Brachiopods are marine animals living on the sea bottom. They have a soft body covered by two valves, one pedicle valve and one brachial valve. The valves are opened and closed by muscles. The edible particles are brought to the mouth by brush-like tentacles attached to two thin calcareous archs. These are called brachia or "arms", hence their latin name. Some brachiopods have a muscular stalk, called pedicle, that attaches the animal to the substrate.
The brachiopods evolved in the early Cambrian, 540 million years ago. In the Palaeozoic era they were a large and important group. More than 30 000 fossil genera of brachiopods are known. Several hundred fossil species are found in the rocks of the Oslo region. There are only a few living brachiopods today; bivalves have taken over their earlier dominance. Three species of brachiopods are presently living in the Oslo fjord.
Brachiopods and Bivalves.
Brachiopods differ from bivalves, even though both groups have two valves:
1. Brachiopods have two valves which are different when seen from the side of the animal. Seen from above, the valves are symmetrical. The bivalves have two valves which are mirror images of each other when seen from the side. Seen from above the valves are not symmetrical.
2. Brachiopods have calcareous arches supporting the brachia. The bivalves lack these.
3. Brachiopods are attached to the substrate by the muscular pedicle. Bivalves use thin threads for attachment.
4. Brachiopods have valves made of calcium carbonate or calcium phosphate. Bivalves have valves only of calcium carbonate.
|The drawing (above) shows the internal structures in brachiopods.|
Publisert 18. mai 2011 16:00