Brachiopods are marine animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection. Two major groups are recognized, articulate and inarticulate. Articulate brachiopods have toothed hinges and simple opening and closing muscles, while inarticulate brachiopods have untoothed hinges and a more complex system of muscles used to keep the two halves aligned. In a typical brachiopod a stalk-like pedicle projects from an opening in one of the valves, known as the pedicle valve, keeping the animal anchored to the seabed but clear of silt that would obstruct the opening.
Lifespans range from three to over thirty years. The larvae of inarticulate brachiopods are miniature adults, with lophophores that enable the larvae to feed and swim for months until the animals become heavy enough to settle to the seabed. The planktonic larvae of articulate species do not resemble the adults, but rather look like blobs with yolk sacs, and remain among the plankton for only a few days before leaving the water column upon metamorphosing.
Brachiopods live only in the sea, and most species avoid locations with strong currents or waves. The larvae of articulate species settle in quickly and form dense populations in well-defined areas while the larvae of inarticulate species swim for up to a month and have wide ranges. Brachiopods now live mainly in cold water and low light. Fish and crustaceans seem to find brachiopod flesh distasteful and seldom attack them. Among brachiopods, only the lingulids have been fished commercially, on a very small scale.