Brachiopods are bivalved marine invertabrates that constitute a separate group, Brachiopoda. Brachiopods were prominent shell fish in many continental-shelf sea environments for about 250 million years, from the Ordovician into the Permian periods. In the Cambrian, some were prominent in nearshore environments. Since the end of the Paleozoic, clams appear to have been the prominent marine benthic bivalved animal in environments previously inhabited by brachiopods. Today, brachiopods are found in many habitats in nearly all of the shelf seas. Brachiopod shells, known as lamp shells because they resemble ancient Roman oil lamps are frequently collected. In some places, brachiopods are used for food.
The two shells of a brachiopod, unlike those of a clam, are diferent. About two-thirds of the shell interior is occupied by a tentacle-bearing arm. This organ functions essentially as a pump, bringing in oxygenated water and food particles and generating currents that take away wastes.
Most brachiopods are attached by a fleshy, muscular stalk to the sea floor for most, if not all, of their life. The body cavity is hollow, containing a rudimentary digestive system and some kidneylike organs. Brachiopods lack discrete respiratory and circulatory systems.
All brachiopods, except for members of the Lingula group, live on the sea floor. As bottom dwellers they are vulnerable to rapid rates of sedimentation and to predation. They have proved valuable as a means of dating rocks. Also, paleontologists working with Paleozoic rocks have used them to establish the depth of the ancient sea floor during different periods; different types appear to have been distributed in bands paralleling ancient shorelines, and the bands of particular types of brachiopods can be correlated with differing sea-floor depths.