Degradation of the radula in the snails Biomphalaria glabrata say and Limnaea stagnalis L. (Gastropoda,Pulmonata)

Summary The radula of snails is formed at the posterior end of the radular gland or pocket, and degraded at the same rate at its anterior end. Degradation is due to different secretory activities of the inferior epithelium of the radular gland. Its secretions seem to degrade enzymatically the matrix of the radular membrane and basal plates of teeth, leaving only chitin containing microfibres and degradation products. The sclerotized parts of the teeth remain unchanged, but as they are now only loosely connected with the radular membrane. they are torn off easily during feeding movements. The rest of the degraded and frayed radular membrane and the subradular membrane are also lost by abrasion during feeding. The cells of the inferior epithelium are connected with each other by septate desmosomes and an elaborate system of deep lateral interdigitation which may provide tensile strength. Extrusion of degraded cells of the inferior epithelium into the subradular membrane takes place, although the thick basal lamina forms a continuous sheath which is closely adjoined to the basal parts of the inferior epithelium. Nerve fibres containing vesicles with electron dense neurosecretory material (deduced from the diameter of 200–250 nm) are attached to this sheath or penetrate into it; they may be involved in the regulation of production and degradation processes during radula replacement. Problems of the forward transport of radula and inferior epithelium are discussed.     

Original molluscan radula: comparisons among Aplacophora,Polyplacophora, Gastropoda,and the Cambrian fossil Wiwaxia corrugata

As the original molluscan radula is not known from direct observation, we consider what the form of the original radula may have been from evidence provided by neomenioid Aplacophora (Solenogastres), Gastropoda, Polyplacophora, and the Cambrian fossil Wiwaxia corrugata (Matthews). Conclusions are based on direct observation of radula morphology and its accessory structures (salivary gland ducts, radular sac, anteroventral radular pocket) in 25 species and 16 genera of Aplacophora; radula morphogenesis in Aplacophora; earliest tooth formation in Gastropoda (14 species among Prosobranchia, Opisthobranchia, and Pulmonata); earliest tooth formation in four species of Polyplacophora; and the morphology of the feeding apparatus in W. corrugata. The existence of a true radula membrane and of membranoblasts and odontoblasts in neomenioids indicates that morphogenesis of the aplacophoran radula is homologous to that in other radulate Mollusca. We conclude from p redness of salivary gland ducts, a divided radular sac, and a pair of anteroventral pockets that the plesiomorphic state in neomenioids is bipartite, formed of denticulate bars that are distichous (two teeth per row) on a partially divided or fused radula membrane with the largest denticles lateral, as occurs in the genus Helicoradomenia. The tooth morphology in Helicoradomenia is similar to the feeding apparatus in W. corrugata. We show that distichy also occurs during early development in several species of gastropods and polyplacophorans. Through the rejection of the null hypothesis that the earliest radula was unipartite and had no radula membrane, we conclude that the original molluscan radula was similar to the radula found in Helicoradomena species.     

Radula formation in two species of Conoidea (Gastropoda)

The radula is the basic feeding structure in gastropod molluscs and exhibits great morphological diversity that reflects the exceptional anatomical and ecological diversity occurring in these animals. This uniquely molluscan structure is formed in the blind end of the radular sac by specialized cells (membranoblasts and odontoblasts). Secretion type, and the number and shape of the odontoblasts that form each tooth characterize the mode of radula formation. These characteristics vary in different groups of gastropods. Elucidation of this diversity is key to identifying the main patterns of radula formation in Gastropoda. Of particular interest would be a phylogenetically closely related group that is characterized by high variability of the radula. One such group is the large monophyletic superfamily Conoidea, the radula of which is highly variable and may consist of the radular membrane with five teeth per row, or the radular membrane with only two or three teeth per row, or even just two harpoon-like teeth per row without a radular membrane. We studied the radulae of two species of Conoidea (Clavus maestratii Kilburn, Fedosov & Kantor, 2014 [Drilliidae] and, Lophiotoma acuta (Perry, 1811) [Turridae]) using light and electron microscopy. Based on these data and previous studies, we identify the general patterns of the radula formation for all Conoidea: the dorsolateral position of two groups of odontoblasts, uniform size, and shape of odontoblasts, folding of the radula in the radular sac regardless of the radula configuration. The morphology of the subradular epithelium is most likely adaptive to the radula type.     

Experimental and comparative morphology of radula renewal in pulmonates (Mollusca,Gastropoda)

Summary The continuous renewal of the pulmonate radula and the histology and regeneration of its concomitant epithelia were studied by light and electron microscopy, autoradiography and electron microprobe analysis. The two species investigated show histological differences and the results were compared with those of a preceding study on a prosobranch radula. The radula is an intricate cuticular structure of the foregut. Only the fully grown part, which is active during feeding, lies in the buccal cavity while it is constantly renewed by the coordinated cooperation of specialized cells forming the radular sheath. The end of the sheath is occupied by cells which produce the organic matrix of the radula. In taeniogloss prosobranchs, seven multicellular cushions of small odontoblasts lie at the end of the sheath and produce the seven teeth of each cross-row. In pulmonates, the multidenticular radula is generated by numerous groups of a few voluminuous cells. Despite these histological differences, prosobranchs and pulmonates generate the radula matrix by microvilli, cytoplasmatic protrusions and apocrine secretions. The epithelia of the radular sheath contribute to the transport, tanning and mineralization of the radula. The concomitant epithelia are replaced in limited proliferation zones at the end of the radular sheath and their cells migrate anteriorly to the buccal cavity. The ultrastructure of the sheath cells and the alterations which they undergo in connection with their functions are discussed. The proliferation zone of the superior epithelium is strictly confined and the cells move together with the radula forward. In prosobranchs, the cells of the superior epithelium begin to degenerate in the middle of the radular sheath and the entire epithelium is simply extruded into the buccal cavity. In pulmonates, the opening of the radular sheath is closed by the cuticular collostylar hood which is generated by a distinct epithelium which is proved to be stationary. When leaving the proliferation zone, the superior epithelium differentiates into supporting cells and mineralizing cells; the latter cause the hardening of the radular teeth and already degenerate in the middle of the sheath. The whole superior epithelium degenerates at the border to the collostylar hood-epithelium. In Lymnaea the degeneration zone is strictly confined whereas in Cepaea the collostylar hood and its generating epithelium extend into the radular sheath and the degeneration zone ranges over a distance of 3–5 rows of teeth. The proliferation zone of the inferior epithelium extends over the posterior half of the radular sheath, but the replacement rate is much lower than in the superior epithelium. Although the inferior epithelium carries the radula, it migrates slower than the radula. Obviously the radula has to be transported actively by apical protrusions of the cells, which penetrate into the radular membrane. At the opening of the radular sheath the inferior epithelium generates the adhesive layer and degenerates. During feeding, the adhesive layer has to maintain the firm mechanical connection between radula and distal radular epithelium. Autoradiographic experiments demonstrate that the distal radular epithelium is stationary. Nevertheless, the radula is known to advance to its degeneration zone. Special attention is paid to this problem. We strongly suspect that the transport of the adhesive layer and the radula is based on pseudopodial movements of apical protrusions characteristic for the distal radular epithelium. These protrusions interdigitate with the lower face of the adhesive layer. The mechanical connection has to be maintained and so the respective structures (tonofilaments and hemi-desmosomes) have to be continually renewed. This needs a high amount of energy and obviously results in the conspicuous concentration of mitochondria near the apical surface.Abbreviations al adhesive layer - ax axon - bc buccal cavity - bce buccal cavity epithelium - bl basal layer - bla basal labyrinth - bm basal membrane - bp basal plate - bpc basal plate cell - c cilia - ch collostylar hood - che collostylar hood-epithelium - cl cuticular layer - col collostyle - cr cell remnant - cts connective tissue sheath - d desmosome - dl upper layer - dre distal radular epithelium - dz degeneration zone - fe front edge - g granula - gol dictyosome - hd hemidesmosome - hl haemolymph - ie inferior epithelium - j jaw - ma tooth matrix - mc mineralizing cell - mem membranoblast - mfb microfibrills - mfl microfilaments - mgb multigranular body - mi mitochondria - mit mitosis - ml middle layer - mt microtubuli - mv microvilli - mw membrane whirl - n nucleus - nc necrotic cluster - nf nerve fibres - nsg neurosecretory granula - o odontophor - od odontoblast - odg odontoblast group - pod pre-odontoblast - rb residual body - rer rough endoplasmatic reticulum - rm radular membrane - rt radula teeth - sc supporting cell - se superior epithelium - sj septate junction - sro subradular organ - ss secretion substance - tf tonofilaments - tsm supramedian tensor muscle - tw terminal web - v vacuole - ves vesicle     

Main patterns of radula formation and ontogeny in Gastropoda

Gastropoda is morphologically highly variable and broadly distributed group of mollusks. Due to the high morphological and functional diversity of the feeding apparatus gastropods follow a broad range of feeding strategies: from detritivory to highly specialized predation. The feeding apparatus includes the buccal armaments: jaw(s) and radula. The radula comprises a chitinous ribbon with teeth arranged in transverse and longitudinal rows. A unique characteristic of the radula is its continuous renewal during the entire life of a mollusk. The teeth and the membrane are continuously synthesized in the blind end of the radular sac and are shifted forward to the working zone, while the teeth harden and are mineralized on the way. Despite the similarity of the general mechanism of the radula formation in gastropods, some phylogenetically determined features can be identified in different phylogenetic lineages. These mainly concern shape, size, and number of the odontoblasts forming a single tooth. The radular morphology depends on the shape of the formation zone and the morphology of the subradular epithelium. The radula first appears at the pre- and posttorsional veliger stages as an invagination of the buccal epithelium of the larval anterior gut. The larval radular sac is lined with uniform undifferentiated cells. Each major phylogenetic lineage is characterized by a specific larval radula type. Thus, the docoglossan radula of Patellogastropoda is characterized by initially three and then five teeth in a transverse row. The larval rhipidoglossan radula has seven teeth in a row with differentiation into central, lateral, and marginal teeth and later is transformed into the adult radula morphology by the addition of lateral and especially marginal teeth. The taenioglossan radula of Caenogastropoda is nearly immediately formed in adult configuration with seven teeth in a row.     

Histology and regeneration of the radula of Pomacea bridgesi (Gastropoda,Prosobranchia)

Summary Histology, physiological regeneration, and degradation of the taenioglossan prosobranch radula and its concomitant epithelia were studied by light and electron microscopy (TEM, SEM), electron microprobe analysis, and autoradiography. Taenioglossa have seven multicellular odontoblastic cushions which produce the tooth matrix by apocrine secretion; many long microvilli are also incorporated. In contrast to pulmonates, the odontoblasts of prosobranchs are capable of division, and their mitoses contribute to the expansion of the cushions, but presumably also to the displacement of degenerating odontoblasts. The seven cushions are isolated from each other by separation cells. The radular membrane is produced from microvilli of membranoblasts and a substance secreted at the base of microvilli.Strands of the supraradular epithelium subsequently move in between the teeth and finally enclose them completely. They effect the hardening and mineralization of the teeth. The strands move together with the radula towards the anterior and are extruded at the opening of the radular sheath; their degeneration, however, has already started in the middle section of the sheath. Epithelial cells are produced by two completely separated mitotic centres which lie dorsolaterally at the end of the sheath.In the subradular epithelium, mitotic activity is scattered over the posterior half of the sheath but is not found in the region where the supramedian radula tensor muscle is inserted. The epithelial cells move forward, but at a much lower rate than the radula. At the opening of the sheath the subradular membrane is generated, while cells of the subradular epithelium lying between the lamellae of the subradular membrane are extruded.The subradular membrane is limited to the functional part of the radula. It is situated on the distal radular epithelium, which is obviously not a continuation of the subradular epithelium. In test animals treated with tritiated thymidine, there is a very strong stationary centre of labeled cells at the beginning of the epithelium, but so far no mitoses have been found in this centre and the labeled cells do not move on continually. In the middle of the distal epithelium mitoses do occur, and the labeled cells permit the assumption that these cells do not migrate at all to the anterior end. At least in Prosobranchia, the distal radular epithelium does not transport the radula to its degradation zone. The transport mechanism for the radula is still unknown.     

A New Genus and Species of the Family Anulidentaliidae (Scaphopoda: Dentaliida) and its Systematic Implications

Shell, radula, and anatomy of Epirhabdoides ivanovi new genusand species are described from a sample of the Russian VitjazExpedition from the Japan Trench. It is distinguished from thesimilar Laevidentalium sominium by shell morphometrics and radulamorphology. The radula is almost identical with that of Anulidentaliumbambusa (Anulidentaliidae). The anatomy of the mantle margins,however, including dorsolateral slits at the anterior mantlemargin and a connective tissue bolster at the posterior mantleopening is that of the family Rhabdidae. In a parsimony analysisEpirhabdoides ivanovi takes an intermediate position betweena basal grade of Gadilinidae and the remaining Dentaliida implyingconvergent evolution of mantle characters. An alternative butless parsimonious tree with E. ivanovi as sister taxon to Rhabdusrequires convergences in radula characters. This is the firstdocumented case of convergent anatomical features among Scaphopodaand enhances the need of radula and soft part investigationof the conchologically little informative, smooth-shelled dentaliidgroups. (Received 16 March 1998; accepted 1 June 1998)     

海洋软体动物齿舌中磁铁物的研究

在扫描电镜下对红条毛肤石鳖（ Ａｃａｎｔｈｏｃｈｉｔｏｎ ｒｕｂｒｏｌｉｎｅａｔｕｓ Ｌｉｓｃｈｋｅ） 齿舌进行了观察,用原子力和磁力显微镜及超导量子干涉器（ＳＱＵＩＤ） 式磁强计对齿舌中的磁铁物Ｆｅ３ Ｏ４ 进行了分析和测量,实验证明齿舌中含有磁铁物Ｆｅ３ Ｏ４ ,每个齿舌约含Ｆｅ３ Ｏ４ ０．２ ｍｇ ,占齿舌重量的１５ ％ ,矿物重量的４０％ ,磁化强度约为０．０２×１０ － ３ Ａｍ２／ 个,相当于１４Ａｍ２／ｋｇ ,并且磁铁物主要存在于第一侧齿的齿尖上,同时Ｆｅ３ Ｏ４ 晶体在齿片表面上的排列及磁畴结构具有方向性。     

A molecular palaeobiological hypothesis for the origin of aplacophoran molluscs and their derivation from chiton-like ancestors

Aplacophorans have long been argued to be basal molluscs. We present a molecular phylogeny, including the aplacophorans Neomeniomorpha (Solenogastres) and Chaetodermomorpha (Caudofoveata), which recovered instead the clade Aculifera (Aplacophora + Polyplacophora). Our relaxed Bayesian molecular clock estimates an Early Ordovician appearance of the aculiferan crown group consistent with the presence of chiton-like molluscs with seven or eight dorsal shell plates by the Late Cambrian (approx. 501-490 Ma). Molecular, embryological and palaeontological data indicate that aplacophorans, as well as chitons, evolved from a paraphyletic assemblage of chiton-like ancestors. The recovery of cephalopods as a sister group to aculiferans suggests that the plesiomorphic condition in molluscs might be a morphology similar to that found in monoplacophorans.     

Magnetic anisotropy of the radula of chiton Acanthochiton rubrolinestus LISCHKE

The magnetic anisotropy of the whole radula, the major lateral radula teeth, and magnetic material in the major lateral radula teeth of the chiton Acanthochiton rubrolinestus LISCHKE have been studied by a magnetic torque meter and superconducting quantum interference device (SQUID) magnetometer. The length and width axes of the teeth are the easily magnetized axes, while the thickness axis is difficult to magnetize. The width and thickness axes of the radula are the easily magnetized axes, and the length axis is difficult to magnetize. The measurement results of the whole radula and the major lateral radula teeth agree well with each other. The magnetic anisotropy of the magnetic material is given as well as a possible distribution of the magnetic material in the major lateral radula teeth.     

The digestive tract of Helicoradomenia (Solenogastres, Mollusca), aplacophoran molluscs from the hydrothermal vents of the East Pacific Rise

Abstract. Species of Helicoradomenia are constantly found at hydrothermal vent sites of the eastern and western Pacific Ocean. The digestive tract of 2 species of the genus was investigated with special focus on the ultrastructure and histochemistry of epithelia and glandular organs. The preoral cavity and foregut epithelia are composed of microvillous main cells, secretory cells producing protein-rich substances, and sensory cells with specialized cilia. The foregut bears a pair of glands with 3 types of extremely long-necked glandular cells surrounded by musculature. Each glandular cell opens directly into the radula pocket without a gland duct. The large radula apparatus consists of pairs of denticulated bars resting on a flexible radular membrane without elaboration of a subradular membrane. The midgut has a narrow, mid-dorsal tract of ciliary cells, but most of the epithelium is composed of digestive cells with a highly developed lysosomal system. The hindgut is lined by ciliated cells and free of glands. The foregut and radula seem to be highly efficient in the capture of relatively large, motile prey. Food contents within the midgut lumen and within some of the large secondary lysosomes indicate a triploblastic metazoan prey of non-cnidarian origin. The digestive tract is not adapted to microvory and there is no indication of a symbiosis with chemoautotrophic bacteria.     

Ciliary ultrastructure of neomeniomorphs (Mollusca, Neomeniomorpha = Solenogastres)

Abstract. The ultrastructure of the ciliary apparatus of multiciliated epidermal cells of the trochophore of Epimenia babai and the adult of Strophomenia scandens was studied. The trochal cirri of E. babai consists of long cilia with unspecialized tips. The surfaces between the trochs are sparsely covered with shorter cilia of similar structure except for length. In the adult of S. scandens , the foot is covered by a dense mat of cilia with blunt electron-dense tips. In both E. babai and S. scandens , all cilia have two perpendicularly orientated rootlets. This condition is similar to that of the Chaetodermomorpha (=Caudofoveata) and Polyplacophora. In other molluscs studied to date, the cilia of multiciliated epidermal cells have a single rootlet or a derivative thereof. The presence of two ciliary rootlets likely represents the basal plesiomorphic state for the Bilateria. The existence of this character in the Neomeniomorpha, Chaetodermomorpha, and Polyplacophora is congruent with the hypothesis of a basal position of these taxa within the Mollusca.     

The salivary papilla of Octopus as an accessory radula for drilling shells

Removal of much of the functional region of the radula of Octopus vulgaris does not prevent the animal from drilling holes in mollusc shells. These drilling activities are carried out by a salivary papilla that lies just below the radula. The papilla is muscular and its anterior face is covered with very small teeth. It is now possible to say that the salivary papilla can function as an accessory radula.     

Mouthparts of the Burgess Shale fossils Odontogriphus and Wiwaxia: implications for the ancestral molluscan radula

The Middle Cambrian lophotrochozoans Odontogriphus omalus and Wiwaxia corrugata have been interpreted as stem-group members of either the Mollusca, the Annelida, or a group containing Mollusca + Annelida. The case for each classification rests on the organisms' unusual mouthparts, whose two to three tooth-rows resemble both the molluscan radula and the jaws of certain annelid worms. Despite their potential significance, these mouthparts have not previously been described in detail. This study examined the feeding apparatuses of over 300 specimens from the 505-million-year-old Burgess Shale, many of which were studied for the first time. Rather than denticulate plates, each tooth row comprises a single axial tooth that is flanked on each side by eight to 16 separate shoehorn-shaped teeth. Tooth rows sat on a grooved basal tongue, and two large lobes flanked the apparatus. New observations-the shape, distribution and articulation of the individual teeth, and the mouthparts' mode of growth-are incompatible with an annelid interpretation, instead supporting a classification in Mollusca. The ancestral molluscan radula is best reconstructed as unipartite with a symmetrical medial tooth, and Odontogriphus and Wiwaxia as grazing deposit-feeders.     

THE FINE STRUCTURE OF THE TRANSITIONAL EPITHELIUM OF RAT URETER

The fine structure of the transitional epithelium of rat ureter has been studied in thin sections with the electron microscope, including some stained cytochemically to show nucleoside triphosphatase activity. The epithelium is three to four cells deep with cuboidal or columnar basal cells, intermediate cells, and superficial squamous cells. The basal cells are attached by half desmosomes, or attachment plates, on their basal membranes to a basement membrane which separates the epithelium from the lamina propria. Fine extracellular fibres, ca. 100 A in diameter, are to be found in the connective tissue layer immediately below the basement membrane of this epithelium. The plasma membranes of the basal and intermediate cells and the lateral and basal membranes of the squamous cells are deeply interdigitated, and nucleoside triphosphatase activity is associated with them. All the cells have a dense feltwork of tonofilaments which ramify throughout the cytoplasm. The existence of junctional complexes, comprising a zonula occludens, zonula adhaerens, and macula adhaerens or desmosome, between the lateral borders of the squamous cells is reported. It is suggested that this complex is the major obstacle to the free flow of water from the extracellular spaces into the hypertonic urine. The free luminal surface of the squamous cells and many cytoplasmic vesicles in these cells are bounded by an unusually thick plasma membrane. The three leaflets of this unit membrane are asymmetric, with the outer one about twice as thick as the innermost one. The vesicles and the plasma membrane maintain angular conformations which suggest the membrane to be unusually rigid. No nucleoside triphosphatase activity is associated with this membrane. Arguments are presented to support a suggestion that this thick plasma membrane is the morphological site of a passive permeability barrier to water flow across the cells, and that keratin may be included in the membrane structure. The possible origin of the thick plasma membrane in the Golgi complex is discussed. Bodies with heterogeneous contents, including characteristic hexagonally packed stacks of thick membranes, are described. It is suggested that these are "disposal units" for old or surplus thick membrane. A cell type is described, which forms only 0.1 to 0.5 per cent of the total cell population and contains bundles of tubular fibres or crystallites. Their origin and function are not known.     

红条毛肤石鳖齿舌主侧齿中铁还原酶分布及与生物矿化的关系

以红条毛肤石鳖Acanthochiton rubrolineatus(Lischke)齿舌为材料,通过切片和酶组织化学技术,在光镜和电镜下对齿舌主侧齿的微结构及高铁还原酶的存在进行观察,从微观角度了解齿舌主侧齿齿尖的矿化机理。结果显示,成熟主侧齿由齿尖和齿基组成。齿尖结构由外至内分为三层,最外层为磁铁矿层,前后齿面磁铁矿层的厚度不等,后齿面约50μm,前齿面约5-10μm。向内依次为棕红色的纤铁矿层,厚约10μm,及略显黄色的有机基质层,有机基质层占据着齿尖内部的大部分结构。高分辨透射电镜下显示磁铁矿由条状四氧化三铁颗粒组成,长约2-3μm,宽约100-150nm。齿舌的矿化是一个连续过程,不同部段处于不同的矿化阶段,齿舌囊上皮细胞沿囊腔分布,并形成齿片。未矿化的新生主侧齿齿尖中存在由有机基质构成的网状结构。随矿化的进行,有机基质内出现矿物颗粒。初始矿化的齿尖外表面有一个细胞微突层,微突的另一端为囊上皮细胞,矿物质经由微突层达齿尖并沉积于有机基质中,齿尖随之矿化并成熟。初始矿化齿尖的外围有大量的三价铁化物颗粒,稍成熟的齿尖外围同时还出现二价铁化物。新生或初始矿化主侧齿齿尖外围的囊上皮细胞中有大量球形类似于铁蛋白聚集体的内容物,直径0.6-0.8μm,球体由膜包围。齿舌囊上皮组织中存在三价高铁还原酶,此酶分布于上皮细胞的膜表面,可能与齿尖表面磁铁矿的生成有一定的关系。       

Variation in radular teeth and acuspid side of the radula in Lacuna pallidula, L. parva and L. vincta (Gastropoda: Littorinidae) from the Isle of Wight, United Kingdom

The variation in the radula of three species of Lacuna has been investigated and the back of the rachidian tooth is proposed as providing a new character set of potentially high taxonomic value. The term basal plate is introduced for the back of the rachidian tooth. Cusp and tooth morphology are closely related to diet and wear, and are subject to considerable homoplasy, whereas the structure of the basal plate of the rachidian tooth provides a more neutral character set. The difference in this character set between the lacunids has been quantified using seven measurements and the exploratory multivariate statistical procedure principal component analysis. The basal plate of the rachidian tooth showed interspecific differences. The taxonomic value of this new character set should be evaluated in further studies of other prosobranchs. Received in revised form: 25 October 2000 Electronic Publication     

A novel respiratory architecture in the Silurian mollusc Acaenoplax

Extant aplacophorans, a group of shell‐less vermiform molluscs, respire through appendages within or projecting from a posterior cavity. Respiratory structures differ between the subclasses Caudofoveata (ctenidia within the cavity) and Solenogastres (folds of the mantle itself). Acaenoplax hayae, a Silurian vermiform mollusc from the Herefordshire Lagerstätte, England, exhibits characteristics of both these groups. While recent work places it within the crown group Aplacophora, near the caudofoveates, initial observations suggested that its respiratory structures were closer to those of the solenogastres. Here, we present new reconstructions of the posterior of Acaenoplax prepared with the aim of resolving features obscured when prior studies were undertaken. These reconstructions detail a novel posterior architecture, not closely comparable to that of either extant aplacophoran group, in which respiratory projections arise from a membrane that partly encloses a central posterior cavity. The posterior membrane is flanked by small spherical projections; both membrane and spherical projections are apparently unique within the Aplacophora. The existence of this previously undocumented respiratory system underlines the diversity of the aplacophoran clade during the Palaeozoic.     

THE MORPHOLOGY OF TOXOGLOSSAN GASTROPODS LACKING A RADULA, WITH A DESCRIPTION OF NEW SPECIES AND GENUS OF TURRIDAE

The morphology of some deep-sea Turridae lacking radulae wasstudied. The main features of their digestive system are theabsence of a radula sac, venom and salivary glands and the reductionor absence of a proboscis. A new genus Teretiopsis including3 new species and T. thaumastopsis (Dautzenberg and Fischer,1896) is described. On the basis of differences of the digestivesystem when compared with other turrids lacking radulae themonotypical subfamily Taraninae Casey, 1904 new status (typegenus Taranis Jeffreys, 1870) is considered. It is shown thatthe process of radula reduction has occurred independently indifferent phylogenetic lines of Toxoglossa—the subfamiliesDaphnellinae and Taraninae among Turridae and the family Terebridae. (Received 8 August 1988; accepted 20 December 1988)