The concept of Gösta Jägersten of a primary biphasic metazoan life‐cycle, consisting of a planktotrophic larva and a benthic adult, forms the basis for several theories on metazoan phylogeny. In this ...paper the assumed planktotrophic life‐style of the larva is critically analyzed and reconsidered. It is shown, in particular for the Mollusca, that a biphasic life‐cycle with a lecithotrophic larva is probably the plesiomorphic condition. Character distribution and structural data suggest a parallel evolution of the downstream collecting system used in planktotrophic larvae or filter‐feeding adults of gastropods, bivalves and other spiralian or aschelminth taxa. In the basic metazoans (Parazoa, Placozoa, coelenterates) direct or lecithotrophic development dominates by far. For the acoelomate (Platyhelminthes, Gnathostomulida) and pseudocoelomate taxa direct development is probably the plesiomorphic condition. The structural similarities of the upstream collecting system in tentaculate and deuterostome phyla may also be explained by parallel events of heterochrony out of an ancestor with adult filter‐feeding. The main conclusion of this survey is that larval planktotrophy is likely to be secondary and not a plesiomorphic condition among the Bilateria. Accordingly, theories which are based on the assumed plesiomorphy of larval planktotrophy of the Bilateria, need careful reevaluation.
The anatomy of Melanodrymia aurantiaca Hickman, 1984, a coiled archaeogastropod from the East Pacific hydrothermal vents, is described based on reconstructions of paraffin and semi‐thin section ...series. Whereas the large protoconch with net‐like sculpture and the lack of a secondary larval shell, the bipectinate gill lacking skeletal rods, the rhipidoglossate rasdula and in particular the hypoathroid nervous system with pedal cords reflect an archaeogastropod level of organization, the single set of pallial organs, the monotocard heart, the single left kidney, the glandular gonoduct and internal fertilization indicated by penis (males) and receptaculum (females) resemble conditions of higher streptoneurans. The latter similarities are regarded as convergences, however.
The unique combination of primitive and advanced features as well as specific peculiarities (e.g. neural sexual dimorphism) prevents an inclusion of Melanodrymia aurantiaca in any of the known archaeogastropod groups. Based on similarity of snout formation and similar radula type several other recently described archaeogastropods (Peltospiroidea) are possibly related to M. aurantiaca.
The anatomy of the neopilinid limpet Laevipilina antarctica is described on the basis of reconstructions of semithin serial section. In general the anatomy of this small (3.2 mm) species resembles ...those of Neopilina galatheae (37 mm) and Vema ewingi (27 mm); small differences occur in the alimentary tract. Corrections of the original descriptions of neopilinid anatomy concern the structure of the mantle margin, the presence and interpretation of cephalic appendages, and details of the nervous system. The excretory organs are clearly separated from each other and there is no connection with the pericardium. The neopilinid ctenidia are considered as ventilatory rather than respiratory organs.
These corrections and clarifications of the neopilinid anatomy provide an improved database for discussion on monoplacophoran affinities. In contrast to popular statements about “living fossils”, the extant Tryblidiida (Neopilinoidea) are regarded as a considerably modified and specialized early conchiferan branch.
Shell pores with cellular content (caeca) are present in various molluscan groups. Up to now, the functions of the caeca are speculative, and also homology is questionable. To clarify the caeca of ...bivalve shell pores (tubules), the ultrastructure of the mantle of several species of arcoid and limopsoid bivalves has been investigated using microwave supported fixation and transmission electron microscopy. In general, the outer mantle epithelium is composed of several cell types: caecal cells have processes that extend to the periostracum; supporting cells have prominent tonofilaments that attach the mantle to the shell, many mucous cells, and enigmatic microvillar pads that may have a photoreceptive function. The hemocoel is vascularized and contains cells that are typical of the bivalve hemocoel such as amoebocytes, rhogocytes (pore-cells), muscle and nerve fibers, and gliointerstitial cells. The inner mantle epithelium is quite homogeneously composed of a partly ciliated, squamous epithelium with a brush border suggesting metabolic and respiratory functions. Structure (monocellular) and mode of development (secondary etching) contradict speculation concerning homology of bivalve caeca with those of other molluscan groups.
The microanatomy and ultrastructure of the larval excretory system of Patella vulgata L., 1758 has been examined by means of semithin and ultrathin serial sections, reconstructions, and transmission ...electron microscopy. The protonephridial system appears after torsion and consists of two terminal flame bulbs with narrow, ciliated ducts. Whereas the polyciliary terminal cells (cyrtocytes) are only slightly asymmetrically placed below the mantle cavity, the distal excretory ducts and their openings show remarkable asymmetry due to torsion. Further larval ultrafiltration sites with identical fine-structure (meandering slits with diaphragms covered by extracellular matrix) are present in the solitary rhogocytes (pore cells). The presence of larval protonephridia is regarded as plesiomorphic for Mollusca and the Trochozoa (Spiralia) as a whole and the specific conditions (asymmetry, simplicity) in Patella are probably plesiomorphic for the Gastropoda.
Recent data on early Cambrian fossils from various sources suggest that the earliest molluscs up to the conchiferan level of organization were animals of very small (1-2 mm) size. These data are in ...contrast to current theories on the origin and evolution of molluscan organ systems, most of which postulate an increase of size at the beginning of molluscan evolution. The evolutionary and functional implications and constraints of an original small body size on molluscan locomotion, respiration, excretion and reproduction are discussed. Based on recent fine-structure work the evolutionary history of the Mollusca is reconsidered. In addition, some alternative ideas are presented to explain the origin of the mantle cavity (originally also used as a fertilization chamber?), ctenidia (originally for ventilation rather than for respiration?), coelom (originally a
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gono-nephrocoel?), and pecularities of the alimentary tract.
Slugs of the genus Limax (Gastropoda: Stylommatophora) show a highly complicated genital system and reproductive behaviour probably triggering radiation and speciation. Pre-studies have revealed two ...so far largely undescribed species groups of Limax in Corsica. In order to clear up the phylogeny and evolutionary history of these radiations, we used a combination of molecular techniques and morphological characters. The two independent species groups of Corsican Limax species are monophyletic, and consist of six to ten species each, most of them new to science. The first species group, the endemic Wolterstorffi-group, can be differentiated by COI-Sequences, whereas COI-sequences fail to discriminate species of the Corsicus-group, which also has representatives in the Apennine Peninsula. This pattern suggests a much younger radiation of the Corsicus-group. Two hitherto unrecognized species on the adjacent islands of Elba and Capraia are described in an appendix.
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