Shell molluscs
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Roman snail ( Helix pomatia ) |
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| Conchifera | ||||||||||||
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Shell molluscs (Conchifera) are all molluscs with a primary, flat one- or two-lobed or shell -like exoskeleton on the (original) back of the body. In addition to this characteristic, the group is mainly justified by the possession of pines and paired statocysts as well as the lack of a thin, predominantly chitinous protective layer ( cuticula ) on the back of the body. The conchifers represent the vast majority of all recent and fossil known molluscs and have a large variety of shapes.
Structure, growth and function of the shell
In the structure of the closed exoskeleton of the Conchifera, also called the shell , a distinction is made between two layers: the periostracum , the outer, very thin layer, also called the membrane, consisting of organic matter ( conchiolin ), and the thick inner layer, consisting of calcium carbonate crystals , which are embedded in a conchiolin matrix. In numerous molluscs ( pearl boats , some snails, some mussels), the inner layer is further divided into the ostracum (prism layer), the outer calcium carbonate layer, located immediately below the periostracum, with relatively large prisms made of aragonite or calcite, oriented perpendicular to the shell surface , and the hypostracum (mother-of-pearl layer), the inner calcium carbonate layer, with very small, tabular aragonite prisms - the corresponding, iridescent mineral-organic composite material is called mother-of-pearl . In other representatives, the calcium carbonate layer is largely homogeneous, with prismatic structures and, in the case of pure aragonite scarfs, cross lamellae (aragonite needles arranged at an angle to one another, covered with a thin "conchiolin" layer) and mother-of-pearl. In addition to calcite and aragonite, mollusc shells can also contain the calcium carbonate mineral vaterite in small quantities.
The shells of the conchifers primarily have a protective function and have an enormous variety of shapes. They can form simple, hat-like or bowl-like "shells" or tubular, often spirally wound housings into which the animal can retreat completely in case of danger (see e.g. → snail shell ). Many casing-bearing conchifers form cover-like structures with which they can close the opening of the casing (aperture), which increases the protective effect (for example the head cap of pearl boats or the opercula of certain snails). Mussels do not have "real" housings, but the mussel shell , which consists of two flaps, fulfills the same function. The formation of the shell is closely linked to the way of life of its owner. So need limpets no "real" housing because they live on rocky ground and protect press the flachkegelige shell on the rock. The generally free-swimming pearl boats and their closer and less close fossil relatives (cf. → nautiloids , → ammonites ) have a chambered housing that is mostly filled with an air-like gas mixture and acts as a buoyant body (cf. → phragmocone ).
The shell is created early in the individual development. Typically, the cells of a region of the ectoderm opposite the original mouth (blastopore) , the so-called shell field, first thicken , and then the central part of the shell field invades into the blastocoel and forms the shell gland. Often part of the shell gland then everts out again like a bud. The shell gland or shell field either only separates the embryonic shell, the watch-glass-shaped protoconch I, after the protuberance or before . The growth of the embryo or the larval stage following hatching increases the size of the shell field to form the larval mantle epithelium and the protoconch I is expanded to form the larval shell, the protoconch II, through the addition of shell material at the edge. A pronounced Protoconch II is shown mainly by the shells of conchifers with relatively long-lived, planktotrophic Veliger larvae. In the case of mussels, the protoconch has two lobes and is therefore also known as the prodissoconch. Even in the adult stage, the surface of the shell only grows at the edges. The part of the shell formed during the adult stage is called the teleoconch. When the shell grows at the edge, the periostracal gland, which is sunk into the edge of the mantle, separates the periostracum and cells of the mantle epithelium secrete the outer layer of calcium carbonate. Since the surface growth is not entirely continuous, but rather in bursts, mollusc shells are characterized by typical growth strips on their outer surfaces. Cells of the mantle epithelium that are further away from the mantle edge secrete the calcium carbonate of the inner shell layers and thus ensure further growth in the thickness of the shell. Therefore, the inner surfaces do not show any growth streaks. Squids have moved or greatly reduced their shell inside the body. With them, the embryonic shell gland is overgrown by the unfolded, surrounding ectoderm and finally tied off.
Systematics
Among the shell molluscs include the Einschaler (Monoplacophora), the snails (Gastropoda), the cephalopods (Cephalopoda) and the Scaphopods (Scaphopoda) and the mussels (Bivalvia). In addition, the extinct Schnabelschaler (Rostroconchia) and the Helcionellidae (Helcionellida) are counted among the shell molluscs.
The traditional division of shell molluscs into curved shell (Cyrtosoma) and stretched shell (Diasoma), among other things based on different trends in shell formation, is no longer relevant today.
As a sister group, the shell molluscs are contrasted with the spiny molluscs (Aculifera). The cladogram below is based on the results of two molecular genetic related analyzes of molluscs published in 2011 in the journal Nature .
| Molluscs |
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literature
- Gerhard Haszprunar, Klaus-Jürgen Götting: Mollusca, molluscs. P. 305–362 in: Westheide, Rieger (Ed.): Special Zoology Part 1: Protozoa and invertebrates. 2nd Edition. Spektrum Akademischer Verlag (Elsevier), Munich 2007, ISBN 978-3-8274-1575-2
Individual evidence
- ^ Nicole Spann, Elizabeth M. Harper, David C. Aldridge: The unusual mineral vaterite in shells of the freshwater bivalve Corbicula fluminea from the UK. Natural sciences. Vol. 97, No. 8, 2010, pp. 743-751, doi: 10.1007 / s00114-010-0692-9 (alternative full text access : carbonateresearch.com ).
- ↑ Gernot Nehrke, Harald Poigner, Dorothee Wilhelms-Dick, Thomas Brey, Doris Abele: Coexistence of three calcium carbonate polymorphs in the shell of the Antarctic clam Laternula elliptica. Geochemistry, Geophysics, Geosystems. Vol. 13, No. 5, 2012, doi: 10.1029 / 2011GC003996 (Open Access).
- ↑ paragraph IEL by Andreas Wanninger, Tim Wollesen: Mollusca. Pp. 103-154 in: Andreas Wanninger (Ed.): Evolutionary Developmental Biology of Invertebrates 2: Lophotrochozoa (Spiralia). Springer, 2015, ISBN 978-3-7091-1870-2 , pp. 131 ff.
- ↑ Kevin M. Kocot, Johanna T. Cannon, Christiane Todt, Mathew R. Citarella, Andrea B. Kohn, Achim Meyer, Scott R. Santos, Christoffer Schander, Leonid L. Moroz, Bernhard Lieb, Kenneth M. Halanych: Phylogenomics reveals deep molluscan relationships. Nature. Vol. 477, 2011 pp. 452-456, doi : 10.1038 / nature10382
- ↑ Stephen A. Smith, Nerida G. Wilson, Freya E. Goetz, Caitlin Feehery, Sónia CS Andrade, Greg W. Rouse, Gonzalo Giribet, Casey W. Dunn: Resolving the evolutionary relationships of molluscs with phylogenomic tools. Nature. Vol. 480, 2011, pp. 364-367, doi : 10.1038 / nature10526