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Notes: Abstract Tolerance experiments on freezing and supercooling (without ice formation) were designed to determine correspondence between tolerance to low temperatures and spatio-temporal distribution of one turbellarian and five polychaete species from sandy beaches of the North Sea island of Sylt. Freezing tolerances are always less than supercooling tolerances. Dinophilus gyrociliatus is significantly more sensitive to freezing (LD50 value after 30 min of freezing:-3°C) than the others, whereas Stygocapitella subterranea is significantly less sensitive (50% mortality at-15.7°C after 30 min). The supercooling tolerances differ considerably among the species. The sequence of tolerances (LD50 values) is as follows: Microphthalmus sczelkowii (-2.9°C after 4 h); D. gyrociliatus (〈1 h at-8°C); M. listensis (5.6 h at-8°C); Protodriloides symbioticus (8.2 h at-8°C); Notocaryoplanella glandulosa (66 h at-8°C); S. subterranea (72 h at-8°C). Species of sand flats (d. gyrociliatus, M. listensis, P. symbioticus) have lower tolerances than those of the beach slope (N. glandulosa, S. subterranea). Among the latter, tolerances increase with distance of the distributional area from low tide level. S. subterranea, a species occurring at the uppermost position in the intertidal, proves to be best adapted to both freezing and supercooling. Species preferring deeper regions of the beach (M. sczelkowii) show lower supercooling tolerances than surface dwelling forms. Northern species usually have higher tolerances to cold than southern ones, reaching their distribution limits near the island of Sylt.

Notes: Summary A comparative anatomical and ultrastructural study of ventral pharyngeal organs (pharyngeal bulbs) was carried out in two species of the Dinophilidae: Dinophilus gyrociliatus and Trilobodrilus axi. Special attention was paid to the fine structure of the stomodeal epithelium, cuticle, glands, muscles, and myoepithelial junctions. The differences between the species are very slight. The pharyngeal organ of the Dinophilidae is characterized by the following features: solid muscle bulbus made up of muscle cells only, bulbus muscle cells with two myofilament systems crossing at an angle of about 90°, gap junctions between these muscle cells, bulbus projects into a pharyngeal sac and bears rostrally a specific epithelium and cuticle, no bulbus glands, no investing (= sagittal) muscles, specific cuticle ultrastructure, cilia of ascending oesophagus with asymmetric tips, specific structure and position of salivary gland openings. The phylogenetic importance of these structures is discussed. Some of these characters are clearly autapomorphic features of the Dinophilidae and no common derived structures to other families with a ventral pharyngeal organ are present. Therefore, it is most likely that the dinophilid pharyngeal organ evolved independently. These findings do not agree with the hypothesis of the unity of the archiannelid families (Polygordiidae, Protodrilidae, Saccocirridae, Nerillidae, Dinophilidae, and Diurodrilidae) established on the basis of an assumed structural similarity of their ventral pharyngeal organs.

Notes: Summary In recently collected specimens of Acanthobdella peledina the nervous system, the genital organs and the coelomic organisation were reinvestigated after complete serial sections. These anatomical results are schematically represented. In addition, the integument, the chaetae and the peripheral muscle layer were investigated by electron microscopy. In general, the results confirm Livanow's classic monograph (1906), with the exception of a few details. The body apparently possesses neither a prostomium nor an achaetous buccal region (peristomium). The number of 29 true segments is concluded from the number of segmental ganglia. The five anteriormost segments, each with four pairs of hookshaped chaetae arranged around the mouth opening, are considered to be functionally equivalent to an anterior sucker. The ultrastructure of the integument and the chaetae generally conforms to the typical annelidan pattern. The muscle cells are of the typical hirudinean type. The outer male genital pore is positioned in segment 10; the female organs open in segment 11 directly behind the septum between segments 10 and 11. The main emphasis is laid on the evaluation of the position of the taxon within the Clitellata, including a discussion of the Branchiobdellida, and the cladograms presented show the Acanthobdellida to be the sister group of the Euhirudinea. Characters shared by the Branchiobdellida and Hirudinea (including A. peledina) are considered to be convergently evolved.

Notes: Summary Transmission electron microscopic studies were carried out on the ventral pharyngeal organs in Ctenodrilus serratus and Scoloplos armiger. The pharyngeal organs are composed of a muscle bulbus and a tongue-like organ. In both species the muscle bulbus consists of transverse muscle fibres and interstitial cells with voluminous cell bodies and dorsoventral tonofilaments; the investing muscle runs into the tongue-like organ; the nuclei of the investing muscle fibres are located in caudal bulges; salivary glands are not present, but numerous gland cells occur in the bulbus epithelium. The tongue-like organ, however, is formed by lateral folds (C. serratus) or a bridge-like structure (S. armiger). The specific structure of the bulbus muscle is probably a homologous characteristic also occurring in several other polychaete families. The phylogenetic importance of this ventral pharynx is discussed and a hypothesis is suggested to explain the differentiation of certain other ventral pharyngeal organs from this probably primitive type.

Notes: Abstract The segmental ocelli (eyes) in specimens of a European and a Chinese “Polyophthalmus pictus” population have been investigated by transmission electron microscopy. The ocelli are situated in corresponding positions in the same segments and reveal similarities in their general structure. They consist of one photoreceptor cell with microvilli-bearing processes and a pigment cup, the receptor processes project into an extracellular cavity formed by the sensory cell and a few supporting cells, the pigment cup is formed by mesodermal cells, and basiepidermal glial cells and gland cells lie above the sensory cell. However, the ocelli differ in size and number of cells, number and dimensions of cellular elements as well as presence or absence of certain cell types associated with the ocelli. There is only little variation in these characters and there is no overlap, so that they distinctly separate the specimens of the two populations. These differences are in the same range, or even larger, as those observed between the ocelli of other closely related polychaete species. Therefore, the specimens from Qingdao, China, are described as a new species of the Opheliidae, Polyophthalmus qingdaoensis sp.n., although specimens from Qingdao, China, and the Island of Giglio, Italy, are almost inseparable by light microscopy except for a few subtle differences.

Notes: Abstract  The ultrastructure of the nuchal organ and cerebral organ is described for the first time in a species of the Sipuncula, Onchnesoma squamatum. The nuchal organ is an unpaired structure lying outside and dorsal to the tentacular crown; furrows give the organ a paired appearance. The cerebral organ is an unciliated pad anterior to the nuchal organ. The nuchal organ consists of ciliated supporting cells, non-ciliated supporting cells and bipolar primary sensory cells. The cerebral organ is composed of unciliated supporting cells and numerous bipolar sensory cells. This clearly favours the hypothesis that this structure has a sensory function in adults rather than being a vestige of a larval organ. The sensory cells are similar in both organs and exhibit features indicative of chemoreception. Since the density of the sensory cells is low in the nuchal organ, an exclusively sensory function is questioned. There is some evidence that the two organs represent a functional unit. The present findings do not support the view that the nuchal organs of Sipuncula and ”Polychaeta” are homologous, but instead suggest that they are convergent structures.

Notes: Summary The “statocysts” inProtodrilus ciliatus, P. oculifer, P. haurakiensis andP. helgolandicus are situated in the prostomium anterior to the palps and have been investigated by electron microscopy. The sensory organs were reconstructed from serial sections, volumes were calculated from areas of consecutive section profiles, and additional data on surface area of distal receptor elements have been determined. In spite of variations in size (diameter 8–20 μm) their structure is nearly identical. The organs consist of one cup-shaped supportive cell, one large bi- or multiciliated sensory cell and two small uni- or biciliated sensory cells forming an extracellular cavity. This cavity is completely filled with microvillus-like or paracrystalline structures and there are no signs of statoliths composed of extracellular material. The most striking feature is the occurrence of paracrystals made up of undulating ciliary membranes extending from the large sensory cell and occupying 75–90% of the cavity inP. ciliatus, P. oculifer andP. haurakiensis. The remaining space is filled with microvilli or dendritic processes of the sensory cells. InP. helgolandicus the ciliary paracrystals are almost completely replaced by microvillus-like branches of cilia of the corresponding sensory cell. Paracrystals fill less than 10% of the cavity and are formed of flattened membranes. These sensory organs enclose large surface areas of membranes (15,000–38,000 μm2). The surface areas of the paracrystals composed of undulating membranes is almost identical to that of densely arranged arrays of microvilli (about 25 μm2 per μm3). These sensory organs are so different from all known statocysts that it is likely that they have another function. Their greatest structural correspondence is to light-receptive organs, especially in the structure and arrangement of microvilli. The role the paracrystals play is discussed: they might bear photopigments or simply represent a lens — a transparent, refractile and crystalline structure. These sensory organs are completely different from pigmented ocelli and phaosomes occurring in some protodrilids and represent a type of sensory organ thus far undescribed in polychaetes.