The structure of the intrapigmented aesthetes and the properiostracum layer in Callochiton achatinus (Mollusca: Polyplacophora)

The ultrastructure of the intrapigmented aesthetes which occur in specific areas of the different valves of Callochiton achatinus are described. The possible functions of the aesthete and the pigmented body are discussed in the light of their ultrastructure and the ecological requirements of the animal. The properiostracum in C. achatinus is unusually thick and has a structure very similar in appearance to that of collagen.     

Fertilization in Callochiton castaneus (Mollusca)

A fine-structural study of fertilization in Callochiton castaneus has revealed that the mechanism of sperm penetration into the egg is intermediate between the primitive condition found in members of the order Lepidopleurida and the more derived condition found in the Chitonida. C. castaneus sperm have the long needlelike nuclear filament and reduced acrosome that characterizes all Chitonida, but they have retained several plesiomorphic features such as an unspecialized mid-piece and a lack of flagellar reinforcement. As in some Lepidopleurida but unlike any Chitonida, the egg hull in this species comprises a thick, smooth jelly coat permeated by pores that permit sperm rapid access to the vitelline layer. The jelly coat is delicate and quickly dissolves when a sperm concentrate is used, suggesting that excess acrosomal enzymes may be responsible. Once the sperm have penetrated the vitelline layer, the long nuclear filament bridges the gap to cups in the egg membrane. However, once the fertilization membrane is raised, the perivitelline space exceeds the length of the nuclear filament, preventing other sperm from penetrating the egg. A fertilization cone forms around the nuclear filament of the penetrating sperm, but it does not appear to engulf the body of the sperm. Rather, the nuclear chromatin is injected into the egg as a long thread. The remaining sperm organelles are apparently abandoned on the egg surface. If this is the case, it would be a significant departure from fertilization in other molluscs and many other metazoans, in which sperm organelles, such as centrioles and mitochondria, enter the egg. New sperm and egg characters, as well as significant differences in fertilization, indicate that Callochitonidae are basal to all other members of the order Chitonida and may warrant separation as the sister taxon to the suborders Chitonina and Acanthochitonina.     

The ultrastructure of the aesthetes of Leptochiton asellus (Polyplacophora: Lepidopleurina)

The ultrastructure of the aesthetes of Leptochiton asellus is described. Comparisons are made with the ultrastructure of the aesthetes of other chiton species and a number of unusual features are highlighted. In particular the perforated structure of the subsidiary caps and the extensive areas of banded molecular strands are discussed.     

Fine structure of the eyes of Onithochiton neglectus (Mollusca: Polyplacophora)

Summary Onithochiton neglectus a common littoral chiton possesses large numbers of small eyes embedded in the outer layer of the shell, the tegmentum. These are arranged in a definite pattern on each shell valve. Each eye lies in a pocket, and is surrounded by pigment laid down in the shell. There is a lens, cup of retina cells and an optic nerve running in an optic canal through the shell. Glial elements are present. The retina cells give rise centrally to a packed array of microvilli, a rhabdom. Cilia are present at the edge of the rhabdom; they have a 9 + 2 arrangement of ciliary filaments and do not appear to be involved in the formation of microvilli. Cells at the periphery of the eye cup give rise to large whorls of membranes, lamellate bodies. These bodies are derived from the membranes of cilia having a 9 + 2 pattern, and form into an extra-cellular space. Nerve processes from the retina cells pass into the optic canal. On the basis of previous work it is thought that the lamellate bodies are also sensory. These structures are discussed in relation to other microvillar and lamellate structures described from photoreceptors.I thank Professor J. E. Morton for his advice in the early stages of this work, and Dr. S. J. Bullivant for the fixation and embedding of material for electron microscopy. To Professor G. A. Horridge I am grateful for advice and the facilities of his laboratory, and to Professor M. S. Laverack, Patricia Holborow and Charles Coleman for much help and encouragement. I am supported by the Science Research Council, and in New Zealand held a Commonwealth Scholarship.     

The valve morphology of Cullochhiton uchutinus (Mollusca: Polyplacophora: Ischnochitonidae)

The valves of Callochiton achatinus comprise four layers which are penetrated by channels carrying extensions of the mantle epithelium known as aesthetes. Three patterns of dorsal sculpturing within defined areas of the various valves are described and these are related to the different types of aesthete channel which perforate the respective areas. The processes involved in valve growth are described and related to the arrangement of the many holes penetrating the eave tissue.     

The chiton gill: Ultrastructure in Chiton olivaceus (Mollusca,Polyplacophora)

Gills of Chiton olivaceus, a primitive mollusc, are relatively simple in their structure and ultrastructure but are well adapted to a life in the intertidal zone. In contrast to some other molluscs, there is no differentiation of the gill epithelium into functional regions other than respiratory ones. Ciliation of the epithelium in certain areas may optimize water flow from the outer to the inner part of the mantle cavity. The hemolymph sinuses are oriented so that hemolymph flows in the opposite direction. Interstitial cells link epithelial cells with nerve endings. Muscle cells as well as the collagenous matrix in the connective tissue differ within the main gill axis and the lateral lamellae. The life cycle of immunoactive cells within the connective tissue and the hemolymph is described.     

The infrastructure of aesthetes in Tonicella marmorea (Polyplacophora; Ischnochitonina) and a new functional hypothesis

The valve surface morphology and aesthete ultrastructure of Tonicella marmorea is described. The possible functions of these very numerous organs are considered with particular reference to the ecological needs of the animal and their similarity to other invertebrate organs. A periostracum-secreting function is proposed as an alternative to the more commonly postulated light receptor theory.     

Fine structure of the subradular Organ of Lepidochitona cinereus (L), (Mollusca,Polyplacophora)

Electron microscopy of the subradular organ of the chiton Lepidochitona cinereus (L) reveals at least three cell types, microvillous, ciliated and mucus-secreting, situated in a single epithelium. The base of the epithelium is abundantly innervated and supplied with muscle cells. The fine structure is consistent with a chemosensory function for the subradular organ.     

The heart Ultrastructure of lepidopleurus asellus (Spengler) and Tonicella marmorea (Fabricius) (Mollusca: Polyplacophora)

Summary The ultrastructure of the auricles, the ostia, and the ventricle of L. asellus and T. marmorea is described. The heart wall consists of an epicardium, a basement membrane, and an inner loose myocardium. The epicardial cells of the auricle are podocytes. The exposed cell body and the branched processes show pedicles. Ventricular epicardium is flat and simple. The slender, unbranched, mononucleated muscle fibres have a peripheral nucleus located midway along the fibre. Mitochondria are peripherally located, leaving the center to longitudinally running thick and thin myofilaments. Dense bodies and attachment plaques make up the Z-material. Sarcomeres and myofibrils are absent, as are transverse tubules and intercalated disks. The sarcoplasmic reticulum consists of peripheral tubules and subsarcolemmal cisternae, some of which radiate, branch, and run between myofilaments. Couplings are lacking. Ventricular fibres in T. marmorea show nexuses and desmosomes; in L. asellus only nexuses. The muscular ostia are tubular, and muscle fibres resemble those of the ventricle; nexuses are detected in T. marmorea and desmosomes in L. asellus. The only nervous elements observed are some nerve processes, structurally similar to those of other molluscs.Supported by grants from the Norwegian Research Council for Science and Humanities     

Chiton integument: Ultrastructure of the sensory hairs of Mopalia muscosa (Mollusca: Polyplacophora)

Summary The dorsal integument of the girdle of the chiton Mopalia muscosa is covered by a chitinous cuticle about 0.1 mm in thickness. Within the cuticle are fusiform spicules composed of a central mass of pigment granules surrounded by a layer of calcium carbonate crystals. Tapered, curved chitinous hairs with a groove on the mesial surface pass through the cuticle and protrude above the surface. The spicules are produced by specialized groups of epidermal cells called spiniferous papillae and the hairs are produced by trichogenous papillae. Processes of pigment cells containing green granules are scattered among the cells of each type of papilla and among the common epidermal cells.The wall or cortex of each hair is composed of two layers. The cortex surrounds a central medulla that contains matrix material of low density and from 1 to 20 axial bundles of dendrites. The number of bundles within the medulla varies with the size of the hair. Each bundle contains from 1 to 25 dendrites ensheathed by processes of supporting cells. The dendrites and supporting sheath arise from epidermal cells of the central part of the papilla. At the base of each trichogenous papilla are several nerves that pass into the dermis. Two questions remain unresolved. The function of the hairs is unknown, and we have not determined whether the sensory cells are primary sensory neurons or secondary sensory cells.     

Vertical distribution of chitons (Mollusca: Polyplacophora) in the rocky intertidal zone of central Chile

The patterns of vertical distribution of chitons in various exposed rocky shore habitats of central Chile are described. The density and size of the individuals were determined at different levels on intertidal rock walls, in pools and on boulders. A group of four species was found on rock walls while another group of four species was found in tidal pools and on boulders. The species showed vertical separation on rock walls and segregation according to a range of increasing frequency of water movement in pools and on boulders. Only the two species with largest adult individuals (Acanthopleura echinata (Barnes) and Chitonlatus Sowerby) showed vertical segregation in size. Small-sized species and small individuals of large-sized species were found higher on rock walls or in zones where water movement is less; large chitons occur in zones of greatest water impact. This suggests that water movement may be a significant factor in setting the lower limits of distribution of these herbivores. Some of the species examined were extremely abundant (600·m^-2) with densities of one to two orders of magnitude greater than for other chiton species elsewhere.     

Ultrastructure of Follicular Epithelia in the Ovary of Lepidochitona cinerea (L.) (Mollusca: Polyplacophora)

In the sac-like ovary of the polyplacophoran mollusc, Lepidochitona cinerea , nutritive tissue arises from the ventral gonadal wall of the organ as prominent folds which support the oocytes during the various stages of their development. Each oocyte is enveloped by the follicular epithelium. Approximately twenty follicle cells surround one full-grown oocyte and by this late stage are connected to it and to each other by desmosomes. The follicle cells contain glycogen, Golgi dictyosomes, mitochondria, lipid droplets, numerous cisternae and vesicles of the rough endoplasmic reticulum, and various kinds of lysosomes. The nutritional function of these cells and their possible role forming the oocytic hulls is discussed.     

Diversity and microstructure of quitons (Mollusca: Polyplacophora) from the Caribbean of Costa Rica

Diversity and microstructure of quitons (Mollusca: Polyplacophora) from the Caribbean of Costa Rica. The polyplacophorans of the coral reef on the Caribbean coast of Costa Rica have been insufficiently studied. The examination of coral rubble accumulated in the shallow sublitoral waters on four collection stations in Provincia Limón revealed a higher diversity of chitons than was documented. From the country eight species were previously known: Ischnochiton erythronotus (C.B. Adams 1845); Ischnoplax pectinata (Sowerby 1840); Stenoplax boogii (Haddon 1886); S. purpurascens (C.B. Adams 1845); Acanthopleura granulata (Gmelin 1791); Chiton marmoratus Gmelin 1791; C. tuberculatus Linnaeus 1758 and Acanthochitona rhodea (Pilsbry 1893). This study added five more species that are reported here for the first time: Callistochiton portobelensis Ferreira 1976; Ischnochiton kaasi Ferreira 1987; I. pseudovirgatus Kaas 1972; Acanthochitona balesae Abbott 1954 and Cryptoconchus floridanus (Dall 1889).     

The junction zone: Initial site of mineralization in radula teeth of the chiton Cryptoplax striata (Mollusca: Polyplacophora)

Elemental composition and distribution in individual teeth of the whole radula of the chiton Cryptoplax striata were analyzed using energy-dispersive spectroscopy. Both the element deposited and its position within the tooth vary according to the stage of mineralization. The initial site of mineralization is the junction zone, the region between the tooth cusp and base. In this region, the first element to be deposited is iron, followed by phosphorus and then calcium. Iron deposition next commences in the tooth cusp cap, where it proceeds rapidly, being virtually complete within 12 tooth rows. By contrast, mineralization in the core of the tooth cusp does not commence until well down the radula and consists initially of iron and phosphorus with the addition of a small amount of calcium 6 rows later. While mineralization in the tooth base commences early in radula development, it continues right through to the fully mature end of the radula. A number of minor elements are also found at various stages of mineralization. The data obtained have been used to construct a schematic of the progression of mineralization along the length of the radula. © 1996 Wiley-Liss, Inc.     

Allometric and morphological characteristics of Tonicella marmorea (Fabricius, 1780) populations (Mollusca: Polyplacophora: Ischnochitonidae)

Allometric and morphological characteristics of population samples of Tonicella marmorea from three sea lochs on the west coast of Scotland are described. The suitability of allometric relationships as taxonomic criteria are investigated by statistical comparisons of various regression constants, scatter diagrams and comparisons of curvature indices. Morphological characteristics such as ctenidia number and numbers of notches in the head and tail valves proved too variable to be regarded as diagnostic but they are useful taxonomic indicators. The need for all taxonomic characteristics to be derived from population samples is stressed and the use of valve and girdle sculpturing together with radula structure is emphasized.     

New records for the shallow-water chiton fauna (Mollusca,Polyplacophora) of the Azores (NE Atlantic)

Published records, original data from recent field work on all of the islands of the Azores (NE Atlantic), and a revision of the entire mollusc collection deposited in the Department of Biology of the University of the Azores (DBUA) were used to compile a checklist of the shallow-water Polyplacophora of the Azores. Lepidochitona cf. canariensis and Tonicella rubra are reported for the first time for this archipelago, increasing the recorded Azorean fauna to seven species.     

Comparative ultrastructural studies of spermiogenesis and spermatozoa in some species of Polyplacophora (Mollusca)

Summary

Comparative data on the ultrastructure of spermiogenesis and spermatozoa of the Polyplacophora Acanthochitona crinita, Chaetopleura angulata and Callochiton septemvalvis are presented in this study. In contrast to what has been described for this and other classes of Mollusca, no acrosome is present in the spermatozoa of these Polyplacophora. The nucleus is extended by a long, thin apical point. In A. crinita and C. angulata the mitochondria are situated at the basal and lateral regions of the nucleus. They do not present a typical middle piece. These species present a pericentriolar process. In C. septemvalvis the mitochondria are situated at the base of the nucleus, surrounding the centrioles, which are orthogonally positioned in all species. The ultrastructural development during spermiogenesis is similar. In middle spermatids of A. crinita, the chromatin is arranged in fine filaments. In C. septemvalvis and C. angulata the chromatin filaments are thicker, forming coarse bands. In late spermatids elongation of the nucleus continues, it becomes rather electron-dense and the chromatin filaments are more condensed. Finally, the nucleus has a uniformly electron-dense appearance, with no signs of filamentous organization. Considering the ultrastructural modifications observed, the Polyplacophora spermatozoa could be included in a modified type.     

The fine structure of the frontal filament complex of barnacle larvae (Crustacea: Cirripedia)

Cell and Tissue Research - The frontal filaments comprise two regions, the internal vesicles and the external filaments. Dendrites of extra-optic protocerebral origin pass ventrally from the brain,...