Characters and Anatomy

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. 

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.