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  • Hard-Part Morphology Of Brachiopoda – Geology Notes – For W.B.C.S. Examination.
    Posted on December 3rd, 2019 in Geology
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    Hard-Part Morphology Of Brachiopoda – Geology Notes – For W.B.C.S. Examination.

    ব্রাকীওপোডার হার্ড-পার্ট মর্ফোলজি – ভূতত্ত্ব নোট – WBCS  পরীক্ষা।

    Brachiopods are marine animals belonging to their own phylum of the animal kingdom, Brachiopoda.Continue Reading Hard-Part Morphology Of Brachiopoda – Geology Notes – For W.B.C.S. Examination.

    Modern brachiopods occupy a variety of sea-bed habitats ranging from the tropics to the cold waters of the Arctic and, especially, the Antarctic. Most live in the relatively stable environments below the low-water mark in sea water of normal salinity, but some have wide salinity tolerances and live in more marginal marine environments, though few can survive the turbulence of the intertidal zone. Although now relatively rare, they may be locally abundant because they tend to be gregarious.

    As with all animals, brachiopods are classified into species and genera whose names must be Latin words or words that have been latinised. The proper scientific name of a particular brachiopod consists of the name of the species, preceded by the name of the genus to which it belongs, plus the name of the first person to describe it and the date of that description.

    Brachiopods are virtually defenceless and their shell, enclosing the animal’s organs, is their only protection. Most are permanently attached by a fleshy stalk (the pedicle) to a hard, sea-floor surface, such as a rock outcrop, boulder or some other shell, and are incapable of actively pursuing food. A few species can attach themselves directly to soft sediment and others remain unattached. The pedicle is the only soft tissue that protrudes outside the shell, which opens and closes to allow food-bearing currents of water to pass through it.

    The shell comprises two valves that are composed of calcite or chitinophosphate (calcium phosphate plus organic matter). The pedicle (or ventral) valve is typically externally convex. The other valve (the brachial or dorsal valve) may be similar but, in some brachiopods, it is extremely concave or more rarely conical.Many brachiopods have their valves hinged together, typically by a pair of ventral teeth and dorsal sockets. The snugly fitting join between the two closed valves is called the commissure. The inner surface of the valves may bear various projections and depressions that reflect the location and form of organs or muscles that open and close the valves. The external surface may bear concentric growth lines, lamellae and wrinkles (rugae), ribs, folds, sulci and spines.

    The space between the two valves is divided into the body cavity (located at the posterior end) and the mantle cavity (located at the anterior end). Most of the mantle cavity is occupied by a fleshy, hollow organ (the lophophore), which has long twisted or coiled arms. Closely spaced filaments are suspended from the arms along their entire length such that they touch those of the adjacent coil and thereby form an enclosed mesh. Lateral cilia attached to the filaments beat rapidly, drawing in food-bearing water currents.

    The water currents carrying food particles (represented by the solid black arrows in the diagram) enter between the brachiopod valves through the lateral inhalant aperture. The food particles are caught in the mesh formed by the filaments on the lophophore arms. The filtered water (represented by the white arrows) leaves via the median exhalant aperture. The filter-feeding mechanism is one of the characteristic features of the phylum, and is responsible for much of the variation seen in brachiopod shell shape.

    Many species grow calcareous supports (brachidia) inside the mantle cavity to support the coiled arms of the lophophore, whose various shapes they mimic. Fossils rarely show indications of the lophophore but many groups show calcareous structures from the posterior end of the brachial valve. Knowledge of the internal features of the valves is needed as an aid to identification.

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