Brachiopoda

Brachiopods (from Latin brachium, arm + New Latin -poda, foot) are a small phylum of benthic invertebrates. Also known as lamp shells (or lampshells), "brachs" or Brachiopoda, they are sessile, two-valved, marine animals with an external morphology superficially resembling bivalves to which they are not closely related. It is estimated by paleobiologists that 99 percent of all documented brachiopod species are both fossils and extinct.

Despite superficial similarities, bivalves and brachiopods differ markedly: Bivalves usually have a plane of symmetry between the valves of the shell, which are mirror images of each other; most brachiopods have a plane of bilateral symmetry through the valves and perpendicular to the hinge. The two brachiopod valves differ in shape and size from one another. Bivalves use adductor muscles to hold their two valves closed, and they open them by means of an external or internal ligament once the adductor muscles are relaxed. Brachiopods use internal diductor muscles to pull their two valves apart; they close the two with adductor muscles.

A second major difference is that most brachiopods are attached to the substrate by means of a fleshy "stalk" or pedicle. In contrast, although some bivalves (pelecypods such as oysters, mussels and the extinct rudists) are fixed to the substrate, most are free-moving, usually by means of a muscular "foot".

Furthermore, brachiopod shells may be made of either Calcium Phosphate or — much more commonly — Calcium Carbonate (CaCO3), as mollusks generally are. Lastly, in contrast to most bivalves, some extinct brachiopods exhibit elaborate flanges and spines.

On July 16, 1986, the Kentucky State Legislature designated the brachiopod to be the official Kentucky state fossil.