Ciliary feeding structures and particle capture mechanism in the freshwater bryozoan Plumatella repens (Phylactolaemata)

Abstract. In contrast to marine bryozoans, the lophophore structure and the ciliary filter‐feeding mechanism in freshwater bryozoans have so far been only poorly described. Specimens of the phylactolaemate bryozoan Plumatella repens were studied to clarify the tentacular ciliary structures and the particle capture mechanism. Scanning electron microscopy revealed that the tentacles of the lophophore have a frontal band of densely packed cilia, and on each side a zigzag row of laterofrontal cilia and a band of lateral cilia. Phalloidin‐linked fluorescent dye showed no sign of muscular tissue within the tentacles. Video microscopy was used to describe basic characteristics of particle capture. Suspended particles in the incoming water flow, set up by the lateral ‘pump’ cilia on the tentacles, approach the tentacles with a velocity of 1–2 mm s^‐1. Near the tentacles, the particles are stopped by the stiff sensory laterofrontal cilia acting as a mechanical sieve, as previously seen in marine bryozoans. The particle capture mechanism suggested is based on the assumed ability of the sensory stiff laterofrontal cilia to be triggered by the deflection caused by the drag force of the through‐flowing water on a captured food particle. Thus, when a particle is stopped by the laterofrontal cilia, the otherwise stiff cilia are presumably triggered to make an inward flick which brings the restrained particle back into the downward directed main current, possibly to be captured again further down in the lophophore before being carried to the mouth via the food groove. No tentacle flicks and no transport of captured particles on the frontal side of the tentacles were observed. The velocity of the metachronal wave of the water‐pumping lateral cilia was measured to be ～0.2 mm s^‐1, the wavelength was ～7 μm, and hence the ciliary beat frequency estimated to be ～30 Hz (～20 °C). The filter feeding process in P. repens reported here resembles the ciliary sieving process described for marine bryozoans in recent years, although no tentacle flicks were observed in P. repens. The phylogenetic position of the phylactolaemates is discussed in the light of these findings.     

Putative sensory structures in marine bryozoans

Abstract. SEM studies of 21 species of marine bryozoans demonstrated that the abfrontal side of the tentacles bears a row of mono- or multiciliated cells, which are presumably sensory. In stenolaemates, the abfrontal cells, as well as the cells at the tentacle tips and the laterofrontal cells, are monociliated. In the 17 gymnolaemate species studied, each tentacle tip bears at least 3 multiciliated cells, each with a tuft of 5–7 stiff cilia of various lengths. On the abfrontal tentacle surface, mono- and multiciliated cells alternate, but all species studied have multiciliated cells at the base and the tip of each tentacle. In live animals, single cilia perform occasional flicks, whereas the tufts of 7–15 cilia on the multiciliated cells are immotile. Length and number of abfrontal cilia vary between species. Two types of multiciliated, putative sensory organs were found on the introvert of some gymnolaemates. One has an apical knob surrounded by a ring of cilia; the other has an apical tuft of cilia. The ultrastructure of the sensory cells of tentacles and introvert was studied in Rhamphostomella ovata . Our observations on both fixed and living material all suggest that these cells are primitive mechanoreceptors. The few species lacking ciliary structures on the introvert have long proximal ciliary tufts on the abfrontal tentacle surface.     

Ciliary sieving and active ciliary response in capture of particles by suspension-feeding brachiopod larvae

Larvae of a brachiopod, Glottidia pyramidata, used at least two ciliary mechanisms to capture algal cells upstream from the lateral band of cilia that produces a feeding/swimming current. (1) Filtration: the larvae retained algal cells on the upstream (frontal) side of a sieve composed of a row of stationary laterofrontal cilia. Movement of the laterofrontal cilia could not be observed during capture or rejection of particles, but the laterofrontal cilia can bend toward the beating lateral cilia, a possible mechanism for releasing rejected particles from the ciliary sieve. (2) Localized changes of ciliary beat: the larvae may also concentrate particles by a local change in beat of lateral cilia in response to particles. The evidence is that the beat of lateral cilia changed coincident with captures of algal cells and that captured particles moved on paths consistent with a current redirected toward the frontal side of the tentacle by an induced local reversal of the lateral cilia. The change of beat of lateral cilia could have been an arrest rather than a reversal of ciliary beat, however. The similar ciliary bands in adult and larval lophophorates (brachiopods, phoronids, and bryozoans) suggest that these animals share a range of ciliary behaviours. The divergent accounts of ciliary feeding of lophophorates could be mostly the result of different authors observing different aspects of ciliary feeding.     

Versatile ciliary behaviour in capture of particles by the bryozoan cyphonautes larva

Cyphonautes larvae of a bryozoan, Membranipora membranacea, used several ciliary mechanisms to capture algal cells upstream from the lateral band of cilia that produces a feeding current. (1) Lateral cilia changed beat and a backcurrent occurred at the time and place that particles were retained. (2) Algal cells were sieved and held stationary at the upstream (frontal) side of a row of laterofrontal cilia that were not beating. (3) Localized extension of cilia toward the inhalant chamber, coincident with particle captures, indicated that laterofrontal cilia flick toward the inhalant chamber. These flicks may aid transport of captured particles toward the mouth. Thus my earlier report that larvae only sieve, in contrast to the adults (which have an active ciliary response) was in error. The similar ciliary bands in adult and larval bryozoans and in other lophophorates (brachiopods, and phoronids) suggest that these animals share a core repertoire of ciliary behaviours in the capture and concentration of suspended food particles.     

Structure and occurrence of cyphonautes larvae (bryozoa,ectoprocta)

We have studied larvae of the freshwater ctenostome Hislopia malayensis with scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM), and LM of serial sections. Some additional observations on larvae of M. membranacea using SEM and CLSM are also reported. The overall configuration of muscles, nerves, and cilia of the two larvae are identical. However, the larva of H. malayensis is much smaller than that of M. membranacea, which may explain most of the differences observed. Although all major nerves and muscle strands are present in H. malayensis, they are generally composed of fewer fibers. The H. malayensis larva lacks the anterior and posterior intervalve cilia. Its pyriform organ is unciliated with only a small central depression. The adhesive epithelium is not invaginated as an adhesive sac and lacks the large muscles interpreted as adhesive sac muscles in the M. membranacea larva. The velum carries two rows of ciliated cells, though the lower “row” consists of only one or two cells. Both rows of ciliated cells are innervated by nerves, which have not been detected in the M. membranacea larva. The ciliated ridge of H. malayensis lacks the frontal cilia. The planktotrophic cyphonautes larvae in a number of ctenostome clades and in the “basal” cheilostome clade Malacostega (and probably in the earliest cheilostomes) support the idea that the cyphonautes larva is the ancestral larval type of the Eurystomata. It may even represent the ancestral larval type of the bryozoans (= ectoprocts). J. Morphol. 271:1094‐1109, 2010. © 2010 Wiley‐Liss, Inc.     

The phylogenetic position of entoprocta, ectoprocta, phoronida, and brachiopoda

Ectoprocts, phoronids and brachiopods are often dealt with underthe heading Tentaculata or Lophophorata, sometimes with entoproctsdiscussed in the same chapter, for example in Ruppert and Barnes(1994). The Lophophorata is purported to be held together bythe presence of a "lophophore," a mesosomal tentacle crown withan upstream-collecting ciliary band. However, the mesosomaltentacle crown of pterobranchs has upstream-collecting ciliarybands with monociliate cells, similar to those of phoronidsand brachiopods, although its ontogeny is not well documented.On the contrary, the ectoproct tentacle crown carries a ciliarysieving system with multiciliate cells and the body does notshow archimery, neither during ontogeny nor during budding,so the tentacles cannot be characterized as mesosomal. The entoproctshave tentacles without coelomic canals and with a downstream-collectingciliary system like that of trochophore larvae and adult rotifersand serpulid and sabellid annelids. Planktotrophic phoronidand brachiopod larvae develop tentacles at an early stage, buttheir ciliary system resembles those of echinoderm and enteropneustlarvae. Ectoproct larvae are generally non-feeding, but theplanktotrophic cyphonautes larvae of certain gymnolaemates havea ciliary band resembling that of the adult tentacles. The entoproctshave typical trochophore larvae and many feed with downstream-collectingciliary bands. Phoronids and brachiopods are thus morphologicallyon the deuterostome line, probably as the sister group of the"Neorenalia" or Deuterostomia sensu stricto. The entoproctsare clearly spiralians, although their more precise positionhas not been determined. The position of the ectoprocts is uncertain,but nothing in their morphology indicates deuterostome affinities."Lophophorata" is thus a polyphyletic assemblage and the wordshould disappear from the zoological vocabulary, just as "Vermes"disappeared many years ago.     

Distribution of catecholamine-containing,serotonin-like and neuropeptide FMRFamide-like immunoreactive neurons and processes in the nervous system of the actinotroch larva ofPhoronis muelleri (Phoronida)

Summary Glyoxylic-acid-induced fluorescence of catecholamines and antibodies against serotonin and FMRFamide were used to study the distribution of putative neurotransmitters in the actinotroch larva ofPhoronis muelleri Selys-Longchamps, 1903. Catecholamines occur in the neuropile of the apical ganglion, in the longitudinal median epistome nerves, in the epistome marginal nerves, and in the nerve along the bases of the tentacles. The tentacles have laterofrontal and latero-abfrontal bundles of processes that form two minor nerves along the lateral ciliary band of the tentacles, and a medio-frontal bundle of processes. Monopolar cells are located on the ventro-lateral part of the mesosome. Processes are located along the posterior ciliary band and as a reticulum in the epidermis. Serotonin-like immunoreactive cells and processes are located in the apical ganglion, in the longitudinal median epistome nerves, and as a dorsal and ventral pair of bundles along the tentacle bases. Processes from the latter extend into the tentacles as the medioabfrontal processes. The latero-abfrontal processes form a minor nerve along the ciliary band. The dorsal bundles forms the major nerve ring along the tentacles and processes extend from it to the metasome. Processes are located along the posterior ciliary band. FMRFamide-like immunoreactive cells and processes are found in the apical ganglion, in the longitudinal median epistome nerves and as a pair of lateral epistome processes projecting towards the ring of tentacles. In the tentacles, a pair of latero-frontal processes are found; these form a minor nerve along the ciliary band. A band of cells can be seen along the tentacle ring.     

草鱼鳃上寄生毛管虫一新种——变异毛管虫的研究

吸管亚纲(Subclass suctoria)中,多数种类具有或长或短的炳,附着它物上营固着生活。寄生在鱼类体表、鳃丝内的毛管虫(Trichophrya)和簇管虫(Erastophrya)的种类中,至今未见报道有固着柄的代表。       

Rejection Mechanism of Plectolophous Lophophore of Brachiopod <Emphasis Type="Italic">Coptothyris grayi</Emphasis> (Terebratulida,Rhynchonelliformea)

Brachiopoda is a relict group of invertebrate filter feeders that used a tentacle organ, lophophore, for capturing food particles from the water column. Brachiopod extinction apparently occurred due to low productivity of their filtering organ in comparison with more advanced filter-feeders. Investigation of the filtering mechanism of modern brachiopods is essential to understanding their evolutionary fate. This study is devoted to the rejection mechanism of large waste particles from the plectolophous lophophore of brachiopod Coptothyris grayi. The waste particles gather inside of the lophophore on the outer side of the brachial fold. The particles form rows along frontal grooves of outer tentacles and are carried successively to the tentacle tips and move along them, slimed by mucus. One portion of the particles comes off the lophophore and falls down the mantle, while another part is carried to the abfrontal surface of the tentacles. Due to repeated reversals of abfrontal cilia, the particles wavily move along the abfrontal surface of tentacles. Such movement contributes to the secretion of mucus and the formation of particle clots. The clots come off the lophophore and fall down the mantle. The particles are transported along the mantle by cilia to the anterior part of the mantle margin. Here the ciliary reversals that facilitate secretion of mucus and formation of pseudofeces also take place. The latter takes away from the mantle cavity. Thus, only outer tentacles participate in the rejection of large waste particles from the lophophore. Ciliary reversals of the abfrontal surface of tentacles and the mantle are discovered in brachiopods for the first time. This facilitates the additional secretion of mucus and formation of pseudofeces, easing their exit from the mantle cavity. The results contribute to the knowledge of lophophore function and evolution of tentacle organs in Bilateria.     

Fine Structure of the Tentacles of Phoronis australis Haswell (Phoronida,Lophophorata)

The epidermis of the tentacles of Phoronis australis consists of six cell types: supporting cells, choanocyte-like sensory cells, both types monociliated, secretory A-cells with a mucous secretion, and three kinds of B-cells with mucoprotein secretions. On cross-sections of the tentacle, one can distinguish four faces: the frontal one, heavily ciliated and located between the two frontolateral rows of sensory cells, the lateral and the abfrontal ones. The orientation of the basal structures of the cilia is related to the direction of their beat. The basiepidermal nervous system is grouped mainly at the frontal and abfrontal faces. The basement membrane is thickest on the frontal face and consists of circular collagen fibrils near the epidermis and longitudinal ones near the peritoneum. All peritoneal cells surrounding the mesocoel are provided with smooth longitudinal myofibrils, and isolated axons are situated between these cells and the basement membrane. The wall of the single blood capillary in each tentacle consists of epitheliomuscular cells with circular myofilaments, lying on a thin internal basal lamina; there is no endothelium.     

Contrasting scaling of ciliary filters in swimming larvae and sessile adults of fan worms (Annelida: Polychaeta)

Abstract. Both larval and adult fan worms capture particles with opposed bands of cilia. While the larvae use one of the opposed bands (the prototroch) for both feeding and swimming, the sessile adults rely partly on ambient currents to bring food particles to the ciliary bands. The scaling of length of prototrochal cilia with larval body size contrasts with scaling of the opposed latero-frontal cilia with adult body size. In the larva of the serpulid Hydroides elegans , the length of prototrochal cilia increased from 28 to 42 μm in early to late-stage larvae. In contrast, latero-frontal cilia did not increase in length (23 μm) during postlarval development of H. elegans. Among adults of 5 fan-worm species, lengths of latero-frontal cilia ranged from 22 to 35 μm and were weakly correlated with body size. The total area of ciliary filter nevertheless increased with increasing body dry weight of worms with an allometric exponent similar to exponents reported for gill and lophophore areas vs. body weight within species of suspension-feeding bivalves, brachiopods, and gastropods. The similar scaling was remarkable given the striking differences in distribution and function of the ciliary filters. In adult fan worms, increases in filter area depended largely on increases in number and length of radioles; differences in branching of radioles had little effect. Radioles were commonly in 2 or more rows in series, implying refiltration in still water by downstream radioles. Since the allometry of worms' filter area with body size depends on filters in series, it depends on ambient currents that overwhelm ciliary currents.     

Studies on the gill ciliation of the American oyster Crassostrea virginica (Gmelin)

Latero-frontal, para-latero-frontal, and frontal ciliary tracts on the gill filaments of Crassostrea virginica (Gmelin) were studied with light microscopy and scanning electron microscopy. Latero-frontal cirri are complex structures composed of varying numbers of paired cilia. The multiple pairs of cilia which constitute a single cirrus are closely appressed for a portion of their length; they then branch laterally from the central axis in a plume-like fashion. Latero-frontal cirri of adjacent gill filaments create a filtration sieve which should be capable of retaining particles smaller than 1 μm in diameter. Para-latero-frontal cilia are short, closely spaced cilia arranged as a staggered row along the frontal side of each tract of latero-frontal cirri. Latero-frontal cirri and para-latero-frontal cilia occur on ordinary, principal, and transitional gill filaments. Frontal ciliary tracts of ordinary filaments are divided into a central, ventrally directed coarse tract, flanked on either side by a dorsally directed fine ciliary tract. The coarse tract is covered by cirri which are comprised of five to eight cilia, while the fine frontal tracts are made up of individually functioning cilia. The frontal ciliary tracts of principal and transitional filaments bear only dorsally directed fine cilia. The unique direction of effective beat of the coarse frontal cirri of ordinary filaments, in combination with the action of fine frontal cilia and the strategic location of mucus producing cells, is used to describe a possible mechanism for the sorting of filtered particles.     

Mechanical sensitivity and cell coupling in the ciliated epithelial cells ofMytilus edulis gill

Summary Transmission electron microscopy has not provided strong evidence for gap junctions inMytilus edulis gill tissue, in spite of extensive physiological evidence for coupled ciliary arrest in lateral cells and coupled activation in abfrontal cells. To investigate the kinds and relative distribution of cell junctions and also to determine whether ciliary membrane particle differences exist in these two types of oppositely mechanically sensitive cells, we analyzed the structure of these and two other ciliated cell types (frontal and laterofrontal) by freeze-fracture replication. Gap junctions occur in all four ciliated cell types, but they are relatively small and of variable morphology, often consisting of elongate, winding complexes of membrane particles. Statistically, such structures rarely would be recognized as gap junctions in thin sections. Gap junctions appear to be most abundant between the highly coupled abfrontal cells, minimal between laterofrontal cells, and not evident in the epithelial cells that separate coupled ciliated cell types. The ciliary necklaces of the mechanically activated abfrontal cilia are typically 4- or 5-stranded while those of the remaining three cell types are mainly 3-stranded. In developing gill tips, ciliated cells have abundant gap junctions and newly formed cilia have a full complement of necklace particles. Nascent lateral cilia are not mechanically sensitive, indicating that the acquisition of mechanosensitivity does not correlate with the presence of ciliary necklace or other membrane particles. Lateral and laterofrontal cells become sensitive to neurotransmitters soon after the appearance of the latter during development, but mechanosensitivity of both lateral and abfrontal cells arises substantially later.     

Ultrastructure of tentacles in the sipunculid worm <Emphasis Type="Italic">Thysanocardia nigra</Emphasis> Ikeda, 1904 (Sipuncula)

The ultrastructure of the tentacles was studied in the sipunculid worm Thysanocardia nigra. Flexible digitate tentacles are arranged into the dorsal and ventral tentacular crowns at the anterior end of the introvert of Th. nigra. The tentacle bears oral, lateral, and aboral rows of cilia; on the oral side, there is a longitudinal groove. Each tentacle contains two oral tentacular canals and an aboral tentacular canal. The oral side of the tentacle is covered by a simple columnar epithelium, which contains large glandular cells that secrete their products onto the apical surface of the epithelium. The lateral and aboral epithelia are composed of cuboidal and flattened cells. The tentacular canals are lined with a flattened coelomic epithelium that consists of podocytes with their processes and multiciliated cells. The tentacular canals are continuous with the radial coelomic canals of the head and constitute the terminal parts of the tentacular coelom, which shows a highly complex morphology. Five tentacular nerves and circular and longitudinal muscle bands lie in the connective tissue of the tentacle wall. Similarities and differences in the tentacle morphology between Th. nigra and other sipunculan species are discussed.Original Russian Text Copyright © 2005 by Biologiya Morya, Maiorova, Adrianov.     

Tentacle structure in freshwater bryozoans

Tentacles are the main food‐gathering organs of bryozoans. The most common design is a hollow tube of extracellular matrix (ECM), covered with ten columns of epithelial cells on the outside, and a coelothelium on the inside. Nerves follow the ECM, going between the bases of some epidermal cells. The tentacle musculature includes two bundles formed by myoepithelial cells of the coelothelium. The tentacles of freshwater (phylactolaemate) bryozoans, however, differ somewhat in structure from those of marine bryozoans. Here, we describe the tentacles of three species of phylactolaemates, comparing them to gymnolaemates and stenolaemates. Phylactolaemate tentacles tend to be longer, and with more voluminous coeloms. The composition of the frontal cell row and the number of frontal nerves is variable in freshwater bryozoans, but constant in marine groups. Abfrontal cells form a continuous row in Phylactolaemata, but occur intermittently in other two classes. Phylactolaemata lack the microvillar cuticle reported in Gymnolaemata. Abfrontal sensory tufts are always composed of pairs of mono‐ and/or biciliated cells. This arrangement differs from individual abfrontal ciliary cells of other bryozoans: monociliated in Stenolaemata and monociliated and multiciliated ones in Gymnolaemata. In all three groups, however, ciliated abfrontal cells probably serve as mechanoreceptors. We confirm previously described phylactolemate traits: an unusual arrangement of two‐layered coelothelium lining the lateral sides of the tentacle and oral slits in the intertentacular membrane. As previously reported, tentacle movements involved in feeding differ between bryozoan groups, with phylactolaemates tending to have slower movements than both gymnolaemates and stenolaemates, and a narrower behavioral repertoire than gymnolaemates. The morphological and ultrastructural differences between the freshwater species we studied and marine bryozoans may be related to these functional differences. Muscle organization, tentacle and coelom size, and degree of confluence between tentacle and lophophore coeloms probably account for much of the observed behavioral variability.     

Ciliated band structure in planktotrophic and lecithotrophic larvae of Heliocidaris species (Echinodermata: Echinoidea): a demonstration of conservation and change

The evolution of lecithotrophic (non-feeding) development in sea urchins is associated with reduction or loss of structures found in the planktotrophic (feeding) echinopluteus larvae. Reductions or losses of larval feeding structures include pluteal arms, their supporting skeleton and the ciliated band that borders them. The barrel-shaped lecithotrophic larva of Heliocidaris erythrogramma has, at its posterior end, two or three ciliated band segments comprised of densely packed, elongate cilia. These cilia may be expressions of the epaulettes that would have been present in an ancestral larval form, represented today by the feeding echinopluteus of H. tuberculata . We compared the development and cellular organization of the larval ciliary structures of both Heliocidaris species to assess whether the ciliary bands of H. erythrogramma are expressions of the feeding ciliated band or epaulettes of an echinopluteus. Epaulette development in feeding larvae of H. tuberculata involves separation of specific parts of the ciliated band from the rest of the feeding ciliated band, hyperplastic addition of ciliated cells and hypertrophic growth of the cilia. Like epaulettes, the ciliated bands of H. erythrogramma are composed of long spindle-shaped cells arranged in a cup-shaped collection that bulges into the blastocoel; and these cells have elongated cilia. In their developmental origin and topological arrangement however, the ciliated bands of H. erythrogramma correspond more closely with parts of the pluteal feeding ciliated band than with epaulettes. The larvae of this echinoid appear to develop epaulette-like bands from parts of the original (but reduced) feeding ciliated band. The evolution of development in H. erythrogramma has thus involved both conservation and change in echinopluteal ciliary structures.     

The sensory epithelium of the tentacles and the rhinophore of Nautilus pompilius L. (cephalopoda, nautiloidea)

Nine intraepithelial ciliated cell types that are presumed to be sensory cells were identified in the epithelium of the pre- and postocular tentacles, the digital tentacles, and the rhinophore of the juvenile tetrabranchiate cephalopod Nautilus pompilius L. The morphological diversity and specialization in distribution of the different ciliated cell types analyzed by SEM methods suggest that these cells include receptors of several sensory functions. Ciliated cell types in different organs that show similar surface features were combined in named groups. The most striking cell, type I, is characterized by a tuft of long and numerous cilia. The highest density of this cell type occurs in ciliary fields in the epithelium of the lamellae of the pre- and postocular tentacles, in the olfactory pits of the rhinophores, and in the lamellae of four pairs of lateral digital tentacles, but not in the epithelium of the medial digital tentacles. The similar morphological data, together with behavioral observations on feeding habits, suggest that this cell type may serve in long-distance chemosensory function. The other ciliated cell types are solitary cells with specific spatial distributions in the various organs. Cell types with tufts of relatively short, stiff cilia (types III, IV, VIII), which are distributed in the lateral and aboral areas of the tentacles and at the base of the tentacle-like process of the rhinophore, are considered to be employed in mechanosensory transduction, while the solitary cells with bristle-like cilia at the margin of the ciliary fields (type II) and at the base of the rhinophore (type IX) may be involved in chemoreception. Histological investigation of the epithelium and the nerve structures of the different organs shows the proportion and distribution of the sensory pathways. Two different types of digital tentacles can be distinguished according to their putative functions: lateral slender digital tentacles in four pairs, of which the lowermost are the so-called long digital tentacles, participate in distance chemoreception, and the medial digital tentacles, whose terminal axial nerve cord may represent a specialized neuromechanosensory structure, appear to have contact chemoreceptive abilities.     

A comparative study of ciliary differentiations in the branchial stigmata of ascidians

In a correlated thin sectioning and freeze-fracturing study, we have examined species belonging to the orders of the ascidian class: Stolidobranchiata (Botryllus schlosseri, Botrylloides leachi, Molgula socialis, Styela plicata), Phlebobranchiata (Ascidiella aspersa, Phallusia ingeria, Ciona intestinalis) and Aplousobranchiata (Clavelina lepadiformis). Though the branchial basket varies in the complexity and filtration efficiency in the three orders, the ciliated epithelia aroand the stigmata contain a common pattern of organization; seven rows of flattened cells, each bearing a single row of long cilia flanked by a single row of microvilli. All the species examined possess ciliary specializations represented by: (a) bridges connecting doublets number 5 and 6 as well as 9.1 and 2; (b) dense material lying between the above mentioned axonemal doublets (5-6 and 1-2) and the ciliary membrane, sometimes in the shape of longitudinal strands or often as lines of dots; (c) a fuzzy coat protruding from the ciliary membrane, consisting of tufts or scattered filaments; (d) intramembrane particles (IMPs) associated with the P-face of the membrane, often arranged in clusters and orderly alignments related to the anderlying axonemal doublets; these IMPs decorate the opposite sides of each cilium facing the adjacent cilia forming the ciliary rows of adjacent cells and are absent on the lateral sides. The stigmatal cilia propel water through the stigmata and their effective strokes follow a line at right angles to the row of cilia in each cell. The usual direction of the effective stroke is toward doublets 5-6. It is possible, therefore, to refer to structure in relation to the ciliary beat cycle. The importance of these specializations is unknown, but the structures appear to vary in the different species. A correlation between the richness of the specializations and the complexity of the branchial basket was not evidenced. It was suggested that the ciliary specializations relate to the peculiar organization of the stigmatal margin and that all are involved in the regulation of the ciliary activity.     

Cilia and the life of ctenophores

Ctenophores, or comb jellies, are a distinct phylum of marine zooplankton with eight meridional rows of giant locomotory comb plates. Comb plates are the largest ciliary structures known, and provide unique experimental advantages for investigating the biology of cilia. Here, I review published and unpublished work on how ctenophores exploit both motile and sensory functions of cilia for much of their behavior. The long‐standing problem of ciliary coordination has been elucidated by experiments on a variety of ctenophores. The statocyst of ctenophores is an example of how mechanosensory properties of motile cilia orient animals to the direction of gravity. Excitation or inhibition of comb row beating provides adaptive locomotory responses, and global reversal of beat direction causes escape swimming. The diverse types of prey and feeding mechanisms of ctenophores are related to radiation in body form and morphology. The cydippid Pleurobrachia catches copepods on tentacles and undergoes unilateral ciliary reversal to sweep prey into its mouth. Mnemiopsis uses broad muscular lobes and ciliated auricles to capture and ingest prey. Beroë has giant smooth muscles and toothed macrocilia to rapidly engulf or bite through ctenophore prey, and uses reversible tissue adhesion to keep its mouth closed while swimming. Ciliary motor responses are calcium‐dependent, triggered by voltage‐activated calcium channels located along the length (reversed beating) or at the base (activation of beating) of ciliary membranes. Ciliary and muscular responses to stimuli are regulated by epithelial and mesogleal nerve nets with ultrastructurally identifiable synapses onto effector cells. Post‐embryonic patterns of comb row development in larval and adult stages are described and compared with regeneration of comb plates after surgical removal. Truly, cilia and ctenophores, like love and marriage, go together like a horse and carriage.     

Ultrastructural evidence for the occurrence of three types of mechanosensitive cells in the tentacles of the cubozoan polypCarybdea marsupialis

Summary The ectodermal cell layer in the tentacles of the cubozoan polypCarybdea marsupialis contains four types of cells (types 1–4) bearing specialized cilia. Epitheliomuscular cells (type 1) are characterized by motile cilia with dynein-decorated axonemes. 200 nm long extramembranous filaments of unknown function are restricted to a belt-like region distal to the transition zone. Up to 40 rn long rigid cilia formed by a slender epithelial cell type (type 2) are surrounded by rings of short microvilli. The axonemes of these cilia are composed of incomplete microtubules and lack dynein. Microvilli and cilia are linked by intermembrane connectors. Microtubuledoublets and ciliary membrane are interconnected by microtubule-associated cross-bridges only within this contact region. At the tip of each tentacle a single nematocyte (type 3) is surrounded by 7–10 accessory cells (type 4). These both cell types are equipped with similar cilium-stereovilli-complexes consisting of a cone-like arrangement of stereovilli and a modified cilium. The axonemal modifications of the cilium, its interconnections with the surrounding stereovilli and the linkages between ciliary axoneme and ciliary membrane are similar to those known from the cnidocil-complexes of hydrozoons and other epithelial mechanosensitive cells of the collar-receptor type. Our data indicate that besides the nematocyte two other types of mechanosensory cells (types 2 and 4) are integrated in the ectodermal cell layer ofCarybdea which possibly affect the triggering mechanism of nematocyst discharge.