Polymorphism is very common in this group. The brachiopod phylum diversified early in the Phanerozoic, and reached its acme in the Paleozoic. While all brachiopods live in marine environments, some can tolerate swings in salinity, or even subarial exposure, and thus can live in the upper intertidal zone along rocky shores. Like their relatives the bryozoans, brachiopods respire and feed using a lophophore , and like bryozoans, they are masters of manipulating water flow. Unlike bryozoans, brachiopods are solitary.
All brachiopods secrete a mineralized shell composed of two bilaterally symmetrical valves.
The valves are manipulated by paired sets of muscles which insert into the interior of the valves, leaving scars. The valve to which the lophophore is attached is called the brachial valve , and is defined as being dorsally oriented. The other valve is called the pedicle valve and is defined as being ventrally oriented. While brachiopods superficially resemble molluscan bivalves, the two groups are easily distinguished from each other by careful observation. The valves of inarticulates are not joined into a hinge structure, like their articulate cousins.