Brachiopods (from Latin bracchium, arm + New Latin -poda, foot) are a phylum of animals. Also known as lamp shells or Brachiopoda, they are sessile, two-shelled, marine animals with an external morphology resembling pelecypod mollusks (i.e. "clams") of phylum Mollusca to which they are not closely related. On July 16, 1986, the Kentucky State Legislature designated the brachiopod to be the Kentucky state fossil. Despite superficial similarities, bivalves and brachiopods differ markedly. Bivalves usually have a plane of symmetry between the shells, whereas most brachiopods have a plane of bilateral symmetry through the shells and perpendicular to the hinge. Both brachiopod shells are symmetrical as individual shells, but they differ in shape from each other. Whereas bivalves use adductor muscles to hold the two shells closed and open them by means of an external or internal ligament once the adductor muscles are relaxed, brachiopods use muscle power (internal diductor and adjustor muscles) to pull the shells apart and to close them (adductor muscles). Most brachiopods are attached to the substrate by means of a fleshy "stalk" or pedicle. In contrast, although some bivalves (such as oysters, mussels and the extinct rudists) are fixed to the substrate, most are free-moving, usually by means of a muscular "foot". Brachiopod shells may be either phosphatic or (in most groups) calcitic. Rarely, brachiopods may produce aragonitic shells. Some fossil forms exhibit elaborate flanges and spines.