Brachiopods are grouped together as a single phylum, usually assigned by invertebrate zoological textbooks to a group of minor coelomate phyla, and they are quite distinct from the bivalve Mollusca, which they resemble only superficially.
The term "Brachiopodes" was first used by the French palaeontologist Cuvier in 1805 and the following year Duméril formalized the name "Brachiopoda" for an order of the Mollusca. The name is derived from the Greek words for "arm" and "foot" and was given in the mistaken belief that the obvious coiled "arms" (what we now call the lophophore) could be extended from the shell and used in locomotion as "feet". Although this notion was soon shown to be erroneous the name survives enshrined in zoological literature and the term "brachial" is used for structures associated with the lophophore. Not only is the name Brachiopoda misleading as a description of the animals but it can be confused with the group of crustaceans called branchiopods.
Brachiopods are entirely marine sessile benthic invertebrates having different dorsal and ventral valves. They are found in waters from below antarctic ice to tropical reefs and from the intertidal to depths of about 6000 m. Most commonly they occur below low-water mark, from between about 200 m to 500 m depth. For this reason, and because the shells break easily once the animal has died, brachiopods are seldom found on sea shores.
Brachiopods are divided into two classes according to the lack or presence of articulating structures between the two valves of the shell - the Inarticulata and the Articulata. Today brachiopods are less diverse than in past ages when their shells were commonly preserved in marine sediments. For this reason fossil brachiopods are commonly more familiar to people than are Recent species (fossil species).
Brachiopods are usually identified from their shells alone, partly because they are studied more by palaeontologists than by zoologists, but also because internal shell morphology reflects the form of many of the principal internal organs.
Source: Brunton and Curry, 1979.