All brachiopods are suspension feeders, and extract food from the water around them using a structure known as a lophophore. The soft tissues of a brachiopod are enclosed by two valves, which can be opened and closed by muscles. All are sessile, and most are attached permanently to the substrate via a fleshy appendage known as a pedicle. Brachiopods superficially resemble bivalves, which are much more common today. Both are suspension feeders and possess shells composed of two valves. The two taxa are however, anatomically very different. Bivalves use a muscle to close their shells and an elastic ligament to open them again. A brachiopod uses muscles to both open and close the shell. Another way of telling the two apart is by a quick inspection of the two valves. In a brachiopod, the valves are different sizes, whereas the valves of a bivalve are usually identical to one another; mirror images. In the bivalves, the line of symmetry runs between the two valves. Also unique to the brachiopods is the lophophore, an organ utilised in both feeding and respiration.
The picture above outlines the main internal morphological features of a brachiopod. The brachiopod featured is Magellania, an articulate brachiopod. Nevertheless, the basic features remain the same. The brachiopod remains attached to the substrate via the stalk-like pedicle. The pedicle valve, ventral in this species, is the valve from which the pedicle exits. The adductor and diductor muscles work to close and open the valves respectively. The lophopore, not indicated in this diagram, is supported by the brachium. Many fossil brachiopods had a calcified support structure for the lophophore. In extant brachiopods, the lophophore is supported purely by hydrostatic pressure. Water is drawn into the valve and food particles are filtered out by the lophophore.
External brachiopod morphology, by Muriel Gottrop.
The above drawing illustrates some of the external features of a brachiopod shell, showing the line of symmetry and the dissimilarity between the two valves. As already mentioned, the line of symmetry in bivalves runs between the two valves.