Phylum Brachiopoda (2)
Brachiopods are bivalved, with the plane of symmetry running through the center of each valve. The pedicle valve is the larger and often has an opening for the pedicle, or stalk, which attaches the shell to the substrate or to another shell. The brachial valve is smaller and contains an internal support for the lophophore. The lophophore is a coiled organ with comblike filaments that are covered with cilia. The cilia keep a current of seawater coming into the body of the brachiopod when the valves are open. The lophophore traps microorganisms, passes them to the mouth, and obtains oxygen from the water. The mantle is the internal organ that secrets calcite for the shell in articulate brachiopods.
Articulates have two muscle systems for shell maneuvering, diductors for opening the valves, and adductors for closing them. Many articulates show a ridge or fold on one valve and a corresponding groove or sulcus on the other. Major brachiopods include the following groups.
Inarticulata have calcite or aragonite shells with poorly developed teeth, sockets, and pedicle. An example is Lingula, "living fossil," which has an apatite shell, and burrows in sediment of shallow shelf waters. It attaches to sediment in its burrow by a long pedicle. Its range is early Cambrian to Recent.
Articulata dominated brachiopod faunas by the Ordovician.