A phylum of solitary, benthic, marine, bivalved, coelomate, invertebrate animals that have existed from the Lower Cambrian to the present day. Brachiopods are commonly attached posteriorly to the sea bed by a stalk (pedicle), but may be secondarily cemented, or free-living (e.g. the fossil form Productus which, like many productids, was spinose, thick-shelled, and lived partly buried in the mud of the sea bed). Usually they consist of two unequal valves: a larger pedicle (ventral) valve and a brachial (dorsal) valve, lined by reduplications (mantle lobes) of the body wall which enclose the large mantle cavity. They are bilaterally symmetrical about the posterior–anterior mid-line of the valves (i.e. through the valves, compare bivalvia). The characteristic feeding and respiratory organ, the lophophore, surrounds the mouth and is covered by ciliated tentacles. It may be a simple horseshoe but more often forms two ciliated arms or brachia that project through the gape (thus giving the phylum its name). The alimentary canal is divided into oesophagus, stomach, and intestine, with or without an anus. The nervous system consists of a circum-oesophogeal ring with a small aggregation of nerve cells on the ventral side. The excretory organs are one or two pairs of nephridia (excretory tubules) also acting as gonoducts (for the release of eggs and sperm). The circulatory system is open, with the contractile vesicle (heart) near the stomach. Considerably more than 3000 fossil species are known and about 330, in ten genera, are alive today; these are widely distributed and occur at all depths. Brachiopods are divided into three classes: Lingulata, Inarticulata, and Articulata.

Brachiopoda