The larval morphology of the marine bryozoan Bowerbankia gracilis (Ctenostomata: Vesicularioidea)

Summary The larval morphology of the marine bryozoan Bowerbankia gracilis has been investigated by light and electron microscopy. The barrel-shaped larva (200 m long and 150 m in diameter) is light yellow without any apparent eyespots, although it is positively phototactic during its brief free-swimming existence. The primary morphological characteristics of the larva are: (1) a large corona that forms most of the larval surface, (2) a small apical disc without blastemas, (3) a deep pallial sinus lined by an extensive pallial epithelium, (4) an internal sac without regional specializations, and (5) a polypide rudiment in the oral hemisphere. This organization is characteristic of larvae of the ctenostome superfamily Vesicularioidea, and differs radically from the organization of all other bryozoan larvae examined. The major morphological differences occur in the size and organization of the apical disc, the pallial epithelium, and the internal sac. In most bryozoans, these regions of the larval epithelium represent rudiments of the polypide and the body wall epidermis of the ancestrula. The oral polypide rudiment, the extensive pallial epithelium, and the reduced internal sac in vesicularioid larvae indicate that their pattern of metamorphosis also differs radically from the metamorphoses of other bryozoans.Figure Abbreviations AB aboral - acr axial ciliary rootlet - ad apical disc - anc aboral nerve cord - ANT anterior - arm apical retractor muscle - b basal body - bf basal foot process - c corona - cc ciliated cleft - ce centriole - ci cilium - cl cupiform layer of the polypide rudiment - cp ciliary pit - cr ciliary rootlet - enr equatorial neural ring - g glandular cells of the pyriform organ - gl glycocalyx - go Golgi complex - gr granule - hcr horizontal ciliary rootlet - ic intercoronal cell - igf inferior glandular field - ip infrapallial cells - is internal sac - jp juxtapapillary cells - l lipid droplets - L lateral - m mesenchyme - m Type I mesenchyme cell - m Type II mesenchyme cell - m Type III mesenchyme cell - mb median band of the polypide rudiment - mc marginal cells of the apical disc - mi mitochondria - mr microridge - mv microvilli - nn nerve nodule - np neural plate - nu nucleus - O oral - oce oral ciliated epithelium - op opening to the internal sac - ovc oral vesicular collarette - p papilla of the pyriform organ - pa pallial cell - pe pallial epithelium - po pyriform organ - POS posterior - pp parasagittal patches of undifferentiated cells - pr polypide rudiment - rer rough endoplasmic reticulum - sc supracoronal cells - sg secretory granules - sgf superior glandular field - sp suprapallial cells - tc terminal cone - tf transitional filaments - u undifferentiated cells - va vacuole - vc vesicular cell - wc wedge-shaped cells of the apical disc - y yolk granule - za zonula adhaerens Caption Abbreviations Gp Glutaraldehyde-phosphate - Os Osmium     

Anatomy of the serotonergic nervous system of an entoproct creeping-type larva and its phylogenetic implications

Abstract. Although the internal phyletic relationships of Spiralia (and Lophotrochozoa) remain unresolved, recent progress has been made due to molecular phylogenetic analyses as well as developmental studies of crucial taxa such as Mollusca, Sipuncula, or Annelida. Despite this progress, the phylogenetic position of a number of phyla, such as Entoprocta, remains problematic, mainly due to their unique morphology, their aberrant mode of development, and their exclusion in most large-scale phylogenetic analyses. In order to extend the morphological dataset of this enigmatic taxon, we herein describe the anatomy of the serotonergic nervous system of the creeping-type larva of Loxosomella murmanica . The apical organ is very complex and comprises six to eight centrally positioned flask cells and eight bipolar peripheral cells. In addition, a prototroch nerve ring, an anterior nerve loop, a paired buccal nerve, and an oral nerve ring are found. Moreover, the larva of L. murmanica has one pair of pedal and one pair of lateral longitudinal nerve cords and thus expresses a tetraneurous condition. Several paired serotonergic perikarya, which form contact with the pedal nerve cords but not with the lateral ones, are found along the anterior–posterior axis. The combination of a complex larval serotonergic apical organ and (adult) tetraneury, comprising one pair of ventral and one pair of more dorsally situated lateral longitudinal nerve cords without ganglia, has so far only been reported for basal molluscs and may be diagnostic for a mollusc–entoproct clade. In addition, the larva of Loxosomella expresses a mosaic of certain neural features that are also found in other larval or adult Spiralia, e.g., a prototroch nerve ring, an anterior nerve loop, and a buccal nervous system.     

Fine structural studies of the nervous system and the apical organ in the planula larva of the sea anemone Anthopleura elegantissima

The nervous system of the planula larva of Anthopleura elegantissima consists of an apical organ, one type of endodermal receptor cell, two types of ectodermal receptor cells, central neurons and nerve plexus. Both interneural and neuromuscular synapses are found in the nerve plexus. The apical organ is a collection of about 100 long, columnar cells each bearing a long cilium and a collar of about 10 microvilli. The cilia of the apical organ are twisted together to form an apical tuft. The ciliary rootlets of the apical organ cells are extremely long, reaching to the basal processes of the cells adjacent to the mesoglea. All three types of sensory cells are tall and slender in profile and are identified by the presence of one or more of the following features: microtubules, small vesicles, membrane-bound granules and synapses. The interneurons are bipolar cells with somas restricted to the aboral end, adjacent to the apical organ. All synapses observed are polarized or asymmetrical. A diagram including all the elements of the nervous system is presented and the possible functions of the nervous system are discussed in relation to larval behavior.     

Mechanisms of rapid morphogenetic movements in the metamorphosis of the bryozoan Bugula neritina (Cheilostomata,Cellularioidea): II. The role of dynamic assemblages of microfilaments in the pallial epithelium

The rapid morphogenetic movements that internalize the transitory larval epithelium and reorient the presumptive adult epidermis during the metamorphosis of the cellularioid cheilostome bryozoan, Bugula neritina, have been examined by light and electron microscopy and analyzed by experimentation with cytochalasin B (CB) and MgC1₂. The pallial epithelium is gradually drawn out over the aboral hemisphere as the larval ciliated epithelium (the corona and the pyriform organ) involutes. At the end of coronal involution the oral margin of the pallial epithelium constricts and the aboral hemisphere is pulled down against the everted sac. Ultrastructural and experimental evidence indicates that an equatorial contractile ring composed of a temporal alignment of CB-sensitive 5.5 nm microfilaments is responsible for the constriction of the oral margin of the pallial epithelium. This morphogenetic movement, in conjunction with the compression of the aboral hemisphere, juxtaposes the pallial epithelium with the oral epithelium of the everted sac. The pallial epithelium adheres to the neck and wall regions of the everted sac and begins a progressive contraction at its aboral margin, pulling the wall epithelium up over the aboral hemisphere. Ultrastructural examination reveals that the pallial cells contain apical bands of microfilaments and associated vesicles at this stage of metamorphosis. The position and time of appearance of the microfilaments in the pallial epithelium support the hypothesis that they generate the force for wall elevation. Histological and experimental data indicate that the compression of the aboral hemisphere at the umbrella stage and the final retraction of the apical disc are muscle-mediated morphogenetic movements. The constriction of the umbrellar margin and the elevation of the wall epithelium, on the other hand, appear to be caused by two distinct populations of microfilaments that assemble in different regions of the pallial epithelium at specific times during metamorphosis.     

Fine structure of the doliolaria larva of the feather star Florometra serratissima (Echinodermata: Crinoidea), with special emphasis on the nervous system

The epidermis of the doliolaria larva of the Florometra serratissima is differentiated into distinct structures including an apical organ, adhesive pit, ganglion, ciliary bands, nerve plexus, and vestibular invagination. All these structures possess unique cell-types, suggesting that they are functionally specialized in the larva, except the vestibular invagination that becomes the postmetamorphic stomodeum. The epidermis also contains yellow cells, amoeboid-like cells, and secretory cells. The enteric sac, hydrocoel, axocoel, and somatocoels have differentiated but are probably not functional in the doliolaria stage. Mesenchymal cells, around the enteric sac and coeloms, appear to be actively secreting the endoskeleton and connective tissue fibers. The nervous system is composed of a nerve plexus, ganglion, and sensory receptor cells in the apical organ. The apical organ is a larval specialization of the anterior end; the ganglion is located in the base of the epidermis at the anterior dorsal end of the larva. The nerve plexus underlies most of the epidermis, although it is more prominent in the anterior region. Here, processes from sensory receptor cells of the apical organ, as well as those from nerve cells, contribute to the plexus. These processes contain one or a combination of organelles including vesicles, vacuoles, microtubules, and mitochondria. The configuration of glyoxylic acid-induced fluorescence, revealing catecholamine activity, correlates to the apical organ, nerve cells, and nerve plexus. Morphological evidence suggests that the nervous system may function in initiation and control of settlement, attachment, and metamorphosis. The crinoid larval nervous system is discussed and compared to that found in other larval echinoderms.     

Embryonic and post-embryonic development of the polyclad flatworm Maritigrella crozieri; implications for the evolution of spiralian life history traits

Background

Planktonic life history stages of spiralians share some muscular, nervous and ciliary system characters in common. The distribution of these characters is patchy and can be interpreted either as the result of convergent evolution, or as the retention of primitive spiralian larval features. To understand the evolution of these characters adequate taxon sampling across the Spiralia is necessary. Polyclad flatworms are the only free-living Platyhelminthes that exhibit a continuum of developmental modes, with direct development at one extreme, and indirect development via a trochophore-like larval stage at the other. Here I present embryological and larval anatomical data from the indirect developing polyclad Maritrigrella crozieri, and consider these data within a comparative spiralian context.

Results

After 196 h hours of embryonic development, M. crozieri hatches as a swimming, planktotrophic larva. Larval myoanatomy consists of an orthogonal grid of circular and longitudinal body wall muscles plus parenchymal muscles. Diagonal body wall muscles develop over the planktonic period. Larval neuroanatomy consists of an apical plate, neuropile, paired nerve cords, a peri-oral nerve ring, a medial nerve, a ciliary band nerve net and putative ciliary photoreceptors. Apical neural elements develop first followed by posterior perikarya and later pharyngeal neural elements. The ciliated larva is encircled by a continuous, pre-oral band of longer cilia, which follows the distal margins of the lobes; it also possesses distinct apical and caudal cilia.

Conclusions

Within polyclads heterochronic shifts in the development of diagonal bodywall and pharyngeal muscles are correlated with life history strategies and feeding requirements. In contrast to many spiralians, M. crozieri hatch with well developed nervous and muscular systems. Comparisons of the ciliary bands and apical organs amongst spiralian planktonic life-stages reveal differences; M. crozieri lack a distinct ciliary band muscle and flask-shaped epidermal serotonergic cells of the apical organ. Based on current phylogenies, the distribution of ciliary bands and apical organs between polyclads and other spiralians is not congruent with a hypothesis of homology. However, some similarities exist, and this study sets an anatomical framework from which to investigate cellular and molecular mechanisms that will help to distinguish between parallelism, convergence and homology of these features.     

New insights on ctenophore neural anatomy: immunofluorescence study in Pleurobrachia pileus (Müller, 1776)

Ctenophores are non-bilaterian animals sharing with cnidarians and bilaterians the presence of sensory receptors, nerve cells, and synapses, absent in placozoans and sponges. Although recent immunofluorescence studies have renewed our knowledge of cnidarian neuro-anatomy, ctenophores have been much less investigated despite their importance to understanding the origin and early evolution of the nervous system. In this study, the neuro-anatomy of the ctenophore Pleurobrachia pileus (Müller, 1776) was explored by whole-mount fluorescent antibody staining using antibodies against tyrosylated -tubulin, FMRFamide, and vasopressin. We describe the morphology of nerve nets and their local specializations, and the organization of the aboral neuro-sensory complex comprising the apical organ and polar fields. Two distinct nerve nets are distinguished: a mesogleal nerve net, loosely organized throughout body mesoglea, and a much more compact “nerve net” with polygonal meshes in the ectodermal epithelium. The latter is organized as a plexus of short nerve cords. This epithelial nervous system contains distinct sub-populations of dispersed FMRFamide and vasopressin immunoreactive nerve cells. In the aboral neuro-sensory complex, our most significant observations include specialized nerve nets underlying the apical organ and polar fields, a tangential bundle of actin-rich fibers (interpreted as a muscle) within the polar fields, and distinct groups of neurons labeled by anti-FMRFamide and anti-vasopressin antibodies, within the apical organ floor. These results are discussed in a comparative perspective.     

Attachment and metamorphosis of the cheilo-ctenostome bryozoan bugula neritina (linné)

The structure, attachment and subsequent metamorphosis of larvae of the marine bryozoan Bugula neritina were studied by light and electron microscopy. Two points of larval anatomy are of special significance to proper interpretation of the metamorphosis:

• 1 Two cytologically similar blastemal tissues, each laden with free ribosomes, occur as parts of the apical organ complex. The upper blastema directly contacts the larval surface, forming the non-ciliated rows of the apical organ. The lower blastema is internal and is oral to and contiguous with the upper blastema.
• 2 The epidermal tissues of the larva are joined in the following sequence, beginning at the aboral pole: a. apical organ complex; b. apical-connecting cell; c. infolded pallial sinus epithelium; d. vesicular-connecting cell; e. aboral vesicular epithelium; f. corona; g. oral vesicular epithelium; and i., j., and k. internal sac neck, wall and roof regions.

The initial stages of metamorphosis involve a complex sequence of morphogenetic movements, including:

• 1 eversion of the internal sac, permanently attaching the larva to the substrate;
• 2 inrolling of the aboral vesicular epithelium, corona, oral vesicular and ciliated epithelia, and neck region of the internal sac into the larval interior; concomitantly the pallial sinus epithelium evaginates;
• 3 loss of connection between the invaginated tissues and the surface;
• 4 fusion of the pallial sinus epithelium with the wall region of the internal sac, maintaining the integrity of the body surface;
• 5 retraction of the apical organ complex and invagination of the pallial sinus epithelium with the simultaneous elevation of the internal sac wall region to the aboral pole.

At the conclusion of these events the preancestrular surface is covered by the wall and roof regions of the internal sac. Cells of the wall region form the epidermis of the body wall except for the attachment disc and secrete a cuticular exoskeleton that is secondarily calcified; the attachment disc is formed by the roof region of the internal sac. Internally, the ectodermal upper blastema differentiates into the lophophore and digestive tract of the ancestrular polypide, while the lower blastema forms the lining of the lophophoral coelom and the splanchnic (but not the somatic) lining of the visceral coelom. The visceral somatic peritoneum is formed from cells that may originate from the mesodermally derived pigmented cells of the larva to which they are similar in pigmentation and cytology. Such a composite derivation of a coelomic lining has not been described previously.     

Nervous and muscle system development in <Emphasis Type="Italic">Phascolion strombus</Emphasis> (Sipuncula)

Recent interpretations of developmental gene expression patterns propose that the last common metazoan ancestor was segmented, although most animal phyla show no obvious signs of segmentation. Developmental studies of non-model system trochozoan taxa may shed light on this hypothesis by assessing possible cryptic segmentation patterns. In this paper, we present the first immunocytochemical data on the ontogeny of the nervous system and the musculature in the sipunculan Phascolion strombus. Myogenesis of the first anlagen of the body wall ring muscles occurs synchronously and not subsequently from anterior to posterior as in segmented spiralian taxa (i.e. annelids). The number of ring muscles remains constant during the initial stages of body axis elongation. In the anterior-posteriorly elongated larva, newly formed ring muscles originate along the entire body axis between existing myocytes, indicating that repeated muscle bands do not form from a posterior growth zone. During neurogenesis, the Phascolion larva expresses a non-metameric, paired, ventral nerve cord that fuses in the mid-body region in the late-stage elongated larva. Contrary to other trochozoans, Phascolion lacks any larval serotonergic structures. However, two to three FMRFamide-positive cells are found in the apical organ. In addition, late larvae show commissure-like neurones interconnecting the two ventral nerve cords, while early juveniles exhibit a third, medially placed FMRFamidergic ventral nerve. Although we did not find any indications for cryptic segmentation, certain neuro-developmental traits in Phascolion resemble the conditions found in polychaetes (including echiurans) and myzostomids and support a close relationship of Sipuncula and Annelida.     

Apical organs in echinoderm larvae: insights into larval evolution in the Ambulacraria

The anatomy and cellular organization of serotonergic neurons in the echinoderm apical organ exhibits class-specific features in dipleurula-type (auricularia, bipinnaria) and pluteus-type (ophiopluteus, echinopluteus) larvae. The apical organ forms in association with anterior ciliary structures. Apical organs in dipleurula-type larvae are more similar to each other than to those in either of the pluteus forms. In asteroid bipinnaria and holothuroid auricularia the apical organ spans ciliary band sectors that traverse the anterior-most end of the larvae. The asteroid apical organ also has prominent bilateral ganglia that connect with an apical network of neurites. The simple apical organ of the auricularia is similar to that in the hemichordate tornaria larva. Apical organs in pluteus forms differ markedly. The echinopluteus apical organ is a single structure on the oral hood between the larval arms comprised of two groups of cells joined by a commissure and its cell bodies do not reside in the ciliary band. Ophioplutei have a pair of lateral ganglia associated with the ciliary band of larval arms that may be the ophiuroid apical organ. Comparative anatomy of the serotonergic nervous systems in the dipleurula-type larvae of the Ambulacraria (Echinodermata+Hemichordata) suggests that the apical organ of this deuterostome clade originated as a simple bilaterally symmetric nerve plexus spanning ciliary band sectors at the anterior end of the larva. From this structure, the apical organ has been independently modified in association with the evolution of class-specific larval forms.     

Larval and adult brains

Apical organs are a well-known structure in almost all ciliated eumetazoan larvae, although their function is poorly known. A review of the literature indicates that this small ganglion is the "brain" of the early larva, and it seems probable that it represents the brain of the ancestral, holopelagic ancestor of all eumetazoans, the gastraea. This early brain is lost before or at metamorphosis in all groups. Protostomes (excluding phoronids and brachiopods) appear to have brains of dual origin. Their larvae develop a pair of cephalic ganglia at the episphere lateral to the apical organ, and these two ganglia become an important part of the adult brain. The episphere and the cerebral ganglia show Otx expression, whereas Hox gene expression has not been seen in this part of the brain. A ventral nervous system develops around the blastopore, which becomes divided into mouth and anus by fusion of the lateral blastopore lips. The circumblastoporal nerve ring becomes differentiated into a nerve ring around the mouth, becoming part of the adult brain, a pair of ventral nerve cords, in some cases differentiated into a chain of ganglia, and a ring around the anus. This part of the nervous system appears to be homologous with the oral nerve ring of cnidarians. This interpretation is supported by the expression of Hox genes around the cnidarian mouth and in the ventral nervous system of the protostomes. The development of phoronids, brachiopods, echinoderms, and enteropneusts does not lead to the formation of an episphere or to differentiation of cerebral ganglia. In general, a well-defined brain is lacking, and Hox genes are generally not expressed in the larval organs, although this has not been well studied.     

The development of the serotonergic and FMRF-amidergic nervous system in<Emphasis Type="Italic"> Antalis entalis</Emphasis> (Mollusca,Scaphopoda)

The morphogenesis of serotonin- and FMRF-amide-bearing neuronal elements in the scaphopod Antalis entalis was investigated by means of antibody staining and confocal laser scanning microscopy. Nervous system development starts with the establishment of two initial, flask-like, serotonergic central cells of the larval apical organ. Slightly later, the apical organ contains four serotonergic central cells which are interconnected with two lateral serotonergic cells via lateral nerve projections. At the same time the anlage of the adult FMRF-amide-positive cerebral nervous system starts at the base of the apical organ. Subsequently, the entire neuronal complex migrates behind the prototroch and the six larval serotonergic cells lose transmitter expression prior to metamorphic competence. There are no strictly larval FMRF-amide-positive neuronal structures. The development of major adult FMRF-amide-containing components such as the cerebral system, the visceral loop, and the buccal nerve cords, however, starts before the onset of metamorphosis. The anlage of the putative cerebral system is the only site of adult serotonin expression in Antalis larvae. Establishment of the adult FMRF-amidergic and serotonergic neuronal bauplan proceeds rapidly after metamorphosis. Neurogenesis reflects the general observation that the larval phase and the expression of distinct larval morphological features are less pronounced in Scaphopoda than in Gastropoda or Bivalvia. The degeneration of the entire larval apical organ before metamorphic competence argues against an involvement of this sensory system in scaphopod metamorphosis. The lack of data on the neurogenesis in the aplacophoran taxa prevent a final conclusion regarding the plesiomorphic condition in the Mollusca. Nevertheless, the results presented herein shed doubts on general theories regarding possible functions of larval "apical organs" of Lophotrochozoa or even Metazoa.     

Peptidergic and serotonergic immunoreactivity in the metamorphosing ophiopluteus of Ophiactis resiliens (Echinodermata, Ophiuroidea)

Abstract. Antibodies against the echinoderm-specific neuropeptide S1 and against 5HT were used to examine the fate of the larval nervous system during metamorphosis in the ophiuroid Ophiactis resiliens . In contrast to most echinoderms, the onset of peptidergic and serotonergic expression was delayed to the advanced ophiopluteus stage, in particular for 5HT. In advanced ophioplutei, peptidergic immunoreactivity was located in simple fibres associated with the ciliated bands, a stomach nerve ring, and cells along the antero-lateral arms. 5HT immunoreactivity was concentrated in 2 oral ganglia in the adoral projections, located at the posterior rim of the mouth. Clusters of 5HT-positive cells were also found along the antero-lateral arms. The ophiopluteus lacked a serotonergic (or peptidergic) anterior ganglion. In echinoids, holothuroids, and crinoids, anterior ganglia are thought to have a sensory role in settlement and metamorphosis. Given that ophioplutei metamorphose in the plankton and that larval structures degenerate before settlement, the absence of apical ganglia correlates with the lack of a functional role for larval structures in substrate selection and settlement. Although most of the larval nervous system degenerated during metamorphosis, the adoral projections and associated oral ganglia appeared to be incorporated into the juvenile mouth, suggesting a potential role for larval neurons in contributing to oral neuronal structures in the adult. S1-positive neurons and fibres in the rudiment developed de novo and in parallel with development of the epineural canal. This structure gives rise to the primordia of the adult circumoral nerve ring and radial nerves, indicating that differentiation of the adult nervous system begins in the early stages of metamorphosis.     

On the serotonergic nervous system of two planktonic rotifers, Conochilus coenobasis and C. dossuarius (Monogononta, Flosculariacea, Conochilidae)

The serotonergic nervous systems of two non-colonial species of Conochilus were examined to obtain the first immunohistochemical insights into the neuroanatomy of species of Flosculariacea (Rotifera, Monogononta). Species of Conochilus, subgenus Conochiloides, were examined using serotonin (5-HT) immunohistochemistry, epifluorescence and confocal laser scanning microscopy, and 3D computer imaging software. In specimens of C. coenobasis and C. dossuarius, the serotonergic nervous system is defined by a dorsal cerebral ganglion, apically directed cerebral neurites, and paired nerve cords. The cerebral ganglion contains approximately four pairs of small 5-HT-immunoreactive perikarya; one pair innervates the posterior nerve cords and three pairs innervate the apical field. The most dorsal pair innervates a coronal nerve ring that encircles the apical field. Within the apical field is a second nerve ring that outlines the inner border of the coronal cilia. Together, both the inner and outer nerve rings may function to modulate ciliary activity of the corona. The other two pairs of perikarya innervate a region around the mouth. Specific differences in the distribution of serotonergic neurons between species of Conochilus and previously examined ploimate rotifers include the following: (a) a lack of immunoreactivity in the mastax; (b) a greater number of apically directed serotonergic neurites; and (c) a complete innervation of the corona in both species of Conochilus. These differences in nervous system immunohistochemistry are discussed in reference to the phylogeny of the Monogononta.     

Evolutionary and structural diversification of the larval nervous system among marine bryozoans

Regardless of the morphological divergence among larval forms of marine bryozoans, the larval nervous system and its major effector organs (musculature and ciliary fields) are largely molded on the basis of functional demands of feeding, ciliary propulsion, phototactic behaviors, and substrate exploration. Previously published ultrastructural information and immunohistochemical reconstructions presented here indicate that neuronal pathways are largely ipsilateral, with more complex synaptic connections localized within the nerve nodule. Multiciliated sensory-motor neurons diversify structurally and functionally on the basis of their position along the axis of swimming largely due to the functional demands of photoklinotaxis and substrate exploration. Vesiculariform, buguliform, and ascophoran coronate larvae all have patches of sensory neurons bordering the pyriform organ's ciliated groove (juxtapapillary cells and border cells) that are active during substrate selection. Despite their simplified form, cyclostome larvae maintain swimming and probing behaviors with sensory-motor systems functionally similar to those of some parenchymella and planula larval types. Considering the evolutionary relationships among the morphological grades of marine bryozoans, particular lineages within the gymnolaemates have independently evolved larval traits that convey a greater range of sensory abilities and increased propulsive capacity. The larval nervous system of bryozoans may be evolutionarily derived from the pretrochal region of a trochophore-like larval form.     

Structure of the nervous system in the tornaria larva of<Emphasis Type="Italic"> Balanoglossus proterogonius</Emphasis> (Hemichordata: Enteropneusta) and its phylogenetic implications

The tornaria larva of hemichordates occupies a central position in phylogenetic discussions on the relationships between Echinodermata, Hemichordata, and Chordata. Dipleurula-type larvae (tornaria and echinoderm larvae) are considered to be primary in the life cycle and thus provide a model for the ancestral animal common to all three taxa (the theory of W. Garstang). If the similarities between tornaria and the larvae in Echinodermata result from homology, their nervous systems should be basically similar as well. The present study utilizes anti-serotonin and FMRFamide antisera together with laser scanning microscopy, and transmission electron microscopy, to describe in detail the nervous system of the tornaria of Balanoglossus proterogonius. Serotonin immunoreactive neurons were found in the apical and esophageal ganglia, and in the stomach epithelium. FMRFamide immunoreactive neurons, probably sensory in nature, were detected in the apical ganglion and in the equatorial region of the stomach epithelium. At the ultrastructural level, the apical organ consists of a columnar epithelium of monociliated cells and includes a pair of symmetrical eyespots. The apical ganglion is located at its base and has a well-developed neuropil. Different types of neurons are described in the apical organ, esophagus, and stomach. Comparison with larvae in Echinodermata shows several significant differences in the way the larval nervous system is organized. This calls into question the homology between tornariae and echinoderm larvae. The possibility of convergence between the two larval types is discussed.     

Serotonergic and FMRFamidergic nervous systems in gymnolaemate bryozoan larvae

A growing body of data from nervous systems of marine invertebrate larvae provides an ideal background for comparisons among higher taxa. The currently available data from Bryozoa, however, do not allow for a consistent hypothesis of an ancestral state for this taxon, which would be necessary for phylogenetic inferences. The larval nervous systems of the four gymnolaemate species Flustrellidra hispida, Bugula fulva, Alcyonidium gelatinosum, and Bowerbankia gracilis are examined by means of antibody staining against the neurotransmitters serotonin and FMRFamide, as well as against acetylated α-tubulin. Despite considerable variation, a comparison reveals a common pattern of the distribution of serotonin. The neurotransmitter is found in at least two cells in the apical organ as well as in paired axial and lateral nerves emerging from a central nerve nodule. A ring nerve is present below the corona and at least two serotonergic cells are found between the corona cells. Serotonergic coronal cells might represent unique bryozoan features, whereas the remaining elements show resemblance to the situation found in most spiralian taxa. The data do not provide support for a closer relationship of Bryozoa to Phoronida or Brachiopoda.     

腔肠动物平衡感觉器官结构的研究进展

本文概述了腔肠动物的平衡感觉器官结构：刺胞动物掌状伞形螅（Corymorpha palma）固着器末端的无纤毛平衡囊、软水母（Leptomedusae）钟形伞缘的开放型和封闭型平衡囊、筐水母（Narcomedusae）外伞表面外伞神经环上方边缘有感觉棍的间囊水母（Aegina）和有感觉乳突及感觉棍的嗜阳水母（Solmissus）的平衡囊、硬水母（Trachymedusae）外伞神经环上方边缘的平衡囊、钵水母（Scyphozoa）伞缘的感觉棍和立方水母（Cubozoa）伞缘稍上方的感觉棍,栉水母（Ctenophore）反口面中央的平衡囊或顶器官。本文内容对理解其他水生无脊椎动物的平衡感觉器官的结构及功能有重要意义,同时也可能作为对现行动物学相关教材内容的有益补充。     

Early development, pattern, and reorganization of the planula nervous system in Aurelia (Cnidaria, Scyphozoa)

We examined the development of the nervous system in Aurelia (Cnidaria, Scyphozoa) from the early planula to the polyp stage using confocal and transmission electron microscopy. Fluorescently labeled anti-FMRFamide, antitaurine, and antityrosinated tubulin antibodies were used to visualize the nervous system. The first detectable FMRFamide-like immunoreactivity occurs in a narrow circumferential belt toward the anterior/aboral end of the ectoderm in the early planula. As the planula matures, the FMRFamide-immunoreactive cells send horizontal processes (i.e., neurites) basally along the longitudinal axis. Neurites extend both anteriorly/aborally and posteriorly/orally, but the preference is for anterior neurite extension, and neurites converge to form a plexus at the aboral/anterior end at the base of the ectoderm. In the mature planula, a subset of cells in the apical organ at the anterior/aboral pole begins to show FMRFamide-like and taurine-like immunoreactivity, suggesting a sensory function of the apical organ. During metamorphosis, FMRFamide-like immunoreactivity diminishes in the ectoderm but begins to occur in the degenerating primary endoderm, indicating that degenerating FMRFamide-immunoreactive neurons are taken up by the primary endoderm. FMRFamide-like expression reappears in the ectoderm of the oral disc and the tentacle anlagen of the growing polyp, indicating metamorphosis-associated restructuring of the nervous system. These observations are discussed in the context of metazoan nervous system evolution.