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.     

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.     

Ciliary filter-feeding structures in adult and larval gymnolaemate bryozoans

Abstract. Ciliary filter-feeding structures of gymnolaemate bryozoans—adults of Flustrellidra hispida and Alcyonidium gelatinosum , larvae of Membranipora sp.—were studied with SEM. In F. hispida and A. gelatinosum , the distal part of each tentacle has a straight row of stiff laterofrontal cilia which carry out "ciliary sieving" to capture suspended food particles that are subsequently transported downward towards the mouth by tentacle flicking; both structure and function resemble those of stenolaemate tentacles. The proximal part of the tentacle and of the ciliary ridge of a cyphonautes larva have strikingly similar structures, except that the laterofrontal cells are monociliate in the adults and biciliate in the larvae. The laterofrontal cells of the tentacles are arranged in a zigzag row and their cilia form two parallel rows, a frontal and a lateral row. The latter probably forms the sieve of stiff filter cilia in front of the water-pumping lateral cilia, whereas the frontal row appears to be held close to the frontal ciliary band of the tentacle. The biciliate laterofrontal cells of the cyphonautes larva have the cilia arranged in similar rows. The detailed morphological similarities between the ciliary bands of adult and larval filtering structures suggest that the feeding mechanisms are similar, contrary to what has been previously thought.     

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.     

Particle capture in ciliary filter‐feeding gymnolaemate and phylactolaemate bryozoans – a comparative study

Riisgård, H.U., Okamura, B. and Funch, P. 2009. Particle capture in ciliary filter‐feeding gymnolaemate and phylactolaemate bryozoans – a comparative study. —Acta Zoologica (Stockholm) 91 : 416–425. We studied particle capture using video‐microscopy in two gymnolaemates, the marine cheilostome Electra pilosa and the freshwater ctenostome Paludicella articulata, and three phylactolaemates, Fredericella sultana with a circular funnel‐shaped lophophore, and Cristatella mucedo and Lophophus crystallinus, both with a horseshoe‐shaped lophophore. The video‐microscope observations along with studies of lophophore morphology and ultrastructure indicated that phylactolaemate and gymnolaemate bryozoans with a diversity of lophophore shapes rely on the same basic structures and mechanisms for particle capture. Our study also demonstrates that essential features of the particle capture process resemble one another in bryozoans, brachiopods and phoronids.     

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.     

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.     

The mussel filter–pump – present understanding,with a re‐examination of gill preparations

Filter feeding in mussels is a secondary adaptation where the gills have become W‐shaped and greatly enlarged, acting as the mussel filter–pump. Water pumping and particle capture in the blue mussel, Mytilus edulis, have been studied over many years. Here, we give a short status of the present understanding of ciliary structure and function of the mussel filter–pump, supplemented with new photo‐microscope and scanning electron microscopy (SEM) pictures of gill preparations. Pumping rate (filtration) and pressure to maintain flow have been extensively studied so the power delivered by the mussel pump to the water flow is known (1.1% of total respiratory power), but the actual cost based on gill respiration is much higher (19%), implying that the cost of maintaining of the large gill pump is considerable and that only relatively little energy can be saved by stopping or reducing the activity of the water‐pumping cilia so that continuous feeding with a ‘minimal scaled’ pump is cheaper than discontinuous feeding with a correspondingly larger pump. According to the present view, the pump proper is the beating lateral cilia (lc) on the gill filaments and particle capture is accomplished by the action of laterofrontal cirri (lfc) transferring particles from the main water current to the frontal gill filament currents driven by frontal cilia (fc). Unexplained aspects include retention efficiency according to particle size and the role of pro‐laterofrontal cilia (p‐lfc) placed between the lfc and fc. The structure of cilia and the mode of ciliary beating have been re‐examined in this study by new high‐resolution light and scanning electron microscopy of isolated gill preparations exposed to serotonin (5‐HT) stimulation which can activate the lc and lfc at low concentrations (10^?6 M), but removes the lfc from the interfilament canals at higher concentrations (10^?5 M).     

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.     

Filter feeding mechanism in the phoronid <Emphasis Type="Italic">Phoronopsis harmeri</Emphasis> (Phoronida,Lophophorata)

Phoronids, like other Lophophorata (Bryozoa and Brachiopoda) are filter feeders. The lophophore performs various functions, the most important of which is the collection and sorting of food particles. The mechanism of sorting has been well studied for many other groups of invertebrate, but until now it has remained obscure for phoronids. With the help of functional morphology data we are proposing a possible scheme of sorting in phoronids on the example of Phoronopsis harmeri. The lower limit of the particle size is defined by the distance between laterofrontal cilia of tentacles and equals 1.2 μm. Larger particles are transferred by frontal cilia to the basis of the tentacles, where they pass into the lophophoral groove. The distance between the epistome and the external row of tentacles regulates the upper limit of the particle size that are suitable for food. Only particles whose size does not exceed 12 μm get into the lophophoral groove and further into the mouth. Larger particles collect in the space above the epistome and are removed from the lophophore. The size of the food particles that phoronids consume by filtration lies in a range 1.2–12 μm. These are bacteria and small phytoplankton organisms. At the same time the significant individual mobility of the phoronid tentacles plays an important role in the expansion of the pabular spectrum to large inactive zooplankton and phytoplankton organisms reaching a size of 50–100 μm.     

Feeding behavior in freshwater bryozoans: function,form, and flow

Bryozoans are impressively active suspension feeders, with diverse feeding behaviors. These have been studied extensively in marine bryozoans, but less so in their freshwater counterparts. Here we identified 16 distinct behaviors in three phylactolaemate species and classified them into behaviors involving separate tentacles, groups of tentacles, lophophore arms, the introvert, or multiple zooids. We examined (1) the repertoire of behaviors in each species, and each behavior's (2) absolute frequency, (3) relative frequency and (4) duration in each of the three species, at two flow velocities (0 and 0.2 cm s^?1). Nine feeding behaviors were shared by all three species, but the occurrence of other behaviors in a given species was limited by its morphology. Behaviors involved in particle capture were the most frequent, and were often faster than the reactions involved in particle rejection. By contrast, the absolute frequency of behaviors varied widely among species without clear associations with species form, or function of the behavior. Flow velocity had only minor effects on the feeding behaviors exhibited by a species, or their frequencies or durations. Our results show that phylactolaemates have the same key feeding behaviors of the individual polypides (especially involving separate tentacles) as previously described in gymnolaemate and stenolaemate bryozoans, although their behaviors tend to be carried out more slowly than those of stenolaemates or gymnolaemates. Feeding behaviors involving multiple zooids were nearly absent in the studied phylactolaemates, but are common in gymnolaemates. Freshwater bryozoans appear to be intermediate between stenolaemate and gymnolaemate bryozoans in terms of richness of the repertoire of feeding behaviors.     

Modern Data on the Innervation of the Lophophore in Lingula anatina (Brachiopoda) Support the Monophyly of the Lophophorates

Evolutionary relationships among members of the Lophophorata remain unclear. Traditionally, the Lophophorata included three phyla: Brachiopoda, Bryozoa or Ectoprocta, and Phoronida. All species in these phyla have a lophophore, which is regarded as a homologous structure of the lophophorates. Because the organization of the nervous system has been traditionally used to establish relationships among groups of animals, information on the organization of the nervous system in the lophophore of phoronids, brachiopods, and bryozoans may help clarify relationships among the lophophorates. In the current study, the innervation of the lophophore of the inarticulate brachiopod Lingula anatina is investigated by modern methods. The lophophore of L. anatina contains three brachial nerves: the main, accessory, and lower brachial nerves. The main brachial nerve is located at the base of the dorsal side of the brachial fold and gives rise to the cross neurite bundles, which pass through the connective tissue and connect the main and accessory brachial nerves. Nerves emanating from the accessory brachial nerve account for most of the tentacle innervation and comprise the frontal, latero-frontal, and latero-abfrontal neurite bundles. The lower brachial nerve gives rise to the abfrontal neurite bundles of the outer tentacles. Comparative analysis revealed the presence of many similar features in the organization of the lophophore nervous system in phoronids, brachiopods, and bryozoans. The main brachial nerve of L. anatina is similar to the dorsal ganglion of phoronids and the cerebral ganglion of bryozoans. The accessory brachial nerve of L. anatina is similar to the minor nerve ring of phoronids and the circumoral nerve ring of bryozoans. All lophophorates have intertentacular neurite bundles, which innervate adjacent tentacles. The presence of similar nerve elements in the lophophore of phoronids, brachiopods, and bryozoans supports the homology of the lophophore and the monophyly of the lophophorates.     

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.     

Cinefilms of particle capture by an induced local change of beat of lateral cilia of a bryozoan

A local disruption of the metachronal wave always accompanies capture of algal cells by tentacles of Flustrellidra hispida (Fabricius). Beat changes for ≈0.2 s over ≈100μm of the ciliated band during capture of a 10-μm particle. The halted parcel of water is therefore larger than the particle of food but much smaller than the flow that continues past the tentacles elsewhere. These events are consistent with the hypothesis that an induced local reversal of beat concentrates particles for those suspension feeders that retain particles upstream from a band of simple cilia (adults or larvae of bryozoans, brachiopods, phoronids, hemichordates, and echinoderms). These events are not explained by other hypotheses that have been advanced for concentration of particles by these suspension feeders. Aerosol filtration models of direct interception are not applicable to this type of ciliary suspension feeder because retention depends on the magnitude of a stimulus and response to it. The stimulus will not be the same function of diameter of the food particle, and response is unlikely below a threshold stimulus.     

Size and shape in the phylum Phoronida

The lophophorate phylum Phoronida consists of about 13 species, which differ in body length and width, number of longitudinal muscles, lophophore geometry and number of lophophore tentacles. In absolute terms large species have a larger body width, more tentacles, more longitudinal muscles and greater coiling of the lophophore than small species. However, size and shape analyses suggest that with increasing size: (I) the body surface area to volume ratio increases because body length increases faster than body width; (2) the relative number longitudinal muscles decreases, and (3) the relative feeding surface area of the lophophore decreases because tentacle diameter is constant while tentacle number increases at the same rate as body length and tentacle length increases more slowly than tentacle number. Coiling and spiraling of the lophophore in large species may be an attempt to compensate for this last relationship. We suggest that the habits, mode of growth and feeding mechanism of phoronids constrain size-related changes in shape.     

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.     

On the organization of the lophophore in phoronids (Lophophorata: Phoronida)

The organization of the lophophore is the main feature used for the identification of phoronid species. The structure of the lophophore and tentacles in seven phoronid species (Phoronis ovalis, P. ijimai, P. hippocrepia, P. svetlanae, P. australis, Phoronopsis harmeri, and Ph. malakhovi) collected in different areas of the World Ocean was studied. Two new patterns of the phoronid lophophore structure were found: “transition to horseshoe-shaped” (as in P. ovalis from Aniva Bay and in P. ijimai from the coast of Iturup Island, Sea of Okhotsk) and “transition to spiral” (in burrowing specimens P. hippocrepia from Aniva Bay, P. svetlanae and Ph. harmeri from Vostok Bay, Sea of Japan). For the first time it was shown that phoronid species with different types of the lophophore structure possess different kinds of tentacles. Thus, five types of phoronid tentacles were identified that vary in the shape of their cross section: rounded, oval, ellipsoid, rectangular, and skittle-shaped. A correlation was found between lophophore organization and the type of tentacles in phoronids. A table of the correlation between body size, lophophore organization, tentacle structure, and mode of life in different phoronid species is proposed.     

Architecture and function of the lophophore in the problematic brachiopod Heliomedusa orienta (Early Cambrian, South China)

The detailed structure of the lophophore is a key diagnostic character in the definition of higher brachiopod taxa. The problematic Heliomedusa orienta Sun and Hou, from the Lower Cambrian Chengjiang Lagerstätte of Yunnan, southwestern China, has a well-preserved lophophore, which is unlike that of any known extant or extinct brachiopods. Based on a comparative study of lophophore disposition in H. orienta and the extant discinid Pelagodiscus atlanticus, the in- and excurrent pattern and shell orientation of H. orienta are described and discussed. Reconstructions of lophophore shape and function are based on numerous specimens and comparison with P. atlanticus. The lophophore is composed of a pair of lophophoral arms that freely arch posteriorly rather than coiling anteriorly as commonly seen in fossil and recent lingulids. The lophophore is attached to the dorsal lobe of the mantle; it has neither calcareous nor chitinous supporting structures, and is disposed symmetrically on either side of the valve midline. The mouth can be inferred to be located at the base of the two brachial tubes, slightly posterior to the anterodorsal projection of the body wall. The lophophoral arms bear laterofrontal tentacles with a double row of cilia along their lateral edge, as in extant lingulid brachiopods. The main brachial axes are also ciliated, which presumably facilitated transport of mucous-bound nutrient particles to the mouth. The unique organization of the lophophore in Heliomedusa is not like any known fossil and living brachiopods. This clearly demonstrates that H. orienta is not a member of any crown group. It is here considered as a member of the brachiopod stem group, which challenges recent interpretations of a close discinid affinity.     

Polymorphism and vestigiality: comparative anatomy and morphology of bryozoan avicularia

Avicularia are polymorphic zooids characteristic of cheilostome bryozoans. Avicularia are assumed to have a defensive role yet ascertaining the presence of sensory structures to support this theory has been overlooked. We examine palatal morphology of the avicularia from five species of cheilostome bryozoans and compare the ultrastructural anatomy of the avicularia from two bugulid species from different habitats. SEM analysis revealed an array of palatal morphologies. Small tufts of cilia emerge from the orifice in the palate of the avicularia of Tricellaria catalinensis, Arachnopusia unicornis and Catenicella pseudoelegans. A ciliated vestigial polypide emerges from the orifice in the palate of Rhynchozoon zealandicum and comprises eleven papillae, or vestigial tentacles, seven of which are covered in microvilli. The vestigial polypide of the bird’s head avicularium of the cosmopolitan Bugula flabellata consists of a mass of ciliated and unciliated cells containing numerous granular vesicles. The avicularium of B. flabellata is capable of detecting tactile stimulation by virtue of the tuft of sensory cilia and is proactive in the capture of invertebrate epibionts. In contrast, in the deep-sea Nordgaardia cornucopioides, the vestigial polypide consists of a ciliated vestigial tentacle encased by glandular secretory cells. Avicularia possess structures derived from a feeding autozooid, and we show how the homologous structures have evolved and suggest that avicularia have been modified to carry out a variety of specific functions.     

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.