Comparative ultrastructure of the pharynx simplex in turbellaria

Summary The simple pharynges in thirteen species of Turbellaria in the orders Macrostomida, Haplopharyngida, Catenulida, and Acoela have been studied by electron microscopy. After consideration of the functional aspects of the pharynx simplex, the relationship of the pharynx simplex ultrastructure to the phylogeny of the above mentioned groups is analyzed.The Haplopharyngida and Macrostomida are united as a group by the following characters: a pharynx transition zone of 1–5 circles of insunk cells with modified ciliary rootlets or no cilia, pharynx sensory cells without stereocilia collars and with a variable number of cilia, a prominent nerve ring with more than 30 axons circling the pharynx at the level of the beginning of the pharynx proper distal to the gland ring, 2 or more gland cell types in the pharynx, with at least two layers of muscle present and the longitudinal muscles derived from regular and special body wall circular muscles and a prominent post-oral nerve commissure. This specific arrangement can be distinguished from the other pharynx simplex types and is called the pharynx simplex coronatus.The catenulid pharynx simplex is characterized by the lack of a prominent nerve ring, no prominent post-oral commissure, a transition zone with epidermal type ciliary rootlets, recessed monociliated sensory cells, and one or no type of pharynx gland cell. The Acoela are specialized because of the epidermal type rootlets in the pharynx proper. They also lack a transition zone and a prominent nerve ring and have monociliated sensory cells different from the catenulid type.Ultrastructural characters of the pharynx simplex support the view that the Haplopharyngida-Macrostomida are monophyletic. The more primitive catenulid pharynx probably arose from a common ancestral pool with the Haplopharyngida and Macrostomida, although it does not appear possible presently to establish a clear monophyletic line for these forms. The various pharynx types within the Acoela appear to indicate independent origins with no clear link to the basic pharynx simplex type in the three other orders.Abbreviations Used in Figures a nerve axon - ar accessory rootlet - bb basal body - bn brain-nerve ring commissure - c caudal rootlet - ce centriole - ci cilium - cm circular muscle - cp ciliary pit - cu cuticle - cw cell web - d dictyosome - dp proximal pharynx proper cell - e epidermis - er rough endoplasmic reticulum - f fibrous rod - g gastrodermis - gc gastrodermal gland cell - he heterochromatin - i intercellular matrix - lc lateral nerve cord - lm longitudinal muscle - m mitochondria - mo mouth - mt microtubules - mv microvilli - n nucleus - nr nerve ring - ns neurosecretory granules - p pharynx proper - ph pharynx - po post-oral commissure - r rostral rootlet - rm radial muscle - s sphincter - sc sensory cell - sj septate junction - sr sensory rootlet - t transition zone - u ultrarhabdite - v vertical rootlet - va food vacuole - za zonula adhaerens - 1 type I gland cell - 2 type II gland cell - 3 type III gland cell - 4 type IV gland cell - 5 type V gland cell - 6 type VI gland cell - 7 type VII gland cell     

Comparative ultrastructure of the epidermal ciliary rootlets and associated structures in species of the Nemertodermatida and Acoela (Plathelminthes)

 The fine morphology of epidermal ciliary structures in four species of the Nemertodermatida and four species of the Acoela was studied, with emphasis on Meara stichopi (Nemertodermatida). The cilium of M. stichopi has a distal shelf and is proximally separated from the basal body by a cup-shaped structure. The bottom of the cup consists of a bilayered dense plate, or basal plate. The basal body consists of peripheral microtubule doublets continuous with those of the cilium. In the upper part of the basal body, the doublets are set at an angle and are anchored to the enclosing cell membrane by Y-shaped structures. The lower part of the basal body tapers eventually. The striated main rootlet arises on the anterior face of the basal body, initially like a flattened strap, and continues along the basal body shaped as a tube which further down becomes solid. The hour-glass-shaped posterior rootlet arises on the posterior face of the basal body. Contrary to the main rootlet, the striations in the proximal part of the posterior rootlet run parallel to the microtubule doublets of the basal body. A pair of microtubule bundles lead from the posterior rootlet to the two main rootlets in the hind ciliary row, and follow these to their lower tip. In the other species of the Nemertodermatida studied, the structure of the ciliary basal body and the ciliary rootlets is similar to that of M. stichopi. Structural differences in the species of the Acoela are that the lowermost end of the basal body is narrow and bent forwards, the proximal part of the main rootlet is trough-shaped, the main rootlet is accompanied by a pair of lateral rootlets and the posterior rootlet with associated microtubule bundles is thin. The epidermal ciliary structures in species of the Nemertodermatida and Acoela have a number of shared characters which are unique within the Plathelminthes. However, almost all of these characters are found in Xenoturbella bocki (Xenoturbellida), and some even in species of other ”phyla” of the ”lower” Metazoa. Hence, these characters cannot be considered apomorphic for the Acoelomorpha. A character seemingly present only in species of the Nemertodermatida and Acoela is the bilayered dense plate. This feature might represent an autapomorphic character state for the Acoelomorpha. Accepted: 7 March 1997     

Ultrastructure of the nerve cells and sensilla of Geocentrophora baltica (Platyhelminthes,Lecithoepitheliata) and the surface sensilla in the Geocentrophora group

Two types of nerve cell could be distinguished ultrastructurally in the central nervous system of Geocentrophora baltica (Prorhynchida, Lecithoepitheliata). Both show invaginations in the plasma membrane, but they differ in the character of the cytoplasm (light or densely stained) and the distribution of the neuronal vesicles (evenly or in groups). Different kinds of vesicles and neuronal release sites are observed. Special features of the synapses are pronounced local thickenings of the presynaptic membrane connected to paramembranous densities. In G. baltica and five endemic Geocentrophora spp. from Lake Baikal six types of surface sensillum were observed at the epidermal surface: 1. those with a long thin rootlet; 2. a short, balloon-shaped cilium with an aberrant axoneme and a reduced rootlet; 3. a rootlet branching into many striated bundles; 4. a thick rootlet; 5. a reduced rootlet and numerous neurotubules;and 6. collared sensilla each with one cilium in a deep pit surrounded by a collar of 11 to 12 microvilli. The variable number of microvilli in the collared sensillum is considered plesiomorphic relative to the stable number of eight microvilli known in sensilla of the Prolecithophora, Proseriata, and Rhabdocoela. The ultrastructure of the collar sensillum indicates that the Lecithoepitheliata is only distantly related to the Prolecithophora and higher turbellarians.     

Structure and evolution of the pharynx simplex in acoel flatworms (Acoela)

The homology of pharynges within the mostly pharynx‐less Acoela has been a matter of discussion for decades and even the basic question of whether a pharynx is a primitive trait within the Acoela and homologous to the pharynx of platyhelminth turbellarians is open. By using fluorescence staining of musculature, as well as conventional histological techniques and transmission electron microscopy, the present study sets focus on the mouth and pharynx (where present) of seven species of Acoela within Paratomellidae, Solenofilomorphidae, Hofsteniidae, Proporidae, and Convolutidae, as well as one species of Nemertodermatida and Catenulida, respectively. It is shown that among the investigated families of acoels there is a great variability in muscle systems associated with the mouth and pharynx and that pharynx histology and ultrastructural characters are widely diverse. There are no close similarities between the acoel pharynges and the catenulid pharynx but there is a general resemblance of the musculature associated with the mouth in the representatives of Paratomellidae and Nemertodermatida. On the basis of the profound differences in pharynx morphology, three major conclusions are drawn: 1) the pharynges as present in Recent acoels are not homologous to the pharynx simplex characteristic for Catenulida and Macrostomida within the Platyhelminthes; 2) the different muscular pharynx types of acoels are not homologous between higher taxa and thus a single acoel‐type pharynx simplex cannot be defined; 3) the presence of a muscular pharynx most likely does not represent the ancestral state. J. Morphol, 2009. © 2008 Wiley‐Liss, Inc.     

Ultrastructure of the sense receptors of adult Multicotyle purvisi (Trematoda, Aspidogastrea)

The following presumptive sense receptors of adult Multicotyle purvisi from the intestine of freshwater turtles in Malaya are described by transmission electron microscopy: disc-like receptor with many electron-dense collars and modified ciliary rootlet forming a 'disc'; non-ciliate receptor with long rootlet; non-ciliate receptor with branching rootlet and dense mass of irregularly arranged microtubules; non-ciliate receptor with rootlet fanning out from basal body, cross-striated in its upper and with electron-dense structures in its lower part; uniciliate receptor with thick layer of cytoplasm around axoneme; receptor with short cilium, at base of deep invagination of tegument; receptor with short cilium terminating in an electron-dense apical cap; and uniciliate receptor with long cilium. In addition, there may be a small non-ciliate receptor with a long ciliary rootlet at the base of the thick dorsal tegument, and uniciliate receptors differing from the uniciliate receptor with long cilium in the number of electron-dense collars and the length of the cilium and ciliary rootlet. Implications of the findings for the phylogeny of the parasitic Platyhelminthes and for evolutionary trends within that group arc discussed. The considerable degree of divergence of receptor types between the species of one family is attributed to the archaic nature of the group.     

Correlation of fluorescence and electron microscopy of F-actin-containing sensory cells in the epidermis of Convoluta pulchra (Platyhelminthes: Acoela)

Phalloidin‐stained whole mounts of acoel turbellarians show brightly fluorescing club‐shaped structures distributed over the epidermis and concentrated especially at the anterior and posterior tips of the body. By correlating electron micrographic images and fluorescence images of Convoluta pulchra, these structures can be seen to be sensory receptors with a central cilium surrounded by a collar of microvilli. The other candidate for showing fluorescence in the epidermis, namely gland necks, can be ruled out since their distribution is too dense to resemble the distribution of the fluorescent structures seen here. The collared sensory receptors were inserted between epidermal cells, and each bore a central cilium surrounded by a collar of 6–18 microvilli and an additional centrally positioned 2–7 microvilli of which 2 or 3 were associated with a modified rootlet called the swallow’s nest. Confocal scanning laser microscopy resolved the core of actin filaments within the microvilli of the collar and their rootlet‐like connections to the base of the sensory cell. Such receptors could also be identified by fluorescence microscopy in several other species of acoel turbellarians.     

Subsurface sense receptors in the larva of Austramphilina elongata Johnston, 1931 (Amphilinidea)

Summary The ultrastructure of tegumental and subtegumental receptors in the larva of Austramphilina elongata is described. The receptors are terminal swellings of dendrites and contain numerous small vesicles and neurofilaments which are predominantly peripheral. Tegumental receptors, together with a sheath consisting of basal lamina and tegument, project into the epidermis, and cross-striated rootlets were sometimes found in them. Subtegumental receptors lie below the tegument and ciliary rootlets were never observed in them. Anterior dendrites contain single centrioles and clusters of centrioles. The possible function of receptors and centrioles is discussed.Abbreviations in figures bl basal lamina - c centriole - d dendrite - ep epidermis - m microvillus - nt neurotubules - r rootlet of cilium - re receptor - st subtegumental receptor - t tegument     

Topography and ultrastructure of the tegument of Lecithochirium musculus (digenea: Hemiuridae), a parasite of the European eel Anguilla anguilla (osteichthyes: Anguillidae)

The tegumental ultrastructure of the stomach fluke Lecithochirium musculus was studied using scanning and transmission electron microscopy. The surface of the tegument was smooth and covered by transverse cytoplasmic ridges. Cobblestone‐like units of the tegument were observed on the ventral surface. Invagination and evagination of the ecsoma induced variations in the tegumental surface. The ultrastructural study revealed that the tegument of L. musculus had a typical syncytial organization with a distal cytoplasm lying over a basal matrix and cytons. Two types of intra‐tegumental sensory structures were observed. Type 1 sensory receptor was a domed‐like fusiform structure consisting of a smooth elevation of the tegument. Four receptors of this type were observed on the anterior dorsal surface of the fluke. Three nerve bulbs filled with electron‐lucent material and mitochondria composed this receptor. Hemispherical electron‐dense collars were observed at the top of the nerve bulbs. Striated rootlets laid just beneath the hemispherical electron‐dense collars. Type 2 sensory receptor presented two morphological variations, i.e., a bulb‐like monolobed structure, and a bulb‐like bilobed structure observed at two different degrees of evagination. For both variations, the nerve bulb enclosed mitochondria, electron‐lucent material, and a conical electron‐dense collar from which extended a striated rootlet. Numerous sensory receptors of this type were observed around the ventral sucker. J. Morphol., 2012. © 2011 Wiley Periodicals, Inc.     

Ultrastructure of multiciliated collar cells in the pilidium larva of Lineus bilineatus (Nemertini)

Summary The ultrastructure of the apical plate of the free-swimming pilidium larva of Lineus bilineatus (Renier 1804) is described with particular reference to the multiciliated collar cells. In the multiciliary collar cells there are several, up to 12, cilia surrounded by a collar of about 20 microvilli extending from the cells' apical surface. The cilia have the typical 9+2 axoneme arrangement and are equipped with striated caudal rootlets extending from the basal bodies. No accessary centriole or rostral rootlet were observed. Microvilli surrounding the cilia are joined in a cylindrical manner by a mucus-like substance to form a collar. In comparison with many sensory receptor cells built on a collar cell plan the multiciliary collar cells of the pilidium larva apical plate are rather simple and unspecialized. In other pilidium larvae monociliated collar cells are found in the apical plate. The possible function and phylogenetic implications of multiciliated collar cells in Nemertini are briefly discussed.List of Abbreviations a axoneme - b basal body - c cilia or flagella - d desmosome - G Golgi apparatus - m mitochondria - mf microfilaments - mu mucus - mv microvilli - n nucleus - nt neurotubules - pm plasma membrane - r rootlet - ri ribosomes - v secretory vesicles     

Epidermal ciliary ultrastructure of adult and larval sipunculids (Sipunculida)

The ultrastructure of the ciliary apparatus of multiciliated epidermal cells in larval and adult sipunculids is described and the phylogenetic implications discussed. The pelagosphera of Apionsoma misakianum has a dense cover of epidermal cilia on the head region. The cilia have a long, narrow distal part and two long ciliary rootlets, one rostrally and one vertically orientated. The adult Phascolion strombus has cilia on the nuchal organ and on the oral side of the tentacles. These cilia have a narrow distal part as in the A. misakianum larva, but the ciliary rootlets have a different structure. The first rootlet on the anterior face of the basal body is very short and small. The second, vertically orientated rootlet is long and relatively thick. The two ciliary rootlets present in the larval A. misakianum are similar to the basal metazoan type of ciliary apparatus of epidermal multiciliated cells and thus likely represent the plesiomorphic state. The minute first rootlet in the adult P. strombus is viewed as a consequence of a secondary reduction. No possible synapomorphic character with the phylogenetically troublesome Xenoturbella was found.     

A revision of the systematics of panther worms (Hofstenia spp., Acoela), with notes on color variation and genetic variation within the genus

Species of the genus Hofstenia are voracious predators and among the largest and most colorful of the Acoela. They are known from Japan, the Red Sea, the North Atlantic islands of Bermuda and the Bahamas, and the Caribbean and in a variety of habitats including the rocky intertidal, among Thalassia sea grass, on filamentous algae and decaying mangrove leaves. Certain color morphs associated with each of these habitats seem to have confused the taxonomy of the group. While brown-and-white banding and spotting patterns of Hofstenia miamia and Hofstenia giselae are distinctive for species associated with mangrove leaves and Thallasia sp. and are likely to be cryptic for these specific environments, we find some evidence to suggest that the coloration is mimicry of a nudibranch with aposematic coloration. The common plan in these patterns is one with three variously solid or spotted lighter cross bands on a dark background. Our examination of museum type material and live specimens of Hofstenia collected from Bahamas, Belize, Bermuda, and Panama revealed no internal morphological differences between the Hofstenia species occurring in the Caribbean. Similarly, our analyses of 18S and 28S molecular sequence data revealed no significant differences among specimens. Accordingly, we declare that Hofstenia giselae is a junior synonym of Hofstenia miamia, the three-banded panther worm. Electronic supplementary material The online version of this article (doi: ) contains supplementary material, which is available to authorized users. Handling editor: K. Martons     

Distinctive features of cilia in metazoans and their significance for systematics.

A comparative study of epidermal cilia in the Turbellaria and Nemertea has revealed features in these organelles that are specific to certain taxonomic groups. Turbellarians of the order Acoela, in particular, have a characteristic pattern of axonemal filament termination in the distal tips of their cilia and a characteristic ciliary rootlet system that is not seen in other turbellarian orders nor in other metazoans. Each epidermal cilium in acoels has a typical 9 + 2 axonemal pattern through the main part of its length, but near its distal tip there is an abrupt shelf-life narrowing at which filaments 4-7 terminate; filaments 1, 2, 8 and 9 continue into the thinner distal-most part of the shaft along with singlet microtubules from the axonemal center. The rootlet system in acoel cilia involves an interconnecting pattern with lateral connectives. The unique structure of these cilia has systematic and phylogenetic significance for the Acoela, and it is argued that ultrastructural characters in general, including characters of organelles, can be validly applied to the phylogeny and systematics of the Metazoa.     

Occurrence and Ultrastructure of Collar Cells in the Stomach Gastrodermis of Polypodium hydriforme Ussov (Cnidaria)

The existence of collar cells lining the stomach gastrodermis in free-living Polypodium hydriforme and their ultrastructure are described. The collar cells are provided with a collar consisting of 9–10 microvilli which encircles a central flagellum and forms a flagellar pit. At the bottom of the pit around the basal part of the flagellum there is fine crystalline material which extends also in the spaces between the microvilli and keeps them straight. The flagellum has a typical axoneme (9+2), its basal body is located below the apical surface of the collar cell and continues into a striated rootlet. An accessory centriole is situated close to the upper part of the rootlet. The cell nucleus is located in the basal part of the cell. Prominent mitochondria with tubular cristae, Golgi cisternae and fragments of rough endoplasmic reticulum are situated mostly in the basal part of the cytoplasm. Discoidal vesicles are abundant in the apical cytoplasm. The collar cells are connected to each other by septate junctions and interdigitations. The ultrastructure of collar cells described here is discussed in comparison to that of other Cnidarians and in connection with the problem of Polypodium's systematic position.     

Fine Structure and Function of Pharynx Cilia in Glossobalanus minutus Kowalewsky (Enteropneusta)

Two kinds of cilia have been observed in the pharynx of Glossobalanus minutus Kowalewsky. From the present study, a ciliary specialization can be found in order to move a determinate substance, i.e. mucus or water. Mucus-moving cilia (type I cilia) have a single basal centriole and poorly developed ciliary rootlets. Their tips are rounded, bearing an inner, asymmetrical cap attached to some tubules. Water-moving cilia (type II cilia) are exclusively located at lateral epithelia of branchial bars, giving rise to the water current through the gills. They have two basal centrioles, proximal and distal, and a complex system of ciliary rootlets made up of a principal rootlet, a secondary or accessory rootlet and a 'fan' rootlet. The tips of type II cilia have a long process with some tubules inside. All basal structures are precisely orientated in order to assure a good coordination of ciliary beat. The possible functional significance of ciliary substructure is also discussed. From these observations a model for mucus and water currents through gill slits is postulated.     

Differences in the utilization of water vapour and free water in two co‐occurring foliose lichens from semi‐arid southern Australia

Hydration and dehydration kinetics were investigated in two xerophytic lichens with contrasting thallus morphologies, Chondropsis semiviridis (F.Muell. ex Nyl.) Nyl. and Xanthoparmelia convoluta (Krempehl.) Hale. Pulse‐modulated chlorophyll fluorescence was used to measure photosynthetic activity in thalli hydrated with either liquid water or water vapour in the laboratory and in the field. Water content (WC) and photosynthetic activity of thalli in both species increased rapidly on contact with liquid water. When exposed to water vapour, C. semiviridis hydrated more rapidly and achieved higher WC than X. convoluta. Both lichens achieved maximum Fv/Fm at low WC, regardless of hydration source. Rates of water loss were slower, and Fv/Fm remained high for longer, in X. convoluta than in C. semiviridis. Light saturated electron transport rates of both lichens were low compared with a homoiohydric plant from the same environment. Our results suggest that X. convoluta, which has a more complex morphology, retains water and remains photosynthetically active for significantly longer periods than C. semiviridis, providing X. convoluta with a potential advantage in the semi‐arid environment in which both species are found.     

Tail regression induced by elevated retinoic acid signaling in amphioxus larvae occurs by tissue remodeling,not cell death

The vitamin A derived morphogen retinoic acid (RA) is known to function in the regulation of tissue proliferation and differentiation. Here, we show that exogenous RA applied to late larvae of the invertebrate chordate amphioxus can reverse some differentiated states. Although treatment with the RA antagonist BMS009 has no obvious effect on late larvae of amphioxus, administration of excess RA alters the morphology of the posterior end of the body. The anus closes over, and gut contents accumulate in the hindgut. In addition, the larval tail fin regresses, although little apoptosis takes place. This fin normally consists of columnar epidermal cells, each characterized by a ciliary rootlet running all the way from an apical centriole to the base of the cell and likely contributing substantial cytoskeletal support. After a few days of RA treatment, the rootlet becomes disrupted, and the cell shape changes from columnar to cuboidal. Transmission electron microscopy (TEM) shows fragments of the rootlet in the basal cytoplasm of the cuboidal cell. A major component of the ciliary rootlet in amphioxus is the protein Rootletin, which is encoded by a single AmphiRootletin gene. This gene is highly expressed in the tail epithelial cells of control larvae, but becomes downregulated after about a day of RA treatment, and the breakup of the ciliary rootlet soon follows. The effect of excess RA on these epidermal cells of the larval tail in amphioxus is unlike posterior regression in developing zebrafish, where elevated RA signaling alters connective tissues of mesodermal origin. In contrast, however, the RA‐induced closure of the amphioxus anus has parallels in the RA‐induced caudal regression syndrome of mammals.     

The ciliated epidermis of Xenoturbella bocki (Platyhelminthes, Xenoturbellida) with some phylogenetic considerations

The epidermis of Xenoturbella bocki Westblad was studied by scanning and transmission electron microscopy. Two cell types predominate in the epidermis: multiciliated epidermal cells and non-ciliated or monociliated gland cells. A conspicuous feature is the dense ciliary coverage and the numerous gland cell openings. Xenoturbella has a characteristic pattern of axonemal filament termination in the distal tips of their cilia. Each epidermal cilium has the typical 9 + 2 patten through the major part of its shaft. Near the tip there is a shelf at which doublets 4–7 terminate. Doublets 1, 2, 3, 8 and 9 continue into the thinner distal part of the cilium. A similar shelf in cilia is known only from the turbellarian orders Nemertodermatida and Acoela, and hence may be an apomorphic feature which indicates a close relationship between Xenoturbellida, Nemertoder-matida and Acoela. The basal body is provided with a so-called basal foot which has a cross-striated appearance and an expanded distal plate that seems to act as a microtubule organizing center. Approximately 15–25 microtubuli radiate from the endplate of the basal foot to the basal bodies caudally. The arrangement of basal foot and ciliary rootlets in Xenoturbella differs from that of Acoela and related orders in that there are two striated rootlets only (an anterior and a posterior one), rather than one main rootlet and two lateral rootlets.     

Morphology and ultrastructure of the pharynx in Solenofilomorphidae (Acoela)

The homology of pharynges within the mostly pharynx-less Acoela has been a matter of discussion for decades. Here, we analyze the pharynges of three members of the Solenofilomorphidae, Myopea sp. and two species of the genus Solenofilomorpha, by means of light and transmission electron microscopy. Special focus is placed on the ultrastructure of the pharyngeal musculature, epidermis surrounding the mouth, pharyngeal epithelium, and junction with the digestive parenchyma. The main goal of this study was to evaluate the usefulness of certain characters for broader comparisons within the Acoela. Among the three species, characters relating to position of the mouth, presence and elaboration of sphincter muscles, presence of pharyngeal glands, and ultrastructure of epitheliosomes proved to be variously species- and genus-specific. The arrangement of pharyngeal muscles and their connection with body wall musculature, ultrastructure of receptor cells, and morphology of a nonciliated glandular region in the posterior pharynx, in contrast, appear to be characteristic of the family Solenofilomorphidae and thus of predominant interest for comparisons with other acoel families.     

Sensory and secretory cells ofOphryotrocha puerilis (Polychaeta)

Summary As revealed by glyoxylic acid induced fluorescence, the protandric polychaeteOphryotrocha puerilis possesses different types of catecholaminergic primary bipolar sensory cells, the perikarya of which are located beneath the epidermis. About 20 of such receptors are situated in each segment but they are mostly found on antennae, palps, urites and parapodial cirri. The dendrites of these sensory neurones run to the cuticle and dilate to form receptive endings. Three different types of dendritic endings could be distinguished: (1) multiciliary receptors with 4–8 cilia and ciliary rootlets, (2) monociliary receptors with microvilli arranged like a funnel and electron-dense cuffs and (3) monociliary receptors of the collar-type with, constantly, ten microvilli surrounding one single central cilium. The latter type is also characterized by rootlet fragments. Dendrites and dilated receptive endings of all three types contain clear (putative secretory) vesicles, multivesicular bodies and mitochondria. Pharmacological treatment (dopamine, reserpine) does not affect the number of secretory vesicles of the receptor neurones. Extra vesicular storage of catecholamines is discussed. Secretory cells of unknown function containing large numbers of electron-dense vesicles are usually found in close association with sensory cells.Abbreviations CA catecholamines - DA dopamine - RE reserpine     

Development and morphology of ciliary urns in the sea cucumber Synaptula hydriformis (Echinodermata: Holothuroidea)

Sea cucumbers (holothuroids) lack the only known echinoderm immune organ, the axial organ. Holothuroids of the families Synaptidae and Chiridotidae have coelomic organs, known as ciliary urns, that gather and excrete waste and, therefore, might function in immunity. Although ciliary urns are widely reported and illustrated in the literature, the process and histology of urn development remain unknown. Development and structure of ciliary urns were examined in Synaptula hydriformis using scanning electron, brightfield, and scanning laser confocal microscopy. Mature urns occurred on all three mesenteries in 10‐tentacled young and later growth stages, and developing urns were found in post‐pentactulae, 10‐tentacled young, and released juveniles. Developing urns were circular clusters of ciliated collar cells protruding from the mesentery. The cells increased in number to form the sessile cushion stage with a shallow lumen. The subsequent spoon‐shaped stage had a stalk and a deepened lumen with an extensive ciliary field where coelomocytes began to accumulate. Mature urns had a thin stalk and cornucopia‐shaped body with an abluminal epithelium of squamous cells and an adluminal epithelium of densely packed ciliated collar cells. Cell boundaries of the rim of mature urns and of the stalk and body of developing urns were outlined on one or both sides by microvilli and an elevated cell membrane. Ciliary urns resembling the cushion‐stage urns of S. hydriformis have been described in the sea star Archaster typicus. If urns in these groups are homologous, it is likely that cushion urns are plesiomorphic and that they are present and have been overlooked in other echinoderms.