisons and evolutionary significance

Although priapulid worms form a relatively small phylum in present-day marine environments, they were important animals in Cambrian endobenthic communities. Two Early Cambrian priapulids, namely Xiaoheiqingella peculiaris and Yunnanpriapulus halteroformis nov. gen., nov. sp. from the Maotianshan Shale Lagerstätte of SW China are revised and described. Several key-features of the body plan of Recent Priapulidae are recognized in these two forms: 1) the four-fold body division (introvert, neck, trunk, and caudal appendage); 2) the well-developed introvert armed with ca. 25 longitudinal rows of scalids; 3) the caudal appendage; 4) the pharyngeal teeth arranged in a pentagonal disposition (Xiaoheiqingella); 5) the ventral nerve cord present in Yunnanpriapulus. This morphology indicates close evolutionary relationships with modern priapulids. Xiaoheiqingella and Yunnanpriapulus nov. gen. are tentatively placed within the recent family Priapulidae. The Priapulidae lineage may therefore have a remote origin (Early Cambrian) much older than was previously assumed (Priapulites; Late Carboniferous). The functional morphology of Xiaoheiqingella and Yunnanpriapulus nov. gen. suggests that these two worms were chiefly carnivorous with possible occasional mud-eating habits.     

Diverse radial symmetry among the Cambrian Fortunian fossil embryos from northern Sichuan and southern Shaanxi provinces,South China

The Cambrian Fortunian fossil embryos exhibit embryonic development of ancient animals and hence have important bearings on evolutionary developmental biology. They have radial symmetry, and may be early representatives of cnidarians. Here we report new material of three-dimensionally phosphatized fossil embryos from the Fortunian Kuanchuanpu Formation and coeval strata in northern Sichuan and southern Shaanxi provinces, South China. The new material includes previously reported fossil embryos assignable to Pseudooides prima with biradial symmetry or pseudo-hexaradial symmetry, Quadrapyrgites quadratacris with tetraradial symmetry, and Olivooides multisulcatus with pentaradial symmetry. Additionally, we recovered two new types of fossil embryos, i.e., Embryo I with hexaradial symmetry and Embryo II with octaradial symmetry, and they are tentatively suggested to represent new cnidarians. In contrast to the diverse radial symmetry of the Fortunian cnidarians, modern cnidarians exhibit stable tetraradial symmetry in medusozoans, biradial symmetry in anthozoans, and bilateral symmetry in siphonophores (Hydrozoa). The current study supports the view that the tetraradial symmetry of modern medusozoans is a surviving remnant of their Fortunian relatives.     

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.     

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.     

Did internal transport,rather than directed locomotion,favor the evolution of bilateral symmetry in animals?

The standard explanation for the origin of bilateral symmetry is that it conferred an advantage over radial symmetry for directed locomotion. However, recent developmental and phylogenetic studies suggest that bilateral symmetry may have evolved in a sessile benthic animal, predating the origin of directed locomotion. An evolutionarily feasible alternative explanation is that bilateral symmetry evolved to improve the efficiency of internal circulation by affecting the compartmentalization of the gut and the location of major ciliary tracts. This functional design principle is illustrated best by the phylum Cnidaria where symmetry varies from radial to tetraradial, biradial and bilateral. In the Cnidaria, bilateral symmetry is manifest most strongly in the internal anatomy and the disposition of ciliary tracts. Furthermore, the bilaterally symmetrical Cnidaria are typically sessile and, in those bilaterally symmetrical cnidarians that undergo directed locomotion, the secondary body axis does not bear a consistent orientation to the direction of locomotion as it typically does in Bilateria. Within the Cnidaria, the hypothesized advantage of bilateral symmetry for internal circulation can be tested by experimental analysis and computer modeling of fluid mechanics. The developmental evolution of symmetry within the Cnidaria can be further explored through comparative gene expression studies among species whose symmetry varies.     

Cuticular structures of the priapulid Halicryptus spinulosus: A scanning electron microscopical study

Summary The surface anatomy and the structures lining the pharynx of Halicryptus spinulosus were examined by scanning electron microscopy (SEM). The structures were compared and contrasted with those reported for other priapulids, particularly those features previously studied with SEM. Buccal papillae and pharyngeal teeth of two types were described. Surface structures observed with SEM were: scalids, abdominal setae, anal papillae, posterior warts and ring papillae. The latter three structures are unique among described priapulids. The anal papillae are composed of several rounded, perhaps pedunculate, structures; the posterior warts are composed of mitriform structures in close association with columnar structures. Both are located in separate depressions in the posterior integument. The ring papillae occur on the annuli close to the posterior end. Halicryptus spinulosus was previously thought to lack these structures.     

The Development of Radial and Biradial Symmetry: The Evolution of Bilaterality

SYNOPSIS. Understanding the evolutionary origin of novel metazoanbody plans continues to be one of the most sought after answersin biology. Perhaps the most profound change that may have occurredin the Metazoa is the appearance of bilaterally symmetricalforms from a presumably radially symmetrical ancestor. The symmetryproperties of bilaterally symmetrical larval and adult metazoansare generally set up during the cleavage period while most "radially"symmetrical cnidarians do not display a stereotyped cleavageprogram. Ctenophores display biradial symmetry and may representone intermediate form in the transition to bilateral symmetry.The early development of cnidarians and ctenophores is comparedwith respect to the timing and mechanisms of axial determination.The origin of the dorsal-ventral axis, and indeed the relationshipsof the major longitudinal axes, in cnidarians, ctenophores,and bilaterian animals are far from certain. The realizationthat many of the molecular mechanisms of axial determinationare conserved throughout the Bilateria allows one to formulatea set of predictions as to their possible role in the originsof bilaterian ancestors.     

The nervous system ofNectonema munidae andGordius aquaticus,with implications for the ground pattern of the Nematomorpha

 The nervous system of Nectonema munida is shown to be composed of a brain, a ventral nerve cord with an anterior and a posterior enlargement, a dorsal nerve cord and a plexus-like basiepidermal nervous system. The ultrastructure of these parts is given. Additionally, the ventral nerve cord of Gordius aquaticus is ultrastructurally described. The results are compared with the literature to work out the ground pattern of the Nematomorpha according to the nervous system. This contains a circumpharyngeal brain with a main subpharyngeal portion and a weak suprapharyngeal portion, a ventral and dorsal intraepidermal nerve cord and a peripheral nervous system. The ground pattern of the nervous system of Nematomorpha is then compared to that of other Nemathelminthes. The form of the brain and the distribution of perikarya are derived characters of the Nematomorpha. The existence of an unpaired ventral and an unpaired dorsal nerve cord and the position of these two cords in epidermal cords are synapomorphies of the Nematomorpha and the Nematoda. Accepted: 7 July 1996     

Symmetry and the tentacular apparatus in Cnidaria

Comparative analysis provides evidence that bilateral symmetry is a primary character of Cnidaria. All anthozoan taxa are characterized by bilateral symmetry. The anthozoan pharyngeal plane is a plane of bilateral symmetry of mesenteries and, at the same time, it is a plane of bilateral symmetry of regulatory gene expression in anthozoan morphogenesis. In Medusozoa, the bilateral symmetry is replaced by radial symmetry, but some hydrozoans (for example, Corymorphidae) demonstrate bilateral symmetry. The bilateral symmetry of Cnidaria is thought to be inherited from the common ancestors of both cnidarians and triploblastic bilaterians. The secondary radial symmetry of Cnidaria evidently is a result of the adaptation to the sessile mode of life. The presence of both the marginal and labial rings of tentacles is supposed to be a plesiomorphic character of Cnidaria. In some groups of cnidarians, one of the tentacle rings may be reduced.     

Embryogenesis of GABAergic elements in the nervous system of Eisenia fetida (Annelida, Oligochaeta)

The appearance and development of the GABA-immunoreactive nervous elements in the central nervous system of Eisenia fetida were studied by immunocytochemistry. The nervous system originates from the neuroectoderm situated on the ventral side of the embryo. The organization of the circumpharyngeal ring starts earlier than that of the ventral cord. In the elementary ring the first GABA-immunopositive neurons can be observed (E1 stage) around the mouth. Later the cell number gradually increases and parallel to this process the elementary ring is separeted into a superficial and a deeper portion. The brain and the subesophageal ganglion will be organized from the superficial ring, while the nervous elements of the deeper ring will give rise for the first GABA-immunoreactive elements of the stomatogastric nervous system. In the early stages of the embryogenesis the immunoreactive cells of the developing brain appear solitary, while from the stage E4 they gradually are observed in groups. According to their position, these cell groups are similar to those observed in the brain of the adult earthworms. During embryogenesis the level of the ventral cord ganglia depends on their position in the ectodermal germ bands. It means, that the more organized ganglia are near the circumpharyngeal ring, mean while less developed ganglia are situated caudally from them. By the end of the embryogenesis all ganglia of the ventral cord will be equally well organized. The nerve tracts of the ganglia are built up from contra- and ipsilateral by projected fibres. From E3 stage the medial tracts, mean while from the E4 stage the lateral tracts begin to be formed. During the next stages, more and more fibres connect to the both tracts. At hatching, the development of the central nervous system of Eisenia fetida is not completed, the process is continued during the postembryonic development.     

Ventral nerve cord in Phoronopsis harmeri larvae

The nervous system organization is considered a phylogenetically important character among metazoans. The phylum Phoronida is included in a supraphyletic taxon known as Lophotrochozoa. Many lophotrochozoans possess a metameric ventral nerve cord as adults or larvae. Phoronids do not exhibit external metamery either as larvae or as adults. The current study describes the ventral nerve cord in the young larva of Phoronopsis harmeri. This structure is apparent both in the serotonergic and FMRF-amidergic nervous system in young larvae. The ventral nerve cord extends from the mouth to the tentacular ridge. Both serotonergic and FMRF-amidergic components consist of two ventrolateral nerves, each with several unipolar neurons. The ventrolateral nerves connect to each other by means of thin repetitive transversal nerves ("commissures"). The abundance of neurons and nerves in the epidermis of the oral field of actinotrocha larva likely reflects the importance of this area in collection of food particles. The ventral nerve cords of the actinotrocha and the metatrochophore differ in their positions with respect to ciliated bands: the cord is located between the preoral and postoral ciliated bands in the actinotrocha but between the postoral ciliated band and telotroch in the metatrochophore. The presence of the ventral nerve cord, which contains repetitive elements (neurons and "commissures"), in the early development of P. harmeri may recapitulate some stages of nervous system development during phoronid phylogeny. The larval nervous system does not contain nervous centers under the tentacular ridge that can correlate with the catastrophic metamorphosis and unique body plan of phoronids.     

Embryology of the thoracic and abdominal ganglia of the large milkweed bug, Oncopeltus fasciatus (Dallas), (Hemiptera, Lygaeidae)

In this study, the condensation of the three thoracic and 11 abdominal segmental ganglia to form a prothoracic and central nerve mass during embryogenesis is described. During katatrepsis, many changes occur in the organization of these ganglia; this study suggests that some of these changes are caused by mechanical forces acting on the ventral nerve cord at this time. The ventral nerve cord begins its anterior migration and coalescence ten hours after katatrepsis and is completed 63 hours later. The central ganglion is made up of the meso- and metathoracic ganglia and seven abdominal ganglia. Intrasegmental median cord nuclei are shown to form glial elements in the median sagittal plane of the neuropile and in the longitudinal connectives. Intersegmental median cord neuroblasts migrate into the posterior gangliomeres but, apparently, degenerate soon after katatrepsis. Lateral cord cells bordering on the neuropile form a glial investment that surrounds this fiber tract region. Peripheral lateral cord cells are shown to form the cells of the outer ganglionic sheath, the perineurium.     

Neuroblast formation and patterning during early brain development in Drosophila

The Drosophila embryo provides a useful model system to study the mechanisms that lead to pattern and cell diversity in the central nervous system (CNS). The Drosophila CNS, which encompasses the brain and the ventral nerve cord, develops from a bilaterally symmetrical neuroectoderm, which gives rise to neural stem cells, called neuroblasts. The structure of the embryonic ventral nerve cord is relatively simple, consisting of a sequence of repeated segmental units (neuromeres), and the mechanisms controlling the formation and specification of the neuroblasts that form these neuromeres are quite well understood. Owing to the much higher complexity and hidden segmental organization of the brain, our understanding of its development is still rudimentary. Recent investigations on the expression and function of proneural genes, segmentation genes, dorsoventral-patterning genes and a number of other genes have provided new insight into the principles of neuroblast formation and patterning during embryonic development of the fly brain. Comparisons with the same processes in the trunk help us to understand what makes the brain different from the ventral nerve cord. Several parallels in early brain patterning between the fly and the vertebrate systems have become evident.     

Absence of sensory input does not affect persistent neurons in Tenebrio molitor metamorphosis (Insecta: Coleoptera)

Summary The ventral nerve cord of holometabolous insects is reorganized during metamorphosis. Certain elements, however, persist from the larval to the adult stage. A group of dorsal unpaired median neurons and a set of thoracic serotonin-immunoreactive interneurons are shown to be persistent elements in the ventral nerve cord of Tenebrio molitor. These persistent neurons retain their bauplan even after deprivation of the sensory neurons projecting onto them. These persistent neurons might provide a network into which newly developing sensory neurons fit during reorganization of the nervous tissue.     

Immunohistochemical and ultrastructural analysis of the muscular and nervous systems in the interstitial polychaete Polygordius appendiculatus (Annelida)

The systematic position of Polygordiidae is still under debate. They have been assigned to various positions among the polychaetes. Recent molecular analyses indicate that they might well be part of a basal radiation in Annelida, suggesting that certain morphological characters could represent primitive character traits adopted from the annelid stem species. To test this hypothesis, an investigation of the muscular and nervous systems by means of immunological staining and confocal laser scanning microscopy and transmission electron microscopy was conducted. With the exception of the brain, the nervous system is entirely basiepidermal and consists of the brain, the esophageal connectives, the subesophageal region, the ventral nerve cord and several smaller longitudinal nerves. These are connected by a considerable number of ring nerves in each segment. The ventral nerve cord is made up of closely apposed longitudinal neurite bundles, a median and two larger lateral ones. Since distinct ganglia are lacking, it represents a medullary cord. The muscular system mainly consists of longitudinal fibers, regularly distributed oblique muscles and strong septa. The longitudinal fibers form a right and a left unit separated along the dorsal midline, each divided into a dorsal and ventral part by the oblique muscles. Anteriorly, the longitudinal musculature passes the brain and terminates in the prostomium. There is no musculature in the palps. In contrast to earlier observations, regularly arranged minute circular muscle fibers are present. Very likely, a basiepithelial and non-ganglionic organization of the ventral nerve cord as well as an orthogonal nervous system represent plesiomorphic characters. The same applies for the predominance of longitudinal muscle fibers.     

The Larval Nervous System of Polygordius lacteus Scheinder, 1868 (Polygordiidae,Polychaeta): Immunocytochemical Data

The nervous system of the planktotrophic trochophore larva of Polygordius lacteus has been investigated using antibodies to serotonin (5-HT) and the neuropeptide FMRFamide. The apical ganglion contains three 5-HT-ir neurons, many FMRFamide-ir neurons and a tripartate 5-HT-ir and FMRFamide-ir neuropil. A lateral nerve extends from each side of the apical ganglion across the episphere and the ventral hyposphere, where the two nerves combine to form the paired ventral nerve cord. These nerves have both 5-HT-ir and FMRFamide-ir processes. Three circumferential nerves are associated with the ciliary bands: two prototroch and one metatroch nerve. All contain 5-HT-ir and FMRFamide-ir processes. An oral nerve plexus also contain both 5-HT-ir and FMRFamide-ir processes develops from the metatroch nerve, and an esophageal ring of FMRFamide-ir processes develops in later larval stages. In young stages the ventral ganglion contains two 5-HT-ir and two FMRFamide-ir perikarya; during development the ventral ganglion grows caudally and adds additional 5-HR-ir and FMRFamide-ir perikarya. These are the only perikarya that could be found along the lateral nerve and ventral nerve cord. The telotroch nerve develops from the ventral nerve cord. The 5-HT-ir and FMRFamide-ir part of the nervous system is strictly bilateral symmetric. and much of the system (i.e. apical ganglion, lateral nerves ventral nerve cord, dorsal nerve and oral plexus) is retained in the adult.     

Immunohistochemical localization of gastrin-cholecystokinin-like material in the central nervous system of the migratory locust

Brain, corpora cardiaca (CC)-corpora allata (CA) complex, suboesophageal ganglion, thoracic and abdominal ganglia of adults, larvae and embryos of Locusta migratoria have been immunohistochemically screened for gastrin cholecystokinin (CCK-8(s]-like material. In adult, numerous immunoreactive neurons and nerve fibres are located, with a marked symmetry, in various parts of the brain and throughout the ventral nerve cord. In the median part of the brain, cell bodies belonging neither to cellular type A1 nor A2 (following Victoria blue-paraldehyde fuchsin staining) are immunopositive; their processes terminate in the upper protocerebral neuropile. In lateral parts of the brain, external cell bodies send axons into CC and some up to CA, other internal have processes which terminate in the neuropile of the brain. Two of these latter cells react also with methionine-enkephalin antiserum. In the ventral nerve cord, in addition to numerous perikarya, immunoreactive arborizations terminate in the neuropile or in close association with the sheath, at the dorsal part of all ganglia. This CCK-8(s) distribution pattern is observed only at the two last larval instars, but is precociously detected in the abdominal nerve cord of embryos, one day before hatching.